How to Install Flashing Around Roof Vents and Chimneys

Flashing is the difference between a roof that sheds water cleanly and one that leaks into your walls and attic. Where pipes, vents, and chimneys break through the roof plane, you need a transition that doesn't just sit on top—it has to be integrated into the shingle layers so water flows harmlessly down and away. Improper flashing is one of the leading causes of attic leaks, and most of those failures happen because the installation was rushed or the overlap sequence was wrong. Done right, flashing lasts as long as the roof itself. Done wrong, you're looking at water damage, mold, and rot inside the house. This is work you can do yourself if you're comfortable on a roof and willing to pay attention to the layering.

1. Measure and prepare the penetration. Identify the exact location of the vent or chimney from inside the attic, then mark it on the roof from above. For vents, measure the diameter or width of the pipe. For chimneys, measure the width and depth of the chase. Clear any debris around the penetration and remove old flashing if you're replacing it—use a pry bar to lift fasteners and cut away old roofing cement carefully.
2. Slide flashing under shingles above the penetration. For roof vents, start with the upper flashing boot (the part that goes above the vent opening). Lift the shingles above the penetration carefully—don't tear them—and slide the flashing boot underneath so it sits flat against the roof deck. The upper portion of the boot should be under the shingles; the lower portion will sit on top of shingles below. For chimneys, start with the upper cricket flashing, which bridges the uphill side of the chimney where water pools.
3. Secure the upper flashing with fasteners. Drive roofing nails through the upper flange of the flashing, into the roof deck—typically two nails per side. Space them about 1 inch from the edge and don't overdrive them; the nail head should sit flush but not crush the flashing. Do not drive nails through the lower portion of the flashing yet—that gets sealed, not fastened.
4. Seal the upper flashing edge with roofing cement. Apply a continuous bead of roofing cement along the upper edge where the flashing sits underneath the shingles, and press the shingles down firmly. Apply a second bead along the line where the upper flashing meets the roof deck. Use a putty knife to smooth it and ensure no gaps remain. Do not glob cement thickly; a 1/8-inch bead is sufficient.
5. Install lower flashing and shingle overlap. For vents, the lower flashing boot sits on top of the shingles below the vent. Slide it down so it overlaps the shingles beneath by at least 4 inches. Nail it to the roof deck with two fasteners in the lower flanges, then cover those nails and the flashing edges with roofing cement. Shingles above this flashing will cover the nails you just drove. For chimneys, repeat this step on the downhill sides and corners, always ensuring each new piece overlaps the one below.
6. Install replacement shingles and seal all edges. Once the flashing is secured and cemented, lay new or existing shingles back over the upper flanges of the flashing. Nail them down (one nail per shingle tab, standard spacing) and seal any exposed fasteners with roofing cement. Run a continuous bead of cement along the edges where flashing meets shingles, especially where shingles butt against the flashing sides. This is the final watertight barrier.
7. Inspect seams and allow cure time. Walk your work and check that all flashing edges are sealed, all fasteners are covered, and no gaps exist between the flashing and the roof deck or between flashing and shingles. Look from multiple angles—light leaks through gaps. Allow roofing cement to cure for 24 hours before foot traffic or rain, and keep the roof clear of debris that could trap moisture and undermine the seals.