The Atlas Roofing Guide To Flashing

Roof-To-Wall Interfaces And Roof Penetrations

Paul Casseri, Atlas Product Manager
Reading Time: 5 minutes
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Wherever an intersection or penetration takes place either between two roof planes, a wall and the roof, or through the shingles themselves, metal flashing (or a polymer alternative) must be installed. Flashing protects the roof against liquid infiltration and directs the flow of water away from the building and into drains and gutters.

The primary flashing areas that roofers need to be aware of are:

  • Side walls, where roof planes terminate against a wall (as seen in dormers and second story protrusions).
  • Front walls, where the incline of the roof comes up against a vertical wall (usually a second story).
  • Roof penetrations, where chimneys, solar panels or ventilation ducts require the removal or cutting of shingles.

This guide will walk you through the proper installation of roof flashing according to ARMA’s “Asphalt Roofing Residential Manual Design And Application Methods.”

Flashing Installation Procedure Along Walls

When dealing with the juncture of a sloped roof plane with a side wall or front wall, rectangular metal flashing measuring at least 10 inches long and at least 2 inches wider than the expected shingle exposure should be used. For example, if the roof shingles have a 5-inch exposure, the width of the flashing should measure a minimum of 7 inches.

NEVER nail the vertical flange of the flashing unit to a wall. This can cause shingle buckling.

Each flashing unit is bent into two equal flanges at a 90-degree angle for side walls or a suitable angle for front walls, along the 10-inch length, allowing for one of the 5-inch flanges to extend over the roof plane. These units can be put aside until needed.

When installing a roof with a side wall or front wall, underlayment should be installed about 3 to 4 inches up the wall to act as the first line of defense against moisture.

Next, the first flashing unit should be placed over the end of the starter strip. The end shingle in the first course is then cemented to the flashing flange and should cover the flashing completely.

The horizontal flange of the flashing (the part extending over the shingles) is then nailed to the roof within 1 inch of the upper edge of that flange. (NEVER nail the vertical flange of the flashing unit to a wall. This can cause shingle buckling.)

Apply the first shingle course right up to the wall and then position the next flashing unit over the end shingle of the first course, just like the first unit was installed over the end of the starter strip.

From there, step the flashing units along the entire length of the wall/roof intersection, fastening each horizontal flange with two nails. The end shingle in the next course should cover the step-flashing unit of the previous course completely each time. If a flashing unit overlaps the one below it, that’s OK.

When the flashing is properly installed, the roof siding can be brought down over the vertical flanges of the flashing units to serve as counter flashing. Bear in mind that some siding may require painting.

When encountering front walls, flashing units are bent accordingly (depending on the angle of the wall to the roof) and are nailed to the last shingle course. Then an additional row of shingles is installed over and cemented to the flanges of the flashing units that extend down the roof plane from the intersection.

Flashing Installation Procedure Around Roof Penetrations

For vent pipes and soil stacks, install the roof shingles up to the penetration. Measure and cut a hole in the next shingle to fit over the pipe. Slide that shingle down the pipe to the roof deck and install the shingle.

Preformed flashing flanges called boots are available to fit most roof penetrations. Fit an appropriate boot over the pipe or stack and cement into place. Continue with roof installation, making sure to cut the subsequent shingle course to fit over the pipe and its boot.

Chimneys require a combination of side-wall and front-wall flashing techniques. Step flashing is installed against the four walls of a chimney the same way they would be in a side-wall or front-wall scenario.

However, differential movements (house settling) come into play where chimneys are concerned, which means that apron flashing needs to be installed into the masonry of the chimney itself (but not fastened to the roof plane). The apron flashing acts as counter flashing for the step flashing.

If installed correctly, neither the roof plane nor the chimney will be fastened to each other via flashing. If and when the house settles, the deeply rooted chimney won’t rip or buckle the shingles on the roof.

Roof Flashing By Atlas Roofing’s Standards

Proper roof flashing is critical to extending the life expectancy of a new roof and protecting a home from unwanted water infiltration. In combination with an Atlas Signature Select® Roofing System, properly installed flashing is a sound investment in the preservation of a home. Contact Atlas today to learn more.