Basement Bathroom Rough-In: Wet Vent, Slab Fall, and Radiant Tubing
A homeowner wants a basement bathroom - water closet, lavatory, and a 36 x 48 shower - in a 1960s ranch. The 4-inch cast-iron building drain runs under the slab, and a 3-inch stack goes to the roof at the far wall. The lavatory and shower are on one wall; the water closet is 8 feet away. The proposed plan: 2-inch shower drain and 1-1/2-inch lavatory drain tying into a 3-inch line that runs 22 feet at 1/8 inch per foot to the existing building drain, with the water closet connecting downstream of the lavatory and wet-venting through the lavatory's 1-1/2-inch vent. The slab is 4 inches over mesh, with hydronic heat tubing stapled to the mesh in that area.
1. Is the proposed wet vent acceptable in principle? What must you verify before anyone cuts concrete?
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The concept can work, but the details as proposed fail.
- Trap-arm length: each trap arm has a maximum developed length for its size (roughly 6 ft for 1-1/2-inch and 8 ft for 2-inch at 1/4 in/ft, longer as pipe size increases). Measure, do not eyeball.
- Vent size: water closet 3 FU + lavatory 1 FU + shower 2 FU = 6 FU, which needs a 2-inch vent. Using the 1-1/2-inch lavatory vent as the wet vent is undersized.
- Fixture order: on a horizontal wet vent the water closet must be the most downstream fixture, with the lavatory upstream, and the connection must be made in the required manner.
- Rise before offset: the vent must rise vertically at least 6 inches above the flood-level rim of the highest served fixture before it turns.
- AHJ: confirm the local inspector accepts wet venting here at all and that an existing 3-inch stack can serve as the vent for the new group.
Answer the sizing and ordering questions first; they decide whether the layout survives at all.
2. The slab contains hydronic tubing. How does that change the plan?
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You cannot saw-cut a 22-foot trench across a heated slab on the assumption that the tubing is where you think it is. Options, best first:
- Re-route to a wall, chase, or existing penetration so the run is shorter and the cut is narrow, or so no slab work is needed.
- Locate before cutting - thermal scan plus the original tubing layout, then hand-chisel only where confirmed clear, with the owner's written acknowledgment of the risk and an agreement on who repairs a nicked loop.
- Avoid the slab entirely with a sewage ejector/lift station set in a small pit at the low corner, discharging up to the building drain. This also solves the elevation problem if the existing invert is too shallow.
Get the decision in writing before the saw comes out. A cut loop in a heated slab is an expensive, slow repair and it becomes your liability the moment the blade goes in.
3. How much fall do you actually have, and is a backwater valve required?
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Fall: 22 ft at 1/8 in/ft = 2.75 inches of drop, before allowing for fitting depth and required cover over the pipe. Compare that to the depth of the existing building drain invert below the slab. If the invert is only a few inches down, a 3-inch line plus a 2-inch shower trap arm will not fit - the answer is a shorter, smaller-diameter run or an ejector system rather than forcing slope.
Backwater protection: a backwater valve is required where the new fixtures sit below the elevation of the next upstream manhole or the street. In a basement that is common. Without it, a surcharged main backs up into the lowest opening - usually the shower, which is also the hardest fixture to clean.
If an ejector is used: it needs its own vent taken up and sized for the pump discharge, a gas-tight cover, and accessible service clearance. Confirm with the AHJ how the discharge is to be connected to the building drain.