The Devil is in the Details
Done poorly, roofing details can shorten the lifespan of your roofA roof construction detail is a smaller section of the overall roof shown at a larger scale in order to depict how the components fit together. Typical roof details include drains, penetrations and edges. Compared to the overall size of a roof, the details account for a small portion of the square footage but a large percentage of the problems. Roof details often include a large proportion of membrane seams which is where a majority of roofing problems occur. Avoiding problems is a matter of understanding best practices, providing clear detail drawings, setting expectations with the applicator and having clear lines of communication. I spoke with roof expert Nick Lovato, senior vice president of Bluefin LLC in Denver, to learn the most common problem details and his recommendations for providing quality design and installation. Drains Roof drains play an important role in quickly moving water off of a low-slope roof. Ponding water can accelerate the deterioration of the roofing membrane and exceed the weight limit of the building structure. The design of the drain detail is critical due to the frequency and volume of water that this part of the roof is subject to. “Leaving the roof drain detail to the discretion of the roofer can lead to poor installation,” says Lovato. “Often the roofing contractor will simply shave down the insulation eight inches from the drain and pull the membrane tight to the drain to act as the flashing.” Lovato said that is not a good drain detail, because it leaves the membrane bridging and susceptible to stress and hail damage. “When the membrane is under stress it is more susceptible to UV degradation,” he continues. “A better detail is to set the drains 2 inches below the finished elevation of the roof and use tapered insulation to make a nice gradual sump that will draw the water into the drain. You avoid bridging of the membrane and have very little if any ponding water.” Roof drain designed and installed with tapered insulation. Roof drain designed and installed with tapered insulation. Scuppers Scuppers tend to be a complicated detail and an important one given their role to get excess water off of the roof. “This is an area that I always pay close attention to at final inspection,” says Lovato. “There are a lot of 90-degree angles and flashing to different materials. You have one chance of getting it right.” Contractors frequently forget to flash to the outside wall weather barrier, according to Lovato. “I often see an attempt to seal to the exterior brick or siding wall cladding,” he says. “Eventually, the sealant used will fail and water will enter the building envelope.” It is important that the scupper sleeve be flashed and sealed at the plane of the weather barrier. This will require coordination between the building envelope trades. Lovato adds that proper flashing to the top of the scupper collector boxes is also critical. “I often see this left wide open,” he says. “This will lead to water intrusion. The water can get behind the collector box, make it to the scupper sleeve and then back inside the building.” Roof scupper detail. Gravel Stop One of the most problematic details for any membrane roof is the low-profile gravel stop, (a.k.a. fascia or drip edge), where water cascades over the edge of the roof. The following problems often occur with this detail:
- Fissures often occur at lap joints shortly after their application, due to fatigue caused by expansion and contraction of the metal, particularly when the deck flange is not adequately restrained by nails.
- The membrane flashing or stripping sometimes delaminates due to improper application, (e.g.: lack of primer or poor adhesion).
- Pinholes will occur at lap joints along the deck flange due to bridging and lack of adhesion.
- Water can seep through “dry” lap joints in the metal gravel stop when sealant or mastic is not bedded in the lap. Leaks are exacerbated when the gravel stop has a high profile that dams water.
- Corrosion of the exposed galvanized metal, resulting in pin-holes.
- Proper closures are not provided at the naked end of the base flashing where the eave meets an adjacent wall (see figure below:)
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