Four Steps to Flash a Dormer
This leak-free approach begins with an impenetrable corner detail.
Synopsis: While the gold standard for roof flashing is soldered copper, the fact is that for economic reasons, the vast majority of roof flashing is done with aluminum. In this article, veteran remodeler Mike Guertin shows how to flash a dormer with bombproof details.
Photo: Mike Guertin
Roof flashing is critical to keeping a house dry, but people often get it wrong. Most roof leaks I’ve investigated are due to either undersize or improperly installed flashing. To keep out water, flashing has to be fabricated from durable materials, sized to keep out wind-driven rain, and installed in a weather-lapped fashion—that is, with the upper layers overlaying the lower layers. Also, it doesn’t hurt to design flashing to be visually appealing.
Because their front and side walls meet the roof in different ways, dormers require several techniques to integrate the flashing with the roofing material as well as with the house’s water-resistive barrier (WRB) of housewrap or felt paper.
Start with the right flashing
Galvanized steel, lead, copper, stainless steel, UV-stabilized PVC, and TPO are all good flashing materials (the latter four are also the right choices for coastal areas), but the most common material is probably painted aluminum. A lot is riding on the flashing, and heavier-gauge metal doesn’t cost much more than light-gauge stock, so don’t skimp on material. The IRC requires that metal flashing be corrosion resistant and have a minimum thickness of 0.019 in. (26 ga.). Some unfinished aluminum sold as flashing doesn’t meet the thickness requirement and is more prone to corrosion than coated aluminum. Peel-and-stick membrane can be an effective backup to the primary rigid flashing, but it won’t stand up to sunlight or to physical damage from sticks, ladders, and debris. Don’t use it in place of rigid flashing.
On this dormer, I used commonly available painted aluminum coil stock for flashing. Because this type of metal flashing can’t be soldered, making a leak-proof joint where the flashing turns the corner at the bottom of the dormer called for special attention, but I have a detail that handled the situation nicely.
For photos and the four steps to flash a dormer, click the View PDF button below.