Roof Flashing Repair: Replace Plumbing Vent Flashing

What You'll Need
Replacement Flashing
Ice and Water Membrane
Hammer and Roofing Nails
Metal Shears
Urethane Sealant or Roofing Tar
Replacement Shingles
Metal Shears or Razor Blade

The area where your building’s vent stacks penetrate the roof is vulnerable to leaks, and it may eventually need roof flashing repair. This is not a difficult process, but it requires a great deal of care to ensure a watertight seal. You will need to remove any existing roofing material first. When installing the new flashing, place an ice and water membrane, followed by half of the new shingles. Then install the flashing piece and lay the rest of the shingles over it.

Step 1 - Order Supplies and Materials

The first step is to measure the inner diameter of the vent pipe. Flashing comes in standard sizes so you should be able to easily obtain a pre-fitted piece. Vent stack flashing can be made of lead or rigid rubber. Rigid rubber is especially good for PVC vent pipes but may not seal correctly to cast iron pipes. However, rubber boots may eventually crack while lead is the longer-lasting solution. Wear gloves if working with lead flashing.

Step 2 - Access Roof Safely

Place the ladder on a level surface and climb to the roof. If the roof is very steep, you may wish to use a foam block to help climb it.

Step 3 - Remove Old Roofing Materials

Use a crowbar to remove the shingles and old vent flashing. Use extra caution to avoid puncturing the roof. Remove the old roofing nails and seal over their holes with roofing tar. If you brought the new flashing with you, you can tuck it under the shingles above you to prevent it from sliding away while you work on this.

Step 4 - Install Ice and Water Membrane

Cut a hole slightly smaller than the diameter of the vent pipe into the ice and water membrane with a razor blade or by tapping it on the pipe with a hammer. This way, the membrane will sit around the base of the pipe. Slide the membrane over the vent stack and then seal around the pipe.

Step 5 - Install Shingles, Flashing and More Shingles

Install the new shingles halfway across the area that will be covered by the flashing, slightly past the vent stack. Cut the receiving flanges from the last row of shingles. Now slide the flashing over the vent stack and secure it against the roof.

At this point you can choose whether to caulk under the bottom of the flashing flange or just around the sides. Caulking the bottom can trap condensation underneath the flashing, but only sealing around the sides can leave the area vulnerable to wind-driven rain. If you have installed the ice and water membrane, the risk of condensation is negligible and you should definitely caulk the bottom of the flashing.

If you use nails to secure the flashing, seal around those penetrations as well. Install the rest of the shingles to cover the flashing.