It’s a basic principle of what building science experts call “bulk water management”: rainwater that strikes a building has to be directed down and out. Gravity provides the “down”; it’s up to you to make certain of the “out.” that is where a reverse lapped step flashing comes in.
In the details above you can see the problem; If step flashing isn’t tucked behind the house wrap on the wall above, rainwater can drain into the walls below.
Reverse Lapped Step Flashing – Building a weather-resistive barrier
-
1. Flashing Over House wrap – Pieces of flashing are sometimes correctly lapped over each other, but incorrectly lapped over the wall’s house wrap. It often happens when roofers are asked to install the flashing after the building paper is already on the wall. The flaw can direct rain behind the flashing into the structure.
2. Flash, Then Wrap – The ideal way to install step flashing is to use a reverse lapped step flashing. Put the vertical leg of the flashing against the wall sheathing, then seal the flashing to the wall with an applied layer of peel-and-stick flexible membrane. The final step is to lap the house wrap down over this sealed joint.
3. Slit and Reseal – If the house wrap is already in place and the step flashing has already been applied, here’s a fix: slit the house wrap above the step flashing and slide the peel-and-stick up under the existing house wrap, lapping it down over the existing step flashing. Cut and fit a house wrap patch to cover.
Reverse Lapped Step Flashing – Building a weather-resistive barrier
The problem can be trickiest when best practice details require the interaction of two trades. That’s the case, for example, where a shingled roof meets a side wall. In production building-and even in custom work-the crews that apply the house wrap and the siding aren’t usually the same as the workers who install the roof shingles. So whose job is the step flashing at the inter section of roof and wall?
Whoever handles this joint, someone has to make sure that the materials are correctly lapped. It’s not enough for the siding material to lap down over the step flashing. The building paper or house wrap behind the siding, what the code calls the “weather-resistive barrier” also must lap over the step flashing. Otherwise, water that reaches the paper can trickle down behind the flashing and into the structure, where it contributes to damaging rot. It’s very tough to keep this part of the building dry, so it’s worth taking extra care to use a reverse lapped step flashing and ensure that the spot will drain correctly, so that it never needs to dry out.
-TED CUSHMAN – Illustrations by Harry Whitver