Key takeaway
Under a raised floor, properly designed hangers maintain slope. Pipe laid on soil or balanced on scraps is not equivalent support.
Safety and jurisdiction
Educational overview, not a site-specific installation plan. Use a qualified, appropriately licensed contractor and required permits. Never enter an unprotected excavation, cut structural framing or a slab without clearance, or conceal piping before the required inspection. Do not enter a crawlspace with standing water, sewage, exposed wiring, pests or suspected gas; arrange safe professional access.
Codes and permit rules vary by city and change over time. Confirm requirements with your local building department and a licensed contractor.
01Separate suspended drains from buried sewers
A raised foundation often has horizontal building drains suspended beneath joists, then a transition to the exterior buried sewer. Calling all of this 'underground plumbing' can hide an important distinction: suspended and buried pipe need different support and inspection details.
The contractor first maps the existing branches, stack locations, foundation openings and the sewer outlet. Available height must allow the required grade without blocking access or compromising the floor framing. If gravity fall is insufficient, a designer must solve it; reducing slope by eye is not a solution.
02Keep the grade between supports
Supports must suit the pipe material, diameter, temperature and manufacturer's requirements. Hangers should maintain alignment without crushing plastic piping, and transitions need support so the joint is not carrying the weight of a long run.
Check the actual pipe grade between hangers as well as at the ends. A shallow sag can hold water even when the overall run drops correctly. Do not hang plumbing from another pipe, an electrical cable or a loose block of wood.
03Connections must remain serviceable
Changes of direction need approved drainage fittings, not tight pressure elbows. Cleanouts must be reachable with real tools and space for servicing. A cleanout buried behind a new perimeter wall or an inaccessible opening offers little practical access.
When connecting new plastic to existing cast iron, the installer must use a listed transition appropriate for both outside diameters and the location. Correct coupling selection, support and tightening requirements come from the product instructions; visual similarity is not enough.
- Hangers match the material and approved spacing
- No sags, improvised supports or strained transitions
- Cleanouts and crawl access remain usable
- Joists and foundation openings remain structurally compliant
04Check the space as well as the pipe
A recurring wet crawlspace is not always a drain leak. Groundwater, surface drainage and a water-supply leak can produce similar symptoms. Diagnose the source before replacing piping, and address sewage contamination through appropriate remediation.
Request photos of the completed supports, transitions and cleanouts, then confirm testing and inspection before access is closed. Have the contractor record the route and any inaccessible portions so future repairs do not start with guesswork.
Sources and further reading
- 2025 California Plumbing Code — chapter directory (opens in new tab)Code reference, not a substitute for the edition and amendments applicable to your permit. Detailed sections may require an account.
- Charlotte Pipe — The dangers of air testing PVC, CPVC and ABS (opens in new tab)Manufacturer safety guidance and a 10-foot water-head testing example. Never transfer a gas test pressure to a plastic drain system.