Preventing shower drain from cracking

A homeowner forum post raises a problem worth knowing about if your house sits on a concrete slab: a shower drain pipe that cracks underneath, letting water leak into the ground instead of draining away. In this case, a newly bought house had an acrylic shower removed because of a suspected leak, and the crew found the PVC drain pipe cracked where it sat in a pit below the shower, dug into the slab. The removal crew said this happens often in slab homes — they think the pipe flexes slightly every time someone steps into the shower, and over time that flexing cracks the pipe against the unyielding concrete underneath.

The fix being considered has two parts. First, wrapping the repaired pipe in a cushioning material like sill seal (a thin foam strip normally used to seal gaps between a house's frame and foundation) before the concrete is poured back in, so the pipe has some give instead of pressing directly against hard concrete. Second, filling the empty pit under the shower with foam, since an open cavity like that can give soil gas, including radon, a path to collect and potentially seep into the house.

For anyone with a slab-on-grade bathroom, this is a reminder that drain pipes under a shower can be vulnerable to this kind of stress crack, and that any open pit beneath a shower pan is worth sealing up rather than leaving as a void.