Short definition
A drain shoe is the elbow under the tub that bolts to the underside of the drain spud and turns toward the tee in the drain-waste-overflow assembly. It connects the bottom of the tub drain to the trap arm. Standard residential tub drain size is 1-1/2 inch.
What it is
Beneath every tub, the drain spud (the threaded fitting visible inside the tub, where the strainer or trip-lever plate sits) screws down through the tub bottom. The drain shoe bolts to the underside of that spud, sealed against the porcelain or acrylic with a fiber gasket above and a beveled rubber washer below. The shoe turns the flow horizontally toward the drain tee, where it joins the overflow tube and the trap-arm tailpiece.
Traditional tubs use chrome-plated brass shoes; modern tubs use PVC or ABS. The 1-1/2 inch standard size matches UPC residential tub-drain minimums.
Why it matters to a homeowner
Most homeowners only meet the term during a tub leak diagnosis. If water is showing up at the ceiling below an upstairs tub, or on the floor next to a first-floor tub, the suspects in order are: drain-shoe gasket ($20 to $80 part for the kit, accessed from below), supply-line connection at the valve, or wax-ring-equivalent at the tub-spout penetration.
The shoe itself rarely fails on its own — gaskets and seals around it do. Plastic shoes can crack from impact during a remodel or from a hard freeze in an unheated structure; brass shoes occasionally cross-thread at the spud connection if a previous repair was forced.