How to Flash a Roof-to-Wall

When building or renovating a house, you must make the roof waterproof.

Flashing prevents leaks where walls meet roofs.Flashing prevents leaks where walls meet roofs.
Leakage through the roof can cause mold, mildew and structural damage, and it often isn't detected until these problems are well advanced. The joint where a roof meets an abutting wall is a common point of leakage. This problem can be prevented by properly installing flashing between the roof and the wall.

Remove old siding from the wall and old shingles from the roof. Check to see whether the underlying sheathing and structure are solid and free from rot. Any rotten pieces should be replaced.

Install pieces of step flashing against the wall at the same time that you shingle the roof, so that each piece of flashing is underneath the shingle above it. This makes the flashing look better and also allows the water to flow down from the shingle to the flashing, keeping it out of the roof and the wall.

Apply roofing tar to the edges of the underside of each piece of flashing as you install it, for extra protection against leakage.

Nail each piece of flashing to the wall using roofing nails and cover the heads of the roofing nails with roofing tar. Each piece of step flashing is made in an L shape. The corner of the L should be pressed into the joint between the roof and the wall, with one leg of the L extending up the wall and the other extending out onto the roof.

Install siding over the top of the flashing, leaving a 2-inch space between the bottom of the siding and the roof. If you install the siding so its bottom is against the roof, it will absorb moisture from the surface of the roof and rot more quickly.

Things You Will Need

  • Step flashing
  • Shingles
  • Roofing tar
  • Roofing nails
  • Hammer
  • Measuring tape

About the Author

Jagg Xaxx has been writing since 1983. His primary areas of writing include surrealism, Buddhist iconography and environmental issues. Xaxx worked as a cabinetmaker for 12 years, as well as building and renovating several houses. Xaxx holds a Doctor of Philosophy in art history from the University of Manchester in the U.K.