|TITLE:||Roof Covering Systems and New Concrete Roof Decks|
|OBJECTIVE:||To provide commentary on the installation and design of low-slope roof systems over new concrete roof deck substrates.|
Roof membrane and roof assembly installations over new concrete roof decks have always been prone to moisture-related problems due to latent moisture release by the concrete as it continues to cure. The problems are due to the release of excess moisture from the concrete into the roof assembly, which can impact roof system securement, the roof membrane, roof insulation, and related components. Moisture-related problems for roof membrane and roof assembly installations over concrete decks have increased significantly in recent years with changes in roofing materials, adhesives, construction practices, and schedules. The extent of these problems is visible in roofing industry bulletins, advisories,[i] and industry technical articles on this subject issued by the Midwest Roofing Contractors Association,[ii] National Roofing Contractors Association,[iii] Asphalt Roofing Manufacturers Association, GAF,[iv] and IIBEC.
The primary changes in roofing materials, adhesives, construction practices, and schedules that affect roof system performance over concrete roof decks include the following:
- Use of alternative methods instead of asphalt as an adhesive for the direct attachment of roof insulation to concrete decks. A continuous film of asphalt properly bonded to the concrete can function as a vapor retarder. Thus, the use of asphalt as an adhesive mitigated the amount of latent moisture release from the concrete into the roof assembly in amounts large enough to cause roof membrane distress or failure. However, alternative attachment methods—specifically the use of ribbon applications of low-rise foam adhesives or mechanical attachment—have resulted in portions of the roof deck directly exposed to the roof insulation, allowing moisture to enter the roof assembly. The use of mechanical attachment or low-rise foam adhesives over concrete decks is primarily a change in low-slope roofing construction practices.
- Use of concrete roof decks with lightweight aggregate instead of normal-weight aggregate. The use of lightweight aggregate in structural concrete has increased as designers look to minimize overall building weight and lessen construction costs. Lightweight aggregate is typically expanded shale, clay, or slate materials, which have been fired in a rotary kiln to create a porous structure, or air-cooled blast surface slag. Because lightweight aggregate is porous, it retains moisture longer than normal-weight aggregate.
Additionally, lightweight aggregates are pre-wetted or stored in water prior to batching the concrete. This is needed to minimize moisture absorption by the aggregate and to allow adequate water to be available for cement hydration. Thus, the use of lightweight aggregate results in concrete with a greater initial moisture content and concrete that will release this excess moisture over a much longer period of time.
- Use of adhesives sensitive to moisture. Increased restrictions on VOC content in products in some regions of the country have resulted in the elimination of many solvent-based adhesives that perform successfully as single-ply-membrane adhesives. Water-based adhesives are much more susceptible to re-emulsification when exposed to moisture, depending on the adhesive formulation.
- Use of a vented or non-vented steel pan deck beneath concrete. Concrete placed over a steel pan deck provides diaphragm resistance against lateral load due to wind and seismic forces and is a common application on many commercial and institutional buildings. However, steel pan decks, whether vented or non-vented, function essentially as a vapor retarder located beneath the concrete, which significantly reduces the amount of drying that can occur from the bottom side of the concrete. Therefore, moisture from the concrete can only dry through the top surface, at perimeter edges, at side/end laps of pan deck, and at penetrations through the slab. This type of construction is more problematic when lightweight aggregate concrete is used. Ventilated decks may allow for some drying of the deck; however, they still provide very little net venting of moisture and provide minimal value with respect to downward drying.
- Accelerated construction schedules. The standard 28 days at which concrete is evaluated is often misconstrued as a suitable “drying” period for the concrete prior to roofing application. The 28-day curing period, however, is commonly what is required to obtain acceptable structural strength and has no significance or correlation to the moisture content of the concrete. Accelerated construction schedules often do not allow for adequate drying of concrete—even with optimal climatic and construction conditions.
- Additional Items: Items such as the mixture proportions, admixtures, curing methods, finishing methods, and curing compounds for the concrete can also affect its moisture content and drying rate. In addition, climatic conditions and the amount of precipitation that occurs after initial concrete placement can significantly impact the moisture content and drying rate of the concrete.
MOISTURE TESTING FOR CONCRETE DECKS
Moisture testing of concrete roof decks has historically included the placement of hot asphalt on the top surface of the concrete deck, plastic mat tests, or the use of non-destructive capacitance moisture meters. The following are test standards for evaluation of moisture in concrete slabs in use today, followed by commentary on each:
- ASTM D4263, Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method. This test method was developed for indicating the presence of capillary moisture in concrete prior to the application of coating systems. This test includes sealing the perimeter of a small, transparent plastic sheet or glass pane to the concrete surface, monitoring it over the required time, and visually inspecting the underside of the sheet for the presence of moisture.
- Commentary: This test method is unreliable due to the difficulty of creating an airtight seal and will often result in a false dry reading. Lightweight aggregate concrete roof decks release moisture over an extended period of time and a “dry” observation with this test method is not indicative of long-term moisture release that can occur after the roof is installed. As such, this test is of little value for determining acceptable moisture levels in concrete roof decks prior to roof installations.
- ASTM F1869, Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride. This test method measures the rate of moisture vapor emitted from concrete and has been commonly used prior to the application of flooring systems. This test method utilizes a cylindrical plastic dish with a transparent cover that contains calcium chloride and that is sealed to the concrete slab at strategic locations. Each dish is weighed prior to placement and after being in place for a pre-determined time to calculate the moisture vapor emission rate.
- Commentary: This test method is not intended for use on exterior roof decks and may be misleading because it is primarily limited to measuring the moisture content in the top ½ inch to ¾ inch of the concrete. In addition, this test method cannot be used to evaluate moisture vapor emitted by lightweight aggregate concrete because the release of its excess moisture is often delayed.
- ASTM F2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes. This test method was originally developed for the flooring industry to measure the moisture in concrete floor slabs prior to the application of flooring systems and coatings. It utilizes small humidity probes that are installed into pre-drilled holes in the concrete and allowed to equilibrate for a minimum of 24 hours. Each probe measures the internal temperature and relative humidity of the concrete. This method is based on testing in an interior conditioned environment.
- Commentary: This method has had great success in the flooring industry and shows the most promise for use in determining the moisture content of concrete roof decks. The test method, however, will yield varying results on a roof deck due to exposure to sunlight and differing climatic conditions (i.e., temperature and humidity). The current test standard requires a controlled conditioned interior space. Additionally, there are no guidelines for this test to adapt it to uncontrolled environments such as roof decks. The NRCA has suggested a maximum relative humidity of 75% for both normal-weight aggregate concrete and lightweight structural concrete prior to roof application, but this number is not validated by research. This amount of moisture, however, may impact moisture sensitive roof system components such as organic materials, paper faced polyisocyanurate, wood fiberboard and water-based adhesives. Further research is necessary to determine ASTM F2170’s effective use on new concrete roof decks and to determine the maximum relative humidity of the concrete prior to roof application.
- Capacitance and Pin Probe Moisture Meters. This type of test is non-destructive because it involves setting a unit directly on the concrete. The meters have both a transmitting and receiving pad that send radio waves into the substrate to measure the capacitance, which is related to the moisture content of the substrate.
- Commentary: This type of test only measures moisture in the concrete in a very small area (all of these tests are performed in “small” areas or spot locations on a deck) near the approximate top 1½ inches of the concrete slab and does not provide information on the amount of moisture in the lower portion of the concrete slab. Because the moisture content of concrete slab surfaces is often dependent on ambient temperature and humidity conditions, these meters do not provide a reliable measure of concrete slab moisture content. In addition, these types of meters can provide false readings in concrete, if there are large pieces of aggregate or reinforcing steel near the surface that impact the capacitance.
ACCEPTABLE MOISTURE CONTENT IN CONCRETE ROOF DECKS
The unanswered question for the roofing industry is what actual moisture content or relative humidity level is acceptable in concrete roof decks for various roof system applications. Presently, no roofing manufacturer or industry association has taken a position on an acceptable criterion for moisture content of concrete roof decks, and there is no accepted industry standard method of determining moisture content in concrete roof decks. Roofing manufacturers, the roofing industry, and the concrete industry require further research to determine what level of moisture in the concrete is acceptable and to develop a field test method that is reliable for use on roof decks.
WIND UPLIFT RESISTANCE
Wind uplift resistance of low-slope roofs often relies on adhesion of roof system components to the roof structure. On concrete roof decks, adhesion is more commonly used for system component securement than mechanical fastening, because the installation of fasteners into concrete is more difficult than into steel or wood decks. As such, adhesives are often used, and they must be able to perform in moist and alkaline conditions. Elevated moisture levels in concrete can negatively impact the adhesion of adhesives, asphalts, and vapor retarders/membranes and must be considered prior to installation.
Designers and installers of insulated roofing assemblies over concrete roof decks should consider the following:
- Concrete Mixture Proportions and Placement:
- Limit the water-to-cementitious materials ratio (w/c ratio) (for normal-weight concrete, the w/c should be less than or equal to 0.40; for lightweight structural concrete, the w/c should be less than or equal to 0.45)
- Avoid hard-troweling the slab, which densifies the newly placed concrete surface
- Use a cover to cure the concrete and avoid film-forming curing compounds
- New Concrete Roof Decks: Allow time in the schedule for drying of the concrete roof deck past the 28-day curing period prior to roof installation. Precipitation on exposed concrete roof decks during and after the initial concrete curing will significantly increase the required drying time that must also be taken into consideration.
- New Lightweight Aggregate Concrete Roof Decks: Concrete utilizing lightweight aggregate can take much longer to release excess moisture than normal-weight aggregate concrete and, as such, should not be expected to be adequately dry for the installation of an insulated roof assembly without a vapor retarder or moisture release system.
- New Normal-weight Aggregate Concrete Roof Decks: Concrete using normal-weight aggregate typically releases moisture much quicker than lightweight aggregate mixtures; however, it should not be expected to be adequately dry on a typical project for the installation of an insulated roof assembly without a vapor retarder.
- Use of Ventilated Steel Pan Decks vs. Non-Ventilated Steel Pan Decks with Concrete: Although ventilated steel pan decks will not allow significant downward drying, specifying ventilated steel pan decks beneath concrete to provide additional moisture release is recommended in lieu of non-ventilated steel pan decks.
- Roofing-Manufacturer-Specific Assemblies Over New Concrete: Some manufacturers have developed roof assemblies specifically for installation over new concrete decks. Such assemblies are designed to allow the release of excess moisture through venting to the exterior. These systems, however, are new to the market and without widespread long-term performance history. As such, it is recommended that designers carefully review the roof performance requirements with the roofing manufacturer and any system performance history prior to selection of a new proprietary system.
- Roof Replacement – Existing Concrete Roof Decks: Existing concrete decks are typically in place for many years prior to a roof replacement. As such, excess moisture from initial concrete placement is often not an issue. However, moisture from roof leaks can result in rewetting of the cured concrete and result in a large quantity of excess moisture in the concrete. Designers of roof replacement systems over existing concrete decks should analyze and consider the moisture content of concrete roof decks at and around leak locations at a minimum, and should consider the impacts of modern methods and systems detailed above for the roof as a whole.
- Moisture Content and Testing of Concrete Roof Decks: Currently there are no roofing industry standard criteria for an acceptable moisture content in concrete roof decks and no accepted standard field testing for measurement of the moisture content. Results from ASTM D4263 and ASTM F1869 are only reflective of surface moisture and are not effective in detecting moisture levels in lower portions of the concrete slab. Field measurement of concrete relative humidity using in-situ probes using ASTM F2170, as currently used in the flooring industry, has possibilities in the future but needs further refinement and standardization for field use on concrete roof decks.
- Use of a Vapor Retarder: Designers should consider installing a Class I vapor retarder with a perm rating of 0.1 or less over both lightweight aggregate and normal-weight aggregate concrete roof decks. The installation of a vapor retarder will mitigate the latent release of excess moisture concrete over time and still require that the moisture content of concrete be low enough to achieve proper adhesion. Field-verify adhesion per manufacturer recommendations and review manufacturer requirements for vapor retarder installation.
- Roof Assembly Wind Uplift: Roof assembly wind uplift resistance with adhered systems (i.e., those that are adhered directly to the roof deck) is dependent on the adhesion of the vapor retarder to the concrete deck and all subsequent layers that are adhered to it. Designers should consider the adhesion performance and installation requirements of vapor retarders over concrete roof decks when designing a roof assembly over a concrete roof deck that relies on the adhesion for wind uplift resistance.
- Roofing Manufacturers: Contact roofing manufacturers for current recommendations and installation practices for their products over concrete roof deck substrates.
[i] Condren, Stephen J., Pinon, Joseph P. & Paul C. Scheiner, What You Can’t See Can Hurt You – Moisture in Concrete Roof Decks Can Result in Premature Roof System Failure, Professional Roofing, August 2012.
[ii] Midwest Roofing Contractors Association, T & R Advisory Bulletin 1/2011, Structural Lightweight Concrete Roof Decks, September 2011.
[iii] National Roofing Contractors Association, Industry Issue Update – Moisture in Lightweight Structural Concrete Roof Decks.
[iv] GAF Technical Advisory Bulletin – Structural Concrete Roof Decks Utilizing Lightweight Aggregate, 1/29/2014. – Will not warrant membranes installed over non-ventilated steel decks. For PVC and TPO, they recommend a vapor retarder over concrete and require a vapor retarder over lightweight aggregate concrete decks over non-permeable pan decks.
This Technical Advisory is intended to serve only as a general resource and to identify potential issues for consideration by industry professionals. Each person using this Technical Advisory is solely responsible for the evaluation of the Technical Advisory in light of the unique circumstances of any particular situation, must independently determine the applicability of such information, and assumes all risks in connection with the use of such information. The materials contained in this Technical Advisory do not supersede any code, rule, regulation, or legislation and are not intended to represent the standard of care in any jurisdiction.