Sealing a Drain Penetration in a Foundation Wall
Low-expansion urethane goam, roof cement, and roof vent flashing is a better process to prevent cracking.
Lately, we’ve been searching for a better way to seal the gap around a drain pipe where it exits a foundation wall. The standard technique that most builders use in our area is to pack the gap with mortar or hydraulic cement. But lately, our plumbing inspectors have been frowning on this practice because any shift in the soil during frost cycles can crack a rigidly installed pipe. So we’ve tried a half-dozen different methods, with mixed results, until we started using the detail shown in the drawing.
Instead of mortar, we now use low-expansion urethane foam to fill the gap around the pipe. This is the same stuff we use to seal cracks and crevices throughout the house to reduce air leaks. Once the foam has cured, we apply a layer of roof cement to the outside of the wall. This is the thick stuff, the kind of roof cement that you spread with a trowel. Then we slip a roof vent-stack flashing over the pipe and bed the flashing in a 1/4-in. thick layer of roof tar. We lap the tar over the edges of the flashing to promote a better seal.
At backfill time, care must be taken to prevent damage to the flashing. This extra care has presented no problems for us, and we’ve had great luck with the results.
Mike Guertin, East Greenwich, RI
Edited and Illustrated by Charles Miller
From Fine Homebuilding #122
View Comments
Now that was a good tip.
Good tip. Bad graphics. Please dispense with the 19th-century cheap looking line drawings.
I used this same technique on my last build (using peel and stick with primer instead of roofing tar to seal the roof jack to the concrete), and was very pleased with the results as well. Great tip! And for the record- I like the hand-drawn line drawings. The aesthetic detracts nothing from the quality of the advice, as long as the illustration is clear.
Love the tip and the drawing!
Good tip.
Nice, clear, easy to understand graphic.
Thanks!
That is ust the way I would draw it, and I'm an illustrator. The idea is clarity with minimal unnecessary embellishment .
Hey, marleyjune - get a life!
Rubber compressible seals are available from several manufacturers that are (almost) foolproof.
I also dump some granular bentonite around the penetration when backfilling. Your basic belt and suspenders approach.