General construction: how buildings go together
Plumbing rough-in overview
Rough-in is the plumbing installed inside walls and floors before finishes cover them. It has three systems: supply, drain and vent. A guide to how they work and what to check before the walls close.
Rough-in is the plumbing installed inside walls and floors before finishes cover them. It has three systems: supply, drain and vent. A guide to how they work and what to check before the walls close.
Three systems
- Supply: pressurised pipes bring hot and cold water to fixtures.
- Drain: gravity pipes carry waste away, so slope matters.
- Vent: pipes bring air into the drain system so water flows freely and trap seals are not siphoned out.
Layout planning
Group wet rooms back to back or stacked so pipes stay short. Plan pipe routes before the structure is built, since holes in joists and studs must stay within what the engineer allows. See the bathroom catalog page.
Materials
Common pipe materials include copper, PEX and CPVC for supply, and PVC or ABS for drain, depending on local code and water quality. Use only materials approved locally.
Test before closing
Pressure-test supply lines and water-test or air-test drains while the pipes are still visible. Take photos of pipe positions before walls close, since they help in future repairs.
Common problems
Insufficient slope, missing vents, pipes unsupported or freezing in exterior walls, and no access to valves and cleanouts.
Quick answers
- What is a P-trap? A curved pipe holding water that blocks sewer gas from entering the room.
- Why do drains need vents? Without air, draining water creates suction that can empty traps and slow flow.
- Can I move a toilet later? It is costly after the slab or floor is closed, so settle the layout at rough-in.
Keep reading
- Bathroom
- Bathroom pods for hospitality projects
- Electrical rough-in overview
- HVAC basics for small buildings
Always check the project drawings and specification, and confirm requirements with the engineer of record and the local authority.
Questions people ask
What is a P-trap?
A curved pipe holding water that blocks sewer gas from entering the room.
Why do drains need vents?
Without air, draining water creates suction that can empty traps and slow flow.
Can I move a toilet later?
It is costly after the slab or floor is closed, so settle the layout at rough-in.
Sources and further reading
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