Most people think of metal roofing being installed on steep slopes, over 2/12 pitch where the roof is visible, adding to the curb appeal of the building. This is not the type of roofing being discussed here. Instead, this article reviews methods to successfully install metal roofing on open-frame structures on slopes as low as ¼ in. per foot where there is no secondary waterproofing. This is where one has to get it right the first time or, in the case of replacing or re-covering a nonperforming roof, this article will provide methods to get it right the second time around. One thing is for sure: An enormous amount of metal roofing has been installed on industrial and warehouse buildings on pitches as low as ¼ in. per foot. The main reason metal is put into these applications is that it costs less than other available systems, and a properly designed metal roof will outlast those other options by a factor of two to three. Logic tells us that if metal is to be installed on a low slope, one should use means and methods that provide the best chance of success. That means eliminating as many places as possible for the roof to leak. The perfect building for a low-slope metal roof would be rectangular in shape, have exterior drainage with 40-ft. panels on a 2-in. slope, and have zero penetrations. Let’s face it: Consultants won’t get a call about that roof—they will be getting calls about the roof in Figure 1. The roof in Figure 1 has it all: open bar joist installation with five 40-ft.-long panels that are lapped together end to end, covering a 200-ft. span, large penetrations that were installed with the roof on the right, new penetrations that were added later on the left, and a liberal application of caulk 6 • I n t e rf a c e A u g u s t 2 0 1 2 Figure 1 – Typical low-slope industrial standing-seam metal roof with end laps, curbs, and evidence of previous attempted repairs. Figure 2 – Coated end lap caused ponding water, which resulted in rust-through. and coatings to stop the water from coming into the building through the lap joints. Consultants might not get the call about those leaks because the maintenance man can smear some more goop to slow them down (Figure 2), but for sure, someone is getting a call about this. There are thousands and thousands of these roofs hiding behind parapet walls with internal gutters and huge condensers, AC units, and vent pipes penetrating them. The question is how to use metal on a low slope and have the greatest chance for success. Most low-slope metal roof leaks can be sourced to one of three things: exposed fasteners penetrating the building envelope; panel end laps, both in the field and around curbs; and alterations after the initial installation because all of these roofs are difficult to modify/repair after they have been installed. Let’s see how to address each of these for the best chances for success. Pan el Choic e When choosing a metal roof panel to be installed on a low slope, one need only consider mechanically seamed structural standing-seam panels designed to go over open purlins. Stay away from snap-together panels on this type of application. The seam height should be over 2 in. on roofs below 1 in.-per-ft. slope. One may choose between a trapezoidal-shaped rib and a vertical rib. Both have their advantages. A mechanically seamed, 3-in.-tall trapezoidal-rib panel has a couple of advantages over a vertical-rib panel. Mainly, trapezoidal panels are the least expensive structural standing-seam panels available. They are very common and they cost less to manufacture, package, and ship than a verticalrib panel. They are great on rectangular buildings with few if any curb penetrations and short eave-to-ridge lengths (under 50 ft.). They come with preformed closures at the ridge and eave and have a 3-in.-tall, watertight seam. Trapezoidal-rib panel advantages diminish as the panel length increases, the roof geometry gets more complicated, and the number of penetrations increases. If the goal is to eliminate places for the roof to leak, a good place to start is to eliminate panel end laps. Trapezoidal panels are rarely site-formed, so it would be extremely difficult to cover a run of more than 60 ft. without introducing end laps into the roof (Figure 3). Independent Standard for aerIal roof MeaSureMent reportS © 2012 EagleView Technologies, Inc. Measure more roofs in one day using EagleView from your smart phone or your computer. It’s that simple! Call 866-447-3741 or visit www.eagleview.com. A u g u s t 2 0 1 2 I n t e rf a c e • 7 8 • I n t e rf a c e A u g u s t 2 0 1 2 Also, while the shape of the seam lends itself nicely to preformed closures at the ridge, it is a hindrance if the panel has to be cut on an angle for a hip or valley condition. In those conditions, it is much more difficult to make a watertight joint between the closure and the panel seam than a vertical-rib panel. Finally, large curb penetrations can be custom made to fit a trapezoidal panel seam, but these panels rely on exposed fasteners penetrating through the roof at all of these conditions, opening up opportunities for water infiltration. If the roof has a longer panel run, complicated geometry, or curbs, then a site-formed vertical-rib panel may be a better choice. Since the goal is to eliminate as many potential leaks as possible, the easiest way to start is to eliminate all end laps. There are two types of verticalribbed structural standing-seam roof panels: those that are asymmetrical and those that are symmetrical. Both have their advantages. An asymmetrical panel is one piece; installs from left to right or right to left; will most likely be seamed to a 180-degree double-lock seam; and, like trapezoidal panels, has two-piece floating clips folded into the seam. There is an abundance of portable and factory roll formers out there that make a 2-in.-tall asymmetrical panel. This seam height is probably okay down to a 1/12 slope. There are a few manufacturers that offer a site- Figure 3 – 24-in.-wide by 3-in.-tall trapezoidal standing-seam roof panel with end lap. Figure 4 – Diagonal headwall on trapezoidal standingseam roof. Figure 5 – Trapezoidal panel closure in a diagonal application. Figure 6 – Vertical rib panel with diagonal closures. formed vertical-rib panel that is over 2 in. tall, and this is what is needed to go under a 1/12 slope. Asymmetrical panels have several advantages over symmetrical panels. They are much more common than symmetrical panels; and they cost less money because there is only one manufactured component, and they use less material to make. The disadvantages of asymmetrical panels are that there is a break in the seam sealant at each clip location, and the amount of thermal movement is limited to the slide mechanism of the two-piece clip. Also, there is a reasonable chance there can be damage done to the roof panel during the double-lock seaming operation; and finally, these panels are extremely difficult to remove, replace, or repair after they have been installed. A T-shaped symmetrical panel is run in two pieces: a panel and a separate seam cover that is machine seamed on the roof. These panels cost more money because they use more material and two component parts—panel and seam cover—are required. Since there is no contact between the seam sealant and the clip, there is no interruption of the seam sealant, yielding a more watertight seam design. Symmetrical panels allow more flexibility during installation; have a higher wind uplift capacity; and can be easily removed, replaced, or repaired after installation. Trap ezoi dal Rib vs . Vertica l Rib Figure 4 shows a misapplication of a trapezoidal panel on a low slope, terminating into a diagonal headwall. Figure 5 shows the actual closure. Figure 6 shows the same diagonal headwall with a 3-in. vertical-rib panel instead. Eav e Con dition When using a vertical-rib panel on a low slope, it is best to fix the panel at the eave and allow it to expand toward the ridge. This is done for several reasons: mainly, this provides a more watertight eave condition than a panel that is hooked on the eave trim such as on a steep-slope architectural panel. Fold the end of the panel straight down so the water runs off of the panel and does not try to work its way back into the eave trim (Figure 7). A u g u s t 2 0 1 2 I n t e rf a c e • 9 layfast SBS roofing underlayment exceeds ASTM D-226 is ICC approved and qualifies for LEED cer tification credit. layfast SBS is the ideal underlayment for all types of roofs, including tile, shake, and shingle, because it is more versatile, economical, and durable than other roofing substrates. The addition of SBS rubber to asphalt gives layfastSBS an elastomeric property, which can extend the life of a roof several years. layfastSBS is the only roofing material on the market today that can lower architectural liability (exceeds Code 226) and in California, qualifies for MR 5.1 LEED certification credits. Because the difference in cost over felt is a drop in the bucket. and save your buckets for something better. Visit www.mbtechnology.com to see why the nation’s finest architects and contractors have called for the installation of more than 250 million ft2 Specify layfast SBS® Roofing Underlayment, Figure 7 – Vertical-rib panel with end folded down at the eave. 1 0 • I n t e rf a c e A u g u s t 2 0 1 2 Next, lay down double- or triple-bead tape seal on the eave trim, then fasten through the panel into the eave trim or 16-gauge offset cleat to get a good compression seal with the fasteners overhanging the gutter (Figures 8-9). Ridge Con dition In regard to ridge conditions on low-slope roofs, if a verticalrib panel is used, then take the opportunity to “bread pan” it behind the closure (Figure 10). This is cheap insurance against leaks. It can be done without cutting the panel seam. Curbs The most difficult place to make a lowslope metal roof watertight is on a curb. At every curb, there is a minimum of four end laps. Fortunately, there is a way to eliminate these end laps. On the roof in Figure 11, there were nearly 700 panel end laps with around 100 occurring around curb penetrations. The overall panel length was nearly 250 feet. The roof in Figure 12 was re-covered with zero end laps! This is because of the use of variable-width, site-formed, vertical- ribbed panels and transverse panels uphill of all large curb penetrations. Transverse panels are not new, but they are seldom used. Transverse panels can be used to eliminate all end laps on a low-slope metal roof. Two panel widths can be used to ensure that the panel seam lands as close as possible to an existing curb penetration. Figure 12 shows the use of both 24-in. and 18-in. panels, which land the seam within 6 in. of the curb. Installing transverse panels is pretty simple. Basically, the roof is sheeted up with a seam landing as close as possible to the curb, then across the low side of the curb, The RCI Foundations – Supporting The Industry RCI Foundation – United States Web site: www.rcifoundation.org E-mail: foundation@rci-online.org RCI Foundation – Canada Web site: www.rcifoundation.ca E-mail: info@rcifoundation.ca Figure 8 – Vertical-rib panel on triple-bead tape seal over eave trim over 16-gauge offset cleat. Figure 9 – Vertical-rib panel, fixed at the eave with panel end folded down. Exposed fasteners over the gutter. and up the other side, leaving a hole in the roof behind the curb that runs all the way to the ridge (Figure 13). Then, add an eave member down each side and a support member down the middle of the hole that runs all the way from behind the curb to the ridge (Figure 14). The support member should be slightly taller than 800.255.4255 • www.prosoco.com Proven where others fail FastFlash Air & Waterproof barrier systems for new construction and window retrofits. to protect the most vulnerable connections in the building envelope from air and water leaks, and condensation from extreme weather nationwide. Created, tested and proven Tested to hurricane conditions Air & Waterproof Barrier Systems RCI August Ad.indd 1 7/10/12 1:01 PM A u g u s t 2 0 1 2 I n t e rf a c e • 1 1 Figure 10 – Panel “bread panned” at ridge prior to installation of closure and ridge cap. Figure 12 – The same roof as in Figure 11, re-covered with no end laps, using site-formed, variable-width panels and transverse panels uphill of curbs. Figure 11 – Large, low-slope metal roof with over 700 end laps, 100 around curbs. the eave member so that the transverse panels have a slight crown. Panels are attached to the eave members and supported in the middle. Transverse panels can be used to raise difficult penetrations above the plane of the roof, isolating them and greatly reducing the chance of having a leak (Figure 15). So in order to increase the chance of success when using metal roofing on a low slope, start by eliminating all of the places for a metal roof to leak: exposed fasteners penetrating the building envelope, panel end laps (both in the field of the roof and around curbs) by using a site-formed panel when lengths are too long to ship, and by using transverse panels uphill of curbs. Also, use a verticalrib panel as the geometry gets more complicated, panel lengths increase, and the number of curbs increases. Finally, consider a symmetrical panel when the likelihood of needing to open the roof to make a change or replace a panel at a later date is reasonably high. This will save a lot of heartburn. Metal used in this manner can be applied not only in new construction, but also to re-cover an existing metal roof or even an existing externally drained modified or single-ply roof that is on a ¼-in. slope. 1 2 • I n t e rf a c e A u g u s t 2 0 1 2 Charlie Smith founded Architectural Building Components (ABC) in 1989, when he purchased the equipment of a small Houston, TX-based metal roofing manufacturing business. Since then, the company has grown into an industry-leading metal roofing and wall system solution provider specializing in using metal to re-cover existing low-slope roofs. ABC was recently acquired by McElroy Metal, a Louisiana-based metal roofing and siding manufacturing company with a national footprint. Over the last 22 years, Smith has used his roofing expertise and creativity to expand ABC’s product line to over 20 roof and wall systems. He also has commissioned several Archzilla® jobsite manufacturing monster trucks. Recently, Smith wrote half of and is teaching the new metal roofing course for RCI. Smith earned a BS in geology from Stephen F. Austin State University, Nacogdoches, TX, in 1982. He is a member of FM Global Group, Mississippi Roofing Contractors Association, National Roofing Contractors Association, Roofing Contractors Association of Texas, and RCI. Charlie Smith Figure 13 – Initial installation of roof in preparation for transverse panels behind curb. Figure 14 – Installing transverse panels around large round pipe penetrations. Figure 15 – Difficult penetrations are raised above the roof plane and can be isolated using transverse panels, reducing a chance for a leak and the volume of water coming into the building if there is a leak.
