Cold climates present a specific kind of challenge to a home's roof. During the cold months when snow and freezing rain fall, a thawing-freezing process can create ice dams. Ice dams happen when liquid water flows across the roof to a point where the temperature drops sufficiently to freeze and create ice. Once the process starts, ice can rapidly build up in one spot and begin to "dam" or hold back liquid water from flowing off the roof. Ice dams can occur in roof valleys and around penetrations, but are especially common at the eaves where the roof is less likely to be insulated, resulting in a roof deck that is relatively colder. For a visual description of ice dams, see the Training Tab.
The main concern with ice dams is that as the water starts to collect and freeze behind the barrier, it can force its way under the shingles, underlayment, and even the flashing. Once the water starts to flow up the roof instead of down the roof, the chance for a leak can be significant. The more water held behind the ice dam, the greater the chance that a leak can occur. Water that seeps into the house through the roof can quickly ruin insulation, create conditions for mold growth and pest invasion, and ultimately cause structural rot. Over time, even the smallest leak in a roof can result in a significant amount of water damage. If water finds an easy path into the home, it can pool and flow from the roof all the way to the foundation.
The following is part of a whole-house effort (which includes attic air sealing) to ensure that water does not find access to the home from the roof eaves:
- Install self-sealing bituminous membrane or the equivalent along all eaves.
- In cold climates (IECC climate zones 5 and higher), extend the self-sealing bituminous membrane from the edge of the roof line to at least 2 feet beyond the interior plane of the exterior wall.
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 manufacturers’ material classifications and installation instructions to ensure that the material you choose will adequately protect the roof from water intrusion.
Figure 1 - Placement of the self-sealing bituminous membrane. In colder climates, the self-sealing bituminous membrane must be applied to extend up the roof deck at least 2 feet beyond the interior plane of the exterior wall. By lapping the membrane up the roof deck, the home will be better protected from water that can collect behind ice dams that build on the roof eaves.
Lapping the self-sealing bituminous membrane up the roof as you would underlayment helps ensure that any water that finds a path under the roofing material due to ice dams will not be able to easily enter the attic space. In colder climates ice dams may grow to be quite large and linger throughout the heating season. Extending the membrane up the roof deck and not just installing at the eaves is important for two reasons:
Most eave designs are not insulated and are generally much colder than the roofing above the attic. As a result, ice is far more likely to form and create an ice dam over an eave. Regardless of how wide the eave is, an ice dam can build up to sufficiently extend the entire depth of the eave.
Because the roofing deck above the attic may be relatively warmer than the eave, it will encourage the melting of snow. Ice dams are not likely to occur beyond 2 feet up the roofing deck beyond the interior plane of the exterior wall, so it is only critical to provide waterproofing coverage with the membrane up to this distance.
Installing Self-Sealing Bituminous Membranes along Eaves
Bituminous membranes are long rolls of heavy, flexible materials 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 the following steps are followed:
Clean the roofing area along the eaves where the material will be applied to ensure that no nails, wires, or 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 that the membrane will adhere properly and have a complete and tight seal when applied.
Measure the length of the eave to be covered.
Cut the self-sealing bituminous membrane to length. For extremely long eaves, 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 applied to the edge of the roof deck.
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 backing, 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 conform to manufacturers’ specifications.
Install the underlayment directly over the membrane and continue with the construction of the roof.
Figure 2 - Installation of the self-sealing bituminous membrane in relation to the drip edge and roof underlayment. First install the drip edge, then the self-sealing membrane, and finally the roofing underlayment. Note that the rake edge covers the membrane and the underlayment.
Installing the Membrane at Least 2 Feet beyond Interior Plane of Exterior Wall
In cold climates (IECC climate zones 5 and higher), the self-sealing bituminous membrane must be installed at least 2 feet beyond the interior plane of the exterior wall. In simple terms, this means that the membrane must be installed all the way from the edge of the roof deck over the eave and up the roof over the attic space at least 2 feet. To successfully do this, use the following overall installation guidance:
Determine how far up the roof deck from the eave edge the self-sealing bituminous membrane should be installed. To do this, measure from the exterior wall inward 2 feet and add the estimated wall depth including interior and exterior sheathing, framing, drywall, and siding. Next simply mark the roof deck directly above your measurement and ensure the membrane is installed to at least that point on the roof deck.
Calculate, based on membrane roll width, how many courses of membrane need to be installed.
Install the membrane starting at the eave edge and working up the roof deck. Courses should overlap per manufacturer's recommendation.
Installation note: Make sure the self-sealing bituminous membrane extends over the metal drip edge. This will ensure that water will not find access at the very edge of the eave.
Exception: Self-adhesive bituminous membranes do not need to be applied in dry climates as defined in IECC Figure 301.1. Refer to the Map of Climate Zones to determine "Dry Climates," and consult code officials when local climate conditions might dictate the need for protection from ice dams.