How do you make a pipe boot for a roof?
Short answer. For a standard plumbing vent you should not fabricate one; manufactured pipe boots in 1.5, 2, 3 and 4 inch sizes cost little and seal far better than anything homemade. For an odd size or a pipe you cannot slide a boot over, use an adjustable or split retrofit boot, or have a sheet metal shop fabricate a soldered metal flashing with a storm collar.
What a boot has to do
A pipe boot has a flat base that tucks under the shingles above it and lies over the shingles below it, plus a raised collar that grips the pipe. Homemade versions usually fail at one of those two jobs: the base gets laid on top of all the shingles and caulked, or the collar is a ring of sealant that cracks within a season or two of Georgia sun.
Better options than making one
- Standard boots: a metal or plastic base with a rubber collar, sized to the pipe. A 3 inch collar on a 2 inch pipe will never seal.
- Adjustable boots: stepped rings you trim to fit several pipe sizes, handy for odd jobs.
- Split or retrofit boots: they open and wrap around the pipe, useful when the pipe cannot be cut or the old boot cannot come off.
- Lead boots: soft lead is folded down into the top of the pipe, so there is no rubber collar to crack. They last a long time on shingle roofs but should not be used on steel roofs.
- Custom metal flashing: for large or unusual penetrations, a sheet metal shop can fabricate a soldered base and sleeve, finished with a storm collar.
If you are stuck for a few days
When a boot cracks and you cannot replace it right away, a slip-over rubber repair collar or a careful wrap of butyl flashing tape can get you through a few storms. Treat it as temporary and get the boot replaced.
See pipe boot leaks for why boots fail, and flashing explained for how penetrations are woven into the roof.