What size is the vent pipe that goes through the roof?

Measuring from the roof

Wrap a string or flexible tape around the pipe above the boot to get the circumference, then divide by 3.14 for the outside diameter, or hold a ruler across the open top. PVC and ABS plastic drain pipe share the same outside diameters, so this list works for either:

• 1 1/2 inch pipe: about 1.9 inches outside
• 2 inch pipe: about 2.375 inches outside
• 3 inch pipe: about 3.5 inches outside
• 4 inch pipe: about 4.5 inches outside

Why it matters

Pipe boots are sold by nominal pipe size, and some cover a range. A collar stretched over a pipe that is too large splits early in the sun, and one that is too loose leaks from day one. Cast iron vent pipes on older homes have slightly different outside diameters, so check the boot's listed range against your measurement.

Other pipes that come through roofs

Not every round pipe is a plumbing vent. A metal pipe with a cap and a collar is usually a flue for a gas water heater or furnace, which uses a different flashing with a storm collar. High efficiency furnaces and water heaters often vent through PVC pipes, sometimes in pairs, that look like plumbing vents but carry exhaust. Their sizes and terminations are set by the appliance maker, so leave changes to the HVAC installer. The plumbing code also requires open vents to extend above the roof surface, generally at least 6 inches. See pipe boot leaks for how the boots on these pipes fail.