How to install roof flashing against a wall

Working as a laborer on a roofing or siding crew is considered an entry-level position in the world of carpentry and home construction. Beginning workers strip shingles off the roof, remove existing siding, and spread asphalt paper or house sheathing before new shingles or siding is added to replace the existing material.

metal roof shingles
Metal flashing creates a waterproof connection between roof and wall by Randy Tucker

That’s the easy part of those two physically demanding occupations, the difficult part arrives when placing vents, closing in the peak of a roof, or perhaps the most challenging, attaching roof flashing to a vertical wall.

The issue with this vertical flashing is that it needs to provide a permanent, water-repellent link between a wall and a roof. This often involves the roof, siding, facia, and soffit as well, and might include chimneys, ornamental designs, and other challenging physical features.

A self taping metal roofing screw – note the rubber gasket at the top –

The key word is waterproof. If you don’t install flashing correctly you’ll have leaks and water damage will soon follow the poor workmanship.

The first rule in correctly installing metal flashing is to think as water would when it flows over a surface. That means placing the flashing under siding on a wall but over the roof below it. You want water to flow over, never under your flashing.

The valleys in a roof all have flashing beneath them to create waterproof drains that don’t allow any moisture under the shingles or metal roofing, wall flashing works the same way above a door, window, or when creating a path for water to flow over a roof or awnin

Flat metal flashing comes in various sizes –

Cut the metal flashing to the appropriate size for the job. Valley tin is a minimum of 20 inches wide for a standard 4/12, 5/12, or 6/12 roof. Wall tin varies with each situation.

Placing valley tin on the front of a chimney is an easy process, just calk the area heavily with tar-based adhesive and screw it into place. The sides of the chimney are more challenging, they require a stairstep method placing small strips of metal bent at a right angle, and equal to the height of the shingle under each shingle touching the chimney. Start at the bottom and place each piece of metal on top of the one below, this allows water to flow over, but not under the flashing.

Flashing comes in all types of pre-made configurations –

Wall flashing that connects siding with an awning or lower roof requires the metal be placed under the siding, but over the roof. Ample amounts of tar-based, waterproof adhesive with evenly spaced fasteners on the top of the metal edge in close sequence to each other create a waterproof barrier. Attaching the metal above the roof, with good amounts of adhesive, screwed into place with rubber-headed self-taping screws insures a leakproof wall to roof flashing.

The greater the slope, the less likely you’ll have water leakage issues.

Poor flashing around a chimney
Chimney’s require expert installation of metal flashing – Photo Credit: Antranias, Pixabay

Always apply adhesive, and always keep the metal flashing flat, without bends, ripples, or bumps against the vertical wall surface, this guarantees a good seal and a good job of applying flashing.