Repair a mistake
1 of 1
Use a 3-in.-to-4-in. transition coupling, either plastic like this or banded rubber, to join a 3-in. to a 4-in. vent pipe.
Someone took a shortcut. First of all, going from
3-in. pipe to 4-in. is unusual, although it may be
required by your local plumbing code. Sometimes vents
like this had to be made larger to keep frost from clogging
the pipe. However, our best guess is that the plumber
stuck a 4-in. pipe through the roof sheathing before the
roofers came so they could flash the vent into the shingles.
The plumber made the inside connections later and
either didn't have a 3-in.-to-4-in. coupling handy or
didn't want the hassle of making a proper connection.
Whatever the explanation, the joint doesn't meet the plumbing code and was
bound to leak. Three-inch pipe doesn't fit tightly into 4-in. Caulk has to fill about
a 1/4-in. gap all around. In addition, this joint creates a ledge inside the pipe where
water from rainfall can collect. As you might expect, when the caulk eventually fails, a trickle of water will leak through and drip down through the ceiling below.
The easiest solution is to saw out the caulked joint, removing about 1 in. of pipe
(two cuts). Then either glue in a proper 3-in.-to-4-in. PVC coupling or insert a
3-in.-to- 4-in. banded rubber fitting. Our plumbing consultants prefer the banded
fitting because gluing the large PVC fitting in a tight attic space can be difficult,
and the solvent fumes in PVC cement can get dangerously intense. (If you use the
cement, be sure to wear a respirator with organic solvent cartridges.) The banded
fitting has a metal sleeve surrounding a rubber gasket. You'll probably have to go
to a plumbing supply store to find the 3-in.-to-4-in. type.
If you work carefully, you won't disturb the roof flashing. Still, it's a good idea
to spray the flashing area on your roof with a garden hose. Then check the attic
to make sure there are no other leaks.