The roof is the house's primary defense against water intrusion from rain, snow, and ice. This "lid" of the home must be structurally sound, and also be designed and constructed to shed water effectively and consistently through all seasons, extreme weather events, and atmospheric conditions. Water that seeps into the house through the roof can quickly ruin insulation, create conditions for mold growth and pest invasion, and even set into motion structural rot. Over time even the smallest leak in a roof can result in a significant amount of water damage, and not just in the attic. If water finds an easy path into the home, it can pool and flow from the roof all the way to the foundation.
Valleys and penetrations through the roof decking are some of the most vulnerable areas for water intrusion and must be addressed carefully. Valley's purposefully divert water into them and must be carefully sealed along all sheathing intersections to keep water flowing down and away from the house. Penetrations in the roof, as with penetrations anywhere in the building envelope, are points of easy access for water unless carefully sealed.
There are two ways to ensure that water does not find access at these potential failure points.
- Install a self-sealing bituminous membrane or the equivalent along all valleys.
- Install a self-sealing bituminous membrane or the equivalent around all penetrations in the roof sheathing and integrate into the adjoining roofing materials properly (EPA 2012).
Note: The U.S. Environmental Protection Agency (EPA) uses the phrase “or equivalent” to indicate that a variety of products can be used. In this case, products must be water impermeable and durably adhere to the roof deck. Please check with the manufacturer’s material classifications and installation instructions to ensure the material you choose will adequately protect the roof from water intrusion (EPA 2011).
Figure 1 - Peel and stick membrane applied to roof valley. These workers are properly installing a self-sealing bituminous membrane to a valley on the roof deck. Note the material is centered along the valley to ensure maximum protection.
Installing Self-Sealing Bituminous Membranes along Valleys on Roof Decks
Bituminous membranes are long rolls of a heavy, flexible material that have been impregnated with a petroleum based solution, like tar, that makes them waterproof. Most are applied to the exterior of the building with "peel and stick" adhesive backing. Once in place, the membrane will help support the overall water management strategy employed on the roof as long as these steps are followed (EPA 2012):
Clean the roofing area in the valley where the material will be applied to ensure that no nails, wires, and debris are in the area. Although the membrane is made from relatively strong material, it can be torn or punctured. Also, it is important that the area be dry and free of oil and water, and that dust be swept away to ensure the membrane will adhere properly and have a complete and tight seal when applied.
Measure the length of the valley to be covered.
Cut the self-sealing bituminous membrane to length. For extremely long valleys, cut the material in shorter, more manageable lengths, and apply from the lowest point to the highest, overlapping the membrane section by 6 inches in terraced shingle like fashion to allow water to flow unobstructed down the length of the valley. The membrane must be straight and centered with the valley line.
Installation note: Some types of peel and stick membranes come with a split backing above the adhesive. This allows you to unroll the material and secure it using one half of the adhesive back, and then fold back the other half and remove the protective cover to secure the material in place. The split back covering can allow for applying the material over longer lengths.
Secure the material in place with a heavy roller making sure no gaps, creases, or folds are present. The material must lay flat and be applied to the manufacturer’s specifications.
Install the underlayment directly over the membrane and continue with the construction of the roof.
Figure 2 - Roof valley membrane protection. This image shows where the self-adhesive bituminous membrane should be installed in relation to the other roofing materials. The membrane must be installed before underlayment (roofing felt) and shingles.
Installing Self-Sealing Bituminous Membranes around Penetrations
Penetrations in the roof deck generally can be classified in two groups: 1) direct penetrations like plumbing stack vents and 2) structural penetrations like a dormer. Around all the penetrations, self-adhesive bituminous membranes should be applied along with flashing to ensure water does not find a way into the home.
Sealing around Direct Penetrations in the Roof
To properly seal the roof area around a direct penetration, follow these steps:
Clean the roofing area after the pipe or vent has been installed through the roofing deck. Make sure no nails, wires, and debris are in the area. Although the membrane is made from relatively strong material, it can be torn or punctured. Also it is important that the area be dry and free of oil and water, and that dust be swept away to ensure the membrane will adhere properly and have a complete and tight seal when applied.
Install the underlayment (roofing felt) up to the penetration.
Measure the area to be covered and cut the membrane to fit. Allow for at least 6 inches of material around all sides of the penetration.
Install the self-adhesive membrane so that it seals tight around the pipe and laps over the roofing felt. Use a heavy hand roller to secure in place. Some manufacturers require that a primer be applied before the membrane is installed
Continue installing underlayment.
Install shingles or roofing material up to the penetration and then install the pre-manufactured pipe flashing cap over the penetration and finish roof material installation.
Figure 3 - Sealing direct roof penetrations—first steps. This image shows where the self-adhesive bituminous membrane should be installed in relation to the other roofing materials. The membrane laps over roofing felt or underlayment and must be cut to fit closely and in direct contact with the pipe over vent.
Figure 4 - Sealing direct roof penetrations—final steps. After the self-adhesive bituminous membrane has been applied and underlayment secured, shingles and a pre-manufactured pipe flashing cap can be installed around the penetration.
Sealing around Structural Penetrations
To properly seal the roof area around a structural penetration like a dormer, follow these steps:
Clean the roofing area around the penetration making sure it is debris free, dry, and clean.
Install underlayment to the edge of the penetration and secure in place.
Install the step flashing and secure in place.
Apply self-adhesive membrane material directly over the vertical rise of the step flashing and up the side of the penetration.
Install the underlayment material around the penetration, securing it in place and attaching it to the membrane with tape.
Continue installing the roofing material.
Figure 5 - Sealing structural penetrations at roof wall intersections. The self-adhesive bituminous membrane has been applied from the step flashing onto a vertical wall. This prevents any water that gets behind the building wrap from getting behind the step flashing and entering the building. Note that the step flashing is first installed, and then the membrane covers it.