Every shower drain needs a trap, but unlike the one under your sink, you never see it. It sits below the floor, usually buried in a joist bay or set into a concrete slab, and it only gets attention when something goes wrong: a sewer smell after a vacation, a slow drain, or a remodel that moves the shower a few feet. Here is what a shower P-trap looks like, what size and type you need, and how one gets installed when the floor is open.
What a shower trap does and where it sits
The shower trap is a 2 inch P-trap connected directly below the drain body. Water from the shower pan or tile base flows down through the drain, fills the U-bend and spills over the weir into the trap arm, which then runs to a vent and on to the main drain. The water left behind in the bend is the seal that keeps sewer gas out of the bathroom, typically 2 to 4 inches deep.
Because the shower floor sits only a few inches above the subfloor, the trap almost always lives in the joist space below or in a pocket cut into a slab. That means access is the biggest planning issue, much more than the trap itself.
Typical shower trap specs
| Item | Typical figure | Notes |
|---|---|---|
| Trap and drain size | 2 in | Most US codes require 2 in for showers, not 1 1/2 in |
| Trap seal | 2 to 4 in | Same range as other fixtures |
| Trap arm length to vent (2 in) | About 6 to 8 ft | Check your adopted code table |
| Trap arm slope | 1/4 in per ft | Toward the vent and drain |
| Material | PVC or ABS | Match the rest of the house, do not mix glue types |
| Trap part cost | About $5 to $20 | Plain 2 in solvent weld P-trap |
| Plumber install, open floor | About $200 to $500 | More on a slab, often $800 to $2,000 with concrete work |
Older homes sometimes have a 1 1/2 inch shower drain. It is usually allowed to stay, but during a remodel most inspectors expect you to bring it up to 2 inches.
Choosing the right trap
For almost every shower you want a plain solvent weld P-trap in the same material as your drain lines. Some fittings come with a cleanout plug at the bottom, which is handy where codes allow it and the trap is reachable from below. Avoid any deep seal or drum style trap, since they are not allowed for new work, and never use a slip joint trap under a shower floor because those joints are meant to stay accessible. If you are curious how drum traps compare, see P-trap vs drum trap.
The drain body on top matters too. A tile shower uses a clamping drain that grips the liner, while a fiberglass or acrylic base uses a compression or glue-in drain. Your choice of grate, from square to linear, is mostly cosmetic and covered in our guide to shower drain cover types.
How to install a shower P-trap step by step
This assumes the floor is open from below or the subfloor is cut back, and the drain line and vent already exist.
- Dry fit the shower base or mark the drain center on the subfloor and cut a hole about 4 to 5 inches across.
- Hang the 2 inch P-trap so its inlet sits directly under the drain center. Support it with a strap or block so it cannot sag.
- Measure from the trap outlet to the drain line, keeping a fall of about 1/4 inch per foot, and cut the trap arm to length.
- Dry fit everything, mark the fittings so they line up, then prime and cement each joint with the correct cement for PVC or ABS.
- Glue a short riser into the trap inlet, long enough to meet the shower drain body once the base is set.
- Set the base, connect the drain body to the riser, and fill the trap with water.
- Before you close the ceiling or floor, run the shower for a few minutes and watch every joint from below.
Most towns want this inspected before it is covered, so book the inspection first.
Common shower trap problems
- Sewer smell: usually a dry trap in a shower that rarely gets used. Run it for a minute every couple of weeks. Persistent odor points to venting, covered in why a P-trap smells.
- Slow drain: hair is the usual culprit, caught in the drain body or the top of the trap. A drain snake from above usually clears it. Our guide to a clogged P-trap covers the methods.
- Gurgling: often a long trap arm or a blocked vent pulling on the seal.
- Leak below: usually the drain body gasket rather than the trap, so check the top connection first.
Safety and code notes
Showers need a 2 inch trap in most jurisdictions, and every trap must be vented within the allowed trap arm distance. Shower traps are usually not allowed to be S-traps or to share a trap with another fixture. Cutting a slab can hit radiant heat lines, post-tension cables or buried pipes, so scan first and hire a pro for slab work. Wear eye and dust protection when cutting, ventilate when using PVC primer and cement, and pull a permit for new or relocated drains.
Frequently asked questions
What size P-trap does a shower need?
Most US codes require a 2 inch trap and drain for a shower. Older 1 1/2 inch drains may need upsizing in a remodel.
Does every shower have a P-trap?
Yes. Every shower drain needs a trap to block sewer gas, even though it is hidden below the floor.
How do I clean a shower P-trap without removing it?
Pull the grate and run a drain snake or hair tool down through the drain body. Most clogs are hair near the top. Our guide on how to clean a P-trap without removing it has more methods.
Why does my shower smell like sewer?
Usually the trap has dried out from lack of use. If it smells after regular use, check the vent and the drain gasket.
How much does it cost to replace a shower trap?
Typically about $200 to $500 with open access, and often $800 to $2,000 or more if a concrete slab needs cutting.