Outside corners on dormers needs special flashing details to prevent water leaks. This article explains two foolproof methods for making sure that corners shed water and stay dry for the life of the roof.
Run the base flashing 4 to 5 in. beyond the dormer and cut the top section along the dormer edge. Bend the flap down and nail along the top edge.
Notch the first step flashing, leaving a 1-in. flap to bend around the front of the dormer. Nail along its upper edge. Add shingles and additional step flashing.
Squeeze a thin bead of caulk over the leftover gap and along the bottom joint.
There are two good ways to handle corner flashing. The first, bending two pieces and caulking the joint (Photos 1 – 3), is the most common because you can do it on the spot with the flashing at hand. If you bend nice crisp corners and nail carefully, you’ll only leave a tiny hole to caulk at the corner (Photo 3). Remember to place the flashing under the weather barrier on the dormer. If you’re flashing a masonry chimney, nail the flashing to the roof as far up the flashing as possible. Cap flashing (counter flashing) inserted into mortar joints will cover the top edge.
Lay soldered corner flashing around the corner and over the base flashing. Nail the flashing to the dormer near the top edge and continue with the shingles and step flashing.
The second, forming and soldering a custom piece to fit the corner, is the best because it doesn’t leave any potential leak spots (at the caulked gaps). Unfortunately, you can’t solder aluminum flashing, and soldering galvanized steel is difficult. You’ll have to measure your roof slope and have a sheet metal or gutter-making shop make one for you. You may have to nail the lower edge to the roof to hold it flat. If so, cover the nail head with a dab of silicone.
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.