As design professionals specializing in forensic and restoration projects involving building enclosures, it is perhaps a familiar concept that our work benefits from excellent project team relationships—with building owners and staff, contractors and subcontractors, material manufacturers and suppliers, and local experts—and from a shared vision among the project team of successful end products and the best possible outcome for clients. With this in mind, a particularly challenging type of project is the reroofing of high-rise buildings in dense urban settings. This case study documents how our team developed goals for a roof system replacement project at a mid-1960s-era high-rise residential apartment building in Chicago’s downtown urban center. The project used a collaborative approach to obtain a successful result while overcoming challenges. As the architect/roof system designer hired by the building owner, our role for this project included assessment of existing conditions at multiple main roof and penthouse areas, preparation of bidding/construction documents, bidding administration, and construction-phase services (see Building History and Project Team sidebar). PRE-DESIGN CONSIDERATIONS AND RELATIONSHIPS The building owner decided to replace the roof system—in part due to a preceding roof anchor installation project that revealed the existing decades-old modified-bitumen (MB) roof system was saturated below the membrane (Fig. 1). Saturated roofs contribute to increased leakage and represent a higher risk of roof system blowoff, especially on high-rise buildings where wind uplift forces are higher. In our experience, high-rise roof systems often have adhered components, as opposed to mechanically fastened components. Mechanical fastening can be cost prohibitive, especially with concrete roof decks, and undesirable from both an acoustic perspective (for occupied buildings) and an energy standpoint (thermal bridging). When roof-system layers become saturated, however, internal components that rely on adhesion can lose strength as moisture causes the adhesive bond or the material itself to deteriorate and thus loose resistance to wind uplift forces. In order to inform our design, we performed a limited assessment of the roof areas to comprehensively evaluate existing conditions and substrates, identify details needed for future bidding documents, and provide recommended roof replacement system options with budgetary estimates of construction costs for the building owner. Such cost estimates, especially for complex projects, should include 32 • IIBEC Interface September 2021 Figure 1. Ponding was observed on the existing roof. consideration of challenges associated with roof area access, and more generally, access to the building that may be constrained due to building site limitations. For this project, we recognized the roof areas to be replaced were approximately 370 to 400 ft above grade and that the building was located in a dense urban area. Drawing upon our experience with similar projects, the design team recognized the estimated roof system replacement cost would be considerably higher compared with the same roof system installed on a low-rise structure. Higher estimated costs can be attributed to several factors, generally pertaining to access challenges and unique detailing required to address limitations posed by existing construction. As is the case with most complex projects, being able to effectively communicate why costs are estimated to be higher than the building owner may have anticipated was key for setting up a successful project with clear expectations. To serve as a tool for discussing project goals and expectations with the client, we prepared an assessment report which included observed conditions as well as recommended roof replacement approaches. Moreover, the report allowed for a more comprehensive conversation with the building owner related to specific challenges that impact performance and cost of roof replacement projects on highrise buildings, and in particular the identified issues that were recommended to be considered and monitored throughout the project. The success of this project was in large part due to the building owner’s desire to pursue long-term roof system performance over prioritizing initial costs, as well as their willingness to work through short-term inconveniences associated with a project with a more robust scope. Such an approach is often most appealing to experienced building owners planning to maintain ownership of a building over the long term. This life-cycle approach is often the least expensive on a cost-per-year basis due to a roof system design that addresses not only the roof system, but also defects in interrelated, roof-system-adjacent building elements such as parapet and penthouse walls, and roof decks that negatively impact overall roof system performance and life span if left unrepaired. Such defects allow moisture to circumvent roof system flashings and infiltrate into the dry side of the roof system, reducing the performance life of system components, making leaks more likely, and possibly negatively affecting wind uplift resistance. Identified during our review of existing Building History and Project Team The structure located at 1130 South Michigan is a 43-story apartment building overlooking Lake Michigan in downtown Chicago’s South Loop area with original construction drawings dated November 1965. The main roof sits approximately 387 ft above grade and is Y-shaped in plan, with a limestone coping that runs along the entire exterior perimeter parapet. A penthouse sits at the center of the main roof and includes a south and north wing, both with roof heights approximately 395 ft above grade. An elevator penthouse rises above the penthouse wings to a height of nearly 410 ft above grade and hosts a large cooling tower. A large metal screen extends over 30 ft from the main roof level to conceal the large cooling tower located at the uppermost penthouse roof. Owner: Draper and Kramer Roofing contractor: Riddiford Roofing Roofing manufacturer: Soprema Architect/Roof system designer: Klein & Hoffman September 2021 IIBEC Interface • 33 conditions were instances of deterioration at wall and parapet areas immediately above roof system penetrations, as well as at underside and slab edge areas of various cantilevered concrete penthouse roof decks. Damage on the underside of roof decks is sometimes indicative of deterioration on the top side. While addressing exposed defects like cracks above flashings is relatively easy, dealing with defects that are exposed only when the existing roof system is removed is significantly more challenging, as often the needed repairs are outside the expertise of roofing contractors. Managing the sequencing of exposing substrates to allow for observation and identification of damaged areas by the design professional or inspector, having the right subcontractor in place to perform required repairs, and doing all of this while ensuring weathertightness requires careful planning and clear communication with all parties. In the case of this project, we first worked with the building owner to develop an overall scope of anticipated repairs based on estimates of deterioration in concealed substrate areas as derived from our initial condition assessment and previous experience with similar project types. We then worked with ownership to determine what work would best be completed by subcontractors under the control of the roofing contractors (the contractor/subcontractor relationship often allows for smoother coordination) and what work should be completed by contractors independent of the roofing contractor as the building was already engaged with masonry and concrete repairs in other parts of the building. Bidding documents identified masonry and concrete work included in the roof replacement scope as a result, requiring a subcontractor for most roofing contractors. After a bidder was selected, the substrate remediation work was further refined and discussed during preconstruction meetings ahead of the roof replacement project. A part of these conversations included how best the roofing contractor could coordinate with contractors outside of their direct control. This was needed because some surfaces covered by roofing needed to be exposed to allow for repairs but also needed to be maintained in a weathertight condition. Ultimately, both the masonry and roofing contractors compromised to some extent to allow for all work to be completed without delay, without additional leaks being created, and without inconveniencing either contractor more than the other. Such arrangements, if done properly, benefit the building owner by maximizing the work completed by contractors already engaged at the project site. The building owner also opted to expand the project scope to include the installation of a new horizontal lifeline (HLL) system, to provide building staff and vendors with additional options for fall restraint (see HLL System sidebar). Our firm had previously designed roof anchors for the building in 2019, and we were able to work with our structural team to design a hybrid HLL system that consisted of a continuous HLL and dual-lanyard HLL, utilizing the previously installed roof anchors, and allowing for a substantial cost savings. Having experienced the harsh and windy environment on the roof of the building, it is clear why the enhanced protection of an HLL system was desired. During our initial investigation, evidence of the effects of wind were observed; examples include displaced concrete pavers and overturned rooftop equipment. In fact, recent local building code updates to roof-system-related code provisions included increased wind uplift resistance requirements; specifically, requirements per the American Society of Civil Engineers (ASCE) Standard 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, 2016 edition (ASCE 7-16). Although the project was permitted just prior to the rollout of stricter wind uplift code provisions, designing for wind uplift was critical for the new system—especially with respect to the extreme wind at higher elevations, and the evidence of prior undesirable rooftop effects due to wind. The design team ultimately decided to confirm with the roof system manufacturer that the planned new two-ply MB roof system (see following section, Roof Design Development, for additional information) had been tested per FM 4474, Standard for Evaluating the Simulated Wind Uplift Resistance of Roof Assembilies, (a standard referenced in the local building code), and found to have a wind uplift resistance in excess of the design wind uplift pressures calculated by the design team per ASCE 7-16. 34 • IIBEC Interface September 2021 HLL System A horizontal lifeline (HLL) is part of a fall protection and arrest system, consisting of a horizontal cable attached to several anchor points that workers utilize in combination with personal protection equipment (harness and lanyard). The intent of installing an HLL system at this building was to provide workers with continuous access to all elements of the roof while being anchored to a fall arrest system. The completion of a roof anchor project in 2019 (also by the design team responsible for the roof system replacement) allowed for the design of a HLL system that utilized the existing anchors. Another factor guiding planning for both design and construction of the replacement roof system was the falcons that reside on the main roof. From a practical standpoint, the falcons could injure or threaten the life of workers who encroached on the birds’ space during certain periods of their nesting cycle. When walking the roof areas for our initial investigation, the resident falcons were quite vigilant when we neared the nest. We even managed to catch a photo as one flew towards us with its talons out (Fig. 2). As a protected species, removal of the bird from the area of work was not an option. Discussions with the building owner began early on to review the impacts the birds would pose on the project schedule and also how best to contend with a deteriorated nest structure (roost) located on the main roof (Fig. 3). The falcons’ home was being monitored by the Field Museum’s Chicago Peregrine Program (see Falcon sidebar). The building owner was interested in updating the roost, and the design team understood the attachment of the new roost should be engineered to resist building code-mandated wind uplift pressures while also not interfering with roof surface drainage. Moreover, our engineered solution had to accommodate the birds’ needs, as well as provide access for members of the Chicago Peregrine Program responsible for monitoring the birds. ACCESS CONSIDERATIONS When working with existing building stock, there are a range of unique conditions that must be carefully evaluated prior to the start of construction and constantly revisited throughout the project. The reroofing of high-rise buildings in dense urban settings brings to the table a particular set of challenges simply because of building location, harsher work environments found at higher elevations above ground level, and existing layout of rooftop elements. Located in Chicago’s loop, access and mobilization were key considerFigure 2. A view of one of the resident falcons from the roof. Figure 3. The falcons’ nest on the existing roof. Falcon The peregrine falcon is a bird of prey that hunts medium-sized birds, dropping down on them from high above. The falcon is a fast flier, with average speeds around 25 to 30 mph, and reaching speeds up to 200 mph when diving towards prey. After their population significantly dropped due to pesticide poisoning in the 20th century, recovery efforts have contributed to a rebound of the birds. As part of recovery efforts, groups continue to monitor the populations, including the Field Museum’s Chicago Peregrine Program. Consisting of volunteers, the group provided valuable insight into the birds’ nesting cycle and periods of aggression that would prevent roof access, as well as the birds’ needs to be considered in the design of a new nest structure. 36 • IIBEC Interface September 2021 ations for this project as they impacted nearly every aspect of work. The roof areas overlook Michigan Avenue to the east, a heavily trafficked loading dock to the west, and frequently occupied amenity decks to the north and south (Fig. 4). To get to the main roof area from ground level required navigating through an occupied building and competing for a freight elevator in high demand with both other contractors and with resident move-ins/outs. It was clear that it would be a challenge not only to get materials up to the roof, but also to take demolition materials down to be disposed of at street level (Fig. 5). With limited space to store demolition debris and waste at roof level, materials had to be removed in an efficient and timely manner, but to do so utilizing the freight elevator would require complex coordination among several parties also using the elevator, as well as hauling materials through a residential corridor. In response, the contractor proposed utilizing a single-line power beam to hoist materials up and down along the west façade of the building between the main roof and loading dock, where proximity to the dumpster and to new shipments from the loading dock was an added benefit. Even so, the contractor had to be mindful of the heavy use of the service entrance by building occupants and vendors, and work with the tightness of the alley and proximity to neighboring buildings. In discussions with the building owner, the contractor agreed to provide a dedicated safety monitor during power beam operations. The roofs of high-rise buildings typically have a smaller area compared to most other building types, and they often will host several interfacing building elements that contractors must protect and work around, and in some cases remove and reinstall for proper roof system detailing. This project was no different; accessing various roof areas was not obstacle free after tackling the challenge of getting workers and materials to the main roof level. Additionally, there are two penthouse wing roofs, a central penthouse roof, a smoke shaft roof, and a stair tower roof. At the center of the main roof was a large metal screen with over 60 primary frame posts penetrating three different September 2021 IIBEC Interface • 37 Figure 4. A view to the northeast looking down from the roof shows the proximity to busy Michigan Ave. Figure 6. Roof configuration showing some of the interfacing challenges. Figure 7. More complex roof configurations. Figure 5. Tear-off debris and waste waiting to be taken to street level. 38 • IIBEC Interface September 2021 roof areas at two different heights (Fig. 6). Rising nearly 30 ft above main roof level, the metal equipment screen segments split each penthouse wing into separate areas and completely encloses the highest central penthouse. The configuration of the overlapping roof areas results in tight working conditions, and difficult-to-navigate areas of work (Fig. 7). As mentioned previously, the environment at this height was often harsh, with the roof and workers subject to high winds. These factors, in combination with low parapet heights at the outer perimeter of the main roof, were important considerations to discuss with prospective bidders, as they had potential to impact project cost, schedule, and overall logistics (see Roof Plan/3D View sidebar). September 2021 IIBEC Interface • 39 Roof Plan / 3D View 40 • IIBEC Interface September 2021 ROOF DESIGN DEVELOPMENT Due to the complexity of existing building elements and conditions, we understood that the design of the replacement roof system must address these challenges with unique atypical details. To ensure our design would adequately address the challenging conditions of this project, we adopted a “basis-of-design” approach for the roof system design. This method allowed us to work with a particular roof system manufacturer’s technical department during the design phase, and we selected a roof system manufacturer that we had worked with in the past and whom we had had successful outcomes with other challenging projects. Ultimately, we pursued a two-ply SBS MB roof system, asphalt-core cover board, rigid polyisocyanurate insulation with coated-glass facers (ASTM C1289, Type II, Class 2) with a MB vapor barrier for our basis of design. Some benefits for specifying such a system include durability of the cap sheet, an especially good compatibility with reinforced liquid-applied flashings (in this case, polymethyl-methacrylate [PMMA]) used to address challenging conditions, compatibility with potential residues left over from the previous roof system, and the ability of the vapor barrier to interface with the roof system membrane to aid in creation of unique details. A main goal in the design of the replacement roof system was getting “buy-in” from the roof system manufacturer regarding the atypical detailing that would be needed and getting assurance that roof system component testing we required in the roof-system specification would be honored during the construction phase; working together was key. Drawing upon the roof system manufacturers’ technical knowledge of their products, and our professional A/E experience with challenging projects requiring unique solutions, we were able to develop details that addressed conditions of the existing construction and contributed to a new roof system that not only meets or exceeds code requirements but also provides long-term value and performance for the building owner. As an example, we solicited the roof system manufacturer’s involvement with performing peel tests ahead of the work to finalize planned attachment methods for the new vapor barrier layer (Fig. 8). The method of testing was informed by the roof system manufacturer’s procedures noted in their published vapor retarder technical manual. As this project was prompted by the discovery of excessive moisture below the existing roofing membrane, it was a reasonable assumption that the concrete deck had potentially taken on some moisture as well. This was an important assumption to consider and plan for, as it had the potential to not only prohibit the prompt installation of the new vapor barrier, but also to compromise the wind-uplift resistance of the new roof system. Ultimately through field testing with the roof system manufacturer, evaluation of existing conditions, and review of recent roofing industry research and reports related to moisture in concrete roof decks, an attachment methodology was determined prior to bidding the project. It is also important to note that testing also done during construction to verify other areas were similar to the first testing location yielded the same results which initially informed our roof system design and attachment methods. In many roof replacement projects, it is not uncommon to encounter an original roof system that was designed to meet less stringent codes in place when the system was designed, designed to meet only the minimum requirements, or designed with little regard to “replaceability.” For example, it was discovered the existing roof system was designed with approximately 1 to 2 in. of above-deck insulation. This thickness of insulation represents a thermal resistance (R-value) that is less than current code requirements. Furthermore, the design of existing roof system terminations, as well as the installation of rooftop elements, were based on the thinner above-deck insulation and total roof system thickness of the original roof system. Such conditions are not always conducive for increasing the thickness of insulation for the new system, as raising these adjacent elements can pose exorbitant costs upon the building owner, as doing so would require modification of adjacent rooftop elements such as through-wall flashings, door thresholds, equipment curb heights, parapet heights, and the like. Challenges of increasing insulation thickness further complicates design of the new system, as tapered insulation for this project also served the purpose of drainage. A prime example of this was found at the perimeter parapet of the main roof, where a core at the existing roof system revealed a distance of roughly 10 in. from top of deck to bottom of the limestone coping (Fig. 9). To put this into perspective, the Figure 8. A view of peel testing being conducted during design. Figure 9. A core of the existing roof system revealed a 10-in. depth from top of deck to bottom of the limestone coping. minimum flashing height, as required by the roof system manufacturer, typically ranges from 8 in. above the top of the roof system for traditional flashings and less for PMMA or similar flashings. Moreover, this would leave 2 to 6 in. for the total roof system thickness (including the insulation, coverboard, and membranes) where it terminates along the entire perimeter of the main roof. Although it is often possible, in our experience, to obtain relief from building departments when such infeasibilities exist, it was still a priority to provide the highest performing system possible. Ultimately, the design team relied on code language developed by the local jurisdiction (see Local Energy Code Exception sidebar) where we increased the R-value compared to the original system but provided less thermal resistance within the roof system than the energy conservation code dictates; basically, we provided the maximum insulation thickness we could while also ensuring an overall roof system thickness that accommodated minimum roof system flashing heights and allowed for rooftop drainage. Doing otherwise would have required raising the height of over 600 ft of parapet, raising penthouse floors and related equipment, making major alterations to glazed brick penthouse walls, drainage systems, etc. CONSTRUCTION-PHASE QUALITY PROCESSES Working with a project team that shares the common goal of a successful project is critical in the success of any project, but it was especially important in overcoming the many expected and unexpected challenges faced during the construction of this complex roof replacement project. It was understood from the start that the construction schedule would be a challenge with work unable to commence until the falcons’ nesting cycle had completed in mid-to-late summer. This delayed start did not necessarily allow for a delayed completion, however, as winter in Chicago would bring extreme weather and temperatures not suitable for the crew nor the materials being installed. For this project, there was the additional complication of the COVID-19 pandemic and the related closures and restrictions, bringing with it a period of uncertainty. There were times where access to the building and surrounding neighborhood was limited, for various reasons, to residents only, which posed schedule-related challenges. With an atypically large percentage of occupants working from their homes within this existing residential building due to COVID-related reasons, there was an increased building Local Energy Code Exception In a reroofing situation, it is sometimes difficult to comply with the minimum required R-value for above-deck roof insulation, as the original roof system design often has a thickness of insulation below the level needed to achieve the code minimum. As mentioned in the body of the article, there were limiting conditions; therefore, the design team reviewed applicable code language to inform our design. For this project, the Illinois Energy Conservation Code, based on the 2018 Edition of the International Energy Conservation Code was in force. The Illinois code contains an amendment to the model code language, in Section C503- ALTERATIONS, where the following exception was added during the code adoption process: “8. Roof replacements for roof systems 2:12 slope or less shall comply with the low slope roof insulation requirements unless the installation of insulation above the structural roof deck, and necessary to achieve the code-require R-Value, is deemed infeasible by the code official to accommodate the added thickness of insulation above the roof deck. Conditions of infeasibility due to flashing heights presented by existing rooftop conditions include, but are not limited to, HVAC or skylight curb, low door or glazing, parapet, weep holes, drainage patterns, cricket or saddle construction. These conditions are subject to manufacturer’s specifications, manufacturers installation instructions and code official approval.” Further, the Chicago Energy Conservation Code contains the following provision in Section 306.1: “2. Roof replacement or roof recover of existing low sloped roofs shall comply with the roof insulation requirements for new construction unless the installation of additional insulation above the structural roof deck is infeasible due to the height of existing parapets, equipment curbs, skylight curbs, window sills, door thresholds, and similar elements with flashing into the roof system. In no case shall a roof replacement or roof recover reduce the insulating value of the roof.” The project team was guided by the above code provisions to find the right balance to improve overall thermal resistance for the roof system versus the existing system while providing a solution for ownership that did not require an infeasible level of modification to adjacent building elements. September 2021 IIBEC Interface • 43 occupant presence to be cautious of—both within the building and at street and amenity deck levels below. This required workers to be more cognizant as they respectfully navigated through the building to get to roof level and gave rise to a more limited tolerance for noisy work due to increased occupancy during typical workday hours. With restrictions posed on indoor dining, the restaurant tenant of the building had an increased presence at ground level and planned to provide outdoor seating, which would bring additional pedestrian presence below the area of work. Such challenges also sparked additional safety measures, as recommended by the contractor and building owner, and coordination to shut down areas for overhead work. The project team worked closely with the building owner, contractor, roof system manufacturer, and the Chicago Building Department to work through the COVID-19-related challenges to allow the project to move forward, while at the same time, doing everything possible to ensure city- and state-mandated procedures were being followed. Additionally, we worked to ensure design team members, building occupants and staff, contractors, and staff from the roof system manufacturer had a comfort level to ensure the project could move forward safely. As previously noted, following our initial condition assessment of the roof areas, early discussions with the building owner evaluated the benefits of addressing observed deterioration at interrelated building elements while mobilized for the roof replacement project. The resulting scope of substrate repairs (many of which are often overlooked with roofing replacement projects) was included as a preventative measure to ensure optimal roof system performance and lifespan of the new replacement Figure 11. Workers conducting quality control and quality assurance on the jobsite. 44 • IIBEC Interface September 2021 Figure 10. An example of a space that required complex coordination among trades to sequence and complete work. system. It made sense to have substrates that were concealed by the existing roof system, such as the masonry wall bases and topside of concrete roof decks, to be captured within the roofing contractor’s scope, as such locations would need to be made weathertight at the close of every workday, and this allowed for smoother coordination between the roofer and their subcontractor. In turn, locations of brick masonry and concrete that were readily accessible prior to removal of the existing system would be addressed by the concrete and masonry contractor, already engaged elsewhere on the building. Although this would allow an opportunity to capture the benefits of specialized skilled labor and increased access to similar repair materials required for masonry contractor’s work elsewhere on the building, the division of labor posed challenges related to dual-prime work. With limited space for workers to maneuver (Fig. 10), and limited time to sequence work, coordination of the two prime contractors was key. One example of this complex coordination can be found in the number of underside slab repairs that were designated at the expansive overhang of the highest concrete roof deck (Roof Zone C). Roofing work could not occur below the area of concrete repairs until such repairs were completed, as access was restricted by the large network of scaffolding needed to reach the overhead concrete, and repair work above the new roof system was to be avoided due to possible damage that could result from overhead work. These concrete repairs also impacted what roofing work could begin above, as the patch for these underside repairs would need to be poured from the top of the roof deck through a core and it was preferred that openings to access these cores be made in the existing roof system, rather than the new roof system. With these repair areas overhanging a portion of nearly every roof area and also impacting the roof area above, we actively worked with both contractors to resolve optimal sequencing of each respective work scope with the shared goal of keeping the project on track. This required an understanding of the sequencing proposed by each contractor for their respective portion of work, how they could impact or delay one another, and how to assist with fitting these pieces of the puzzle together without assuming each contractor’s individual responsibility for schedule. In facilitating this portion of the project, the design team made sure to have an increased presence on site to designate work, review repairs, and resolve questions to maintain the project’s momentum. Transparent communication in the facilitation of quality control (what the contractor does to ensure quality) and quality assurance (what the design team does during construction to review the installation of roof system materials) processes can be critical for the success of any project, but it becomes especially important when working with the complexities of a high-rise roof replacement project. Understanding the contractor’s quality control practices to identify and correct errors in the work allowed for many concerns to be addressed early on and limited potential costly and disruptive corrective action. As part of our quality assurance practices, performed to ensure contractors’ work meets the design intent, we met with key project managers on the contractors’ teams to review details on site and ensure our design intent was not lost in translation. The design team also performed regular visits to observe roof system components being installed. During each visit, we discussed concerns with the project foreman and issued timely field reports after each visit to the building owner and contractor to keep all informed of issues and resolutions (Fig. 11). Through this rapport, an excellent working relationship was established between all parties that allowed for September 2021 IIBEC Interface • 45 THE ORIGINAL HIGH PERFORMANCELIQUID FLASHINGIdeal for roofing, waterproofing & building envelope applicationsFast, cost effective install with up to 50% labor savingsCompatible with a variety of roofing substrates, no special primers neededSolid monolithic & waterproof configurationUse on vertical or horizontal applicationsAvailable in multiple sizes & containers 515 LIQUID FLASH™ – PERMANENT DETAIL SEALANTWWW.APOC.COM • (800) 562-5669 • INFO@ICPGROUP.COMAPOC IS NOW A PART OF 46 • IIBEC Interface September 2021 Figure 12. A mock-up of the railing post is shown. often quick resolutions to issues that arise on all construction projects. We also required the contractor to coordinate semi-regular site visits with the roofing system manufacturer to confirm proper installation methods, particularly at uncommon conditions, so as to ensure proper adhesion and performance of the new roof system. Due to the collection of unique detailing required for this work, PMMA was used as it can be applied to a wide range of typically limiting conditions, and it is compatible with the new MB roof system. While properly installing PMMA can be challenging, we were fortunate to be working with a contractor that had experience with the intricacies of PMMA. Evaluation by the design team, contractor, and roof system manufacturer’s quality control and quality assurance processes was important to our success. The project’s collection of unique details, focused on providing watertight terminations at atypical conditions, required further collaboration among the design team, contractor, and roof system manufacturer through the process of mock-ups. For instance, existing railings along the edge of the highest roof had to remain in place but created a condition along the edge of the roof slab that could not be flashed per typical manufacturer’s recommended detailing (Fig. 12). Upon review of newly exposed conditions during tear-off, alternate means of detailing the railing posts along the roof edge were evaluated. We met on site with the contractor to observe installation of the proposed detailing, and to resolve any questions and concerns. A series of sketches were exchanged to document the final detailing decision so that the roof system manufacturer could review and provide insight regarding product performance in this atypical application. While an experienced roof system designer may be talented at anticipating and planning for unique and unforeseen conditions in the design phase, there will almost always be issues encountered during construction that will require in-the-field tweaks. Coordination and communication were critical in both the review and the development of unique solutions that not only satisfied roof system manufacturer requirements, but also fit within the project schedule and budget. For instance, following installation of the new roof system at one of the penthouse wings, it became apparent that localized portions of the concrete roof deck were not uniformly level, as there was a large area of observed ponding, despite the newly installed tapered insulation (Fig. 13). Our solution had to address the ponding in the most economical way and also be feasible within Figure 13. Ponding was observed at one of the penthouse wings, despite the newly installed tapered insulation. September 2021 IIBEC Interface • 47 While an experienced roof system designer may be talented at anticipating and planning for unique and unforeseen conditions in the design phase, there will almost always be issues encountered during construction that will require in-the-field tweaks. the short window of workable weather we had remaining. Removing the newly installed roof system as needed to provide additional slope had the potential to be a costly and lengthy fix. Since this location was not easily accessible or heavily trafficked, installation of a field of PMMA was a potential solution that would provide additional protection to the new MB membrane from occasional ponding (Fig. 14). Fortunately, in anticipation of unforeseen conditions, the design team had included unit cost allowances for various repair materials, including PMMA, with estimated quantities on the project bid form. This provided the ability to invoke a quick solution, without a significant impact, if any, to the overall contract sum, as the contractor’s original price included costs for such potential work. FINAL THOUGHTS While this project included both anticipated and unanticipated challenges, a successful result was achieved principally due to: • The building owner’s openness to adopt a life-cycle approach that prioritized both direct roof system component replacement and adjacent surface repairs that minimize future water infiltration into the roof system; such an approach is often more expensive in the short term but less expensive over the long term when evaluated based on performance life of the new roof system vs. the shorter useful life of a lesser roof system—especially when substrate repairs are deferred. 48 • IIBEC Interface September 2021 Jason Wilen, RRO, AIA, CDT, is a senior associate II with the Chicago structural and architectural engineering firm Klein & Hoffman. He has a degree in architecture from the Illinois Institute of Technology and is a licensed architect specializing in forensics, restoration, and building envelope commissioning. Wilen is a roof systems expert with 28 years of experience and a background in building and energy code and standards development. Prior to Klein & Hoffman, he served for seven years as a director of technical services at the National Roofing Contractors Association (NRCA), where he helped develop best practices for the roofing industry nationwide. Violet R. LaBrosse, AIA, LEED Green Associate, is an associate II with the Chicago structural and architectural engineering firm Klein & Hoffman. She holds bachelor’s and master’s degrees in architecture from the University of Illinois at Urbana-Champaign and is a licensed architect specializing in forensics and restoration. Her passion for creative problem-solving and experience with both roofing and façade projects have enhanced her ability to comprehensively evaluate existing conditions and develop details that address the complexity of interfacing building system components. Jason Wilen, RRO, AIA, CDT Violet R. LaBrosse, AIA, LEED Green Associate • Selection of a contractor experienced with high-rise roof system replacement in dense, inter-city locations; all contractors on the bid list had a history of success with similar projects. • A design team with an established working relationship with the building owner and experienced with similar projects and systems, allowing for matching of optimal material replacement selections. (Our relevant experience also helped us to anticipate possible unforeseen conditions and better plan for them/have an action plan in our arsenal.) • Identification of unique project challenges early on in the process and incorporating project-specific detailing and testing that was vetted with the roof system manufacturer prior to construction. • Flexibility with all involved to deal with unexpected challenges. Figure 14. Polymethyl-methacrylate was installed to protect the new modified-bitumen membrane from occasional ponding at the penthouse wings.
