Tech Briefs

The Devil is in the Details

December 20, 2019 - by rnunez
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By Jay Thomas, Building Technology Editor

Done poorly, roofing details can shorten the lifespan of your roof

A 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:
  1. 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.
  2. The membrane flashing or stripping sometimes delaminates due to improper application, (e.g.: lack of primer or poor adhesion).
  3. Pinholes will occur at lap joints along the deck flange due to bridging and lack of adhesion.
  4. 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.
  5. Corrosion of the exposed galvanized metal, resulting in pin-holes.
  6. Proper closures are not provided at the naked end of the base flashing where the eave meets an adjacent wall (see figure below:)
Improper base flashing termination at a wall intersection. These deficiencies will allow water to infiltrate into the roofing system and building. For this reason, Lovato says a gravel stop edge detail demands greater maintenance than other areas of the roof system. This perimeter detail should be inspected more thoroughly and repaired as needed to prevent water intrusion through the building envelope. A gravel stop or drip edge flashing at a gutter is a vulnerable detail that requires a specific design to avoid problems. First, the deck flange should be fastened to the perimeter blocking four-inch on-center with pan-head ring-shank nails. Next, the lap joints should be sealed with a non-skinning sealant. And finally, the vertical lip should be secured with a continuous hook strip that is nailed a maximum of six-inch on-center along the face to prevent wind damage. Rooftop Equipment Mounting Roof top equipment mounted to curb side fastened rails. In the list of roofing do’s and don’ts, mounting of rooftop equipment looms large. “When mounting small heat pumps on top of metal cap curbs, don’t let the contractor run a lag bolt through the metal cap because that becomes an avenue for water,” says Lovato. “The water is going to go under the equipment foot and then hit the screw hole and come into the curb. A better approach is to mount the equipment to two parallel rails that are anchored to either side of the curb with an L-shaped bracket.” Roof top equipment mounted to curb side fastened rails. Odd Shaped Roof Penetrations Any odd shape penetration through the roof can be a challenge to detail and a potential for problems if not addressed correctly. Clusters of pipes and Unistrut are two typical examples. “Other trades will run these things through the roof and assume that the roofing contractor will figure out how to waterproof this,” says Lovato. “Often the go-to solution is the two-piece pitch pocket. This detail is prone to problems, is an ongoing maintenance issue and frequently not included under the manufacturer warranty. I try to avoid them wherever possible.” Pitch pockets rely on two-part epoxy resin. If not mixed correctly the resin never sets up properly and then can shrink away from the sides of the pan or penetration resulting in an avenue for water intrusion. “A better detail for pipe clusters is a hooded curb,” says Lovato. “This approach results in a much easier to waterproof box shape. It is definitely a best practice for these difficult roof penetrations and pipe clusters.” Hooded curb pipe penetration detail. Hooded curb pipe penetration detail. Regardless of the quality of the roof details, problems will still occur without good communication and setting clear expectations with the roofing contractor. “Just designing the details correctly is half the battle,” says Lovato. “The rest has to do with making sure that communication is good and expectations are clearly explained.” Lovato says a lot of contractors are used to doing things a certain way. Their attitude is that as long as the manufacturer guarantees the roof, that's good enough. “They need to understand that if the detail is not done per the drawing then they will have to tear it out and do it again,” he continues. “I think if the project managers and superintendents from the roofing contractor understand what is expected then they will carry through with it. But if the communication is poor you could end up with a mediocre job at best.” Lovato is very sensitive to the fact that during the submittal process things can change. The interpretation of the intent of the specification can change when the contractor gets a set of submittals back. An experienced reviewer such as a consultant or an architect will know when a contractor is trying to cheapen the job or do something that wasn't part of the intent in the specification. In the case of roofing, an ounce of prevention really is worth a pound of cure. With today’s requirements for vapor retarders and air barriers it's more important than ever to make a roof watertight. A leak into these roof assemblies on a concrete deck is going to hold water in the system with a lot of accumulation before the leak is found. By that time the roof may be a complete loss. “We are going to go through a period of time where all the professions including the general contractors and architects begin to realize that they need roofing experts to avoid these very expensive roofing problems,” says Lovato. “The cost of lawsuits and going back and fixing the problems for valued customers is prohibitive. You’re going to go back and tear off the roof and redo it if you want to keep the customer. Design assistance from a roofing expert can help architects avoid these dangerous waters.”
Done well, roofing details should last the life of the roof. Done poorly, they will, at a minimum, lead to ongoing maintenance issues and even premature roof failure. Hiring a roof consultant to pay attention to the details can pay dividends to your reputation as a designer and the satisfaction of the building owner.

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