Water intrusion at roof-to-wall intersections is the movement of water into the wall or ceiling cavity where a roof and wall meet.
Where a wall terminates at the roof, any water that has penetrated the exterior finish must be able to drain from the wall's drainage plane to the roofing. Intersecting walls, including dormer and chimney walls, make the home look great, but they also make the roof vulnerable to water intrusion. The more roof-to-wall intersections a home has, the more vulnerable areas exist, making it increasingly critical to employ an effective, consistent approach to water management.
Flashing at the roof-to-wall intersection
Without the proper flashing, any water on the wall's drainage plane will continue downward into the home to wet the sheathing and framing. It's critical to make sure flashing is in place at the roof-to-wall intersection and that it's properly integrated with the wall's housewrap.
There are two types of roof-to-wall intersections: roof rake-to-wall and shed roof-to-wall. The first type—roof rake-to-wall—is a sloped intersection between a roof and a wall. To prevent water intrusion at this intersection, step flashing has to be installed. Step flashing consists of individual pieces of flashing that are layered with each shingle, allowing water to kick out after every shingle. Step flashing guides water from the wall's drainage plane onto the shingles, where it can drain off the roof.
The second type—shed roof-to-wall—is a horizontal intersection between a shed roof and a wall. To prevent water intrusion at this intersection, apron flashing has to be installed. Unlike step flashing, apron flashing isn't layered individually with each shingle. Rather, the flashing leg that projects out from the wall is layered over the shingles, allowing water to drain freely on top of the shingles. Apron flashing directs water from the wall's drainage plane to the shingles, where it can then drain down and away.
The reveal between the exterior finish and the shingles
Keeping a reveal between the exterior finish and the roofing shingles is critical in managing water at roof-to-wall intersections. A reveal is a gap between the exterior finish and surface of the shingles. Without a reveal, any water moving down the roof can wick up the exterior finish, leading to water damage.
Cause 1:
Improper integration of roof-to-wall flashing with housewrap
If the roof-to-wall flashing isn't properly integrated with the housewrap, water will run behind the flashing and reach the roof sheathing, causing moisture damage.
Solution
The roofing underlayment should layer up the wall surface a minimum of 12". Next, install the roof-to-wall flashing. Then install the housewrap over the vertical leg of the flashing, ensuring a continuous path for water to drain from the wall, down the roof, and to the gutters.
ProTip!Wrapping the roofing underlayment up the wall ensures that wind-driven rain won't penetrate the roof sheathing.
Cause 2:
Insufficient reveal between the exterior finish and roofing
If there isn't enough reveal between the exterior finish and the roofing, any water moving down the roof can wick up the exterior finish, potentially deteriorating it.
Solution
The exterior finish should never touch the shingles. To prevent the finish from wicking water, the exterior finish should stop 1½" above the shingles.
