Which pipe should I use for waste and drainage?
Which pipe should I use for waste and drainage?
Solvent-weld or push-fit PVC-u or polypropylene, at the right diameter and fall — and ABS and PVC cements are not interchangeable.
Two things sink more waste installations than material choice: fall and trap seals. Too little fall and solids settle; too much fall and water runs away leaving solids behind, which is why the standard guidance is a modest gradient of the order of 1 in 40 for small waste pipes rather than as steep as possible. Trap seals are the other half — the water in a trap is all that stops drain gases entering the room, and it is lost by evaporation in unused fixtures or siphoned out by a long unventilated run, which is why air admittance valves and vented stacks exist. Any change in a soil or waste layout in a dwelling is usually subject to building control, and the diameters and gradients are set out there rather than left to judgement.
The options, ranked
| Option | Good at | Bad at |
|---|---|---|
| Solvent-weld PVC-u | Permanent, slim joints that fit in tight spaces, and no seals to perish. | No allowance for expansion unless expansion couplings are included, and no way to dismantle. |
| Push-fit polypropylene | Quick, demountable and accommodates expansion at each joint — ideal behind a bath where access is limited. | Bulkier, and the seals can be displaced if the pipe is pushed in dry or at an angle. |
| ABS | The common black waste material in North America, tough and easy to solvent-weld without primer. | ABS cement and PVC cement are different products, and the transition between the two materials is code-restricted. |
| Cast iron soil pipe | Dramatically quieter than plastic, which matters for a soil stack running past a bedroom. | Heavy, expensive and needs proper support. |
Technique that changes the outcome
- Keep the fall consistent — roughly 1 in 40 for small waste, checked with a level
- Wet the seal and push square on push-fit joints, and mark the insertion depth
- Leave expansion allowance on long solvent-weld runs
- Support the pipe at the intervals the manufacturer specifies so it cannot sag into a belly
What goes wrong
- Mixing ABS and PVC with either cement
- Excessive fall, which strands solids
- Connecting a new appliance into a stack without checking trap seal protection and venting
Compiled practical guidance rather than a published standard. Product formulations vary by brand and market, so read the label — and where the work is structural, electrical or gas, the regulations that apply where you are govern rather than anything here.
Other pipe questions
| For | Use |
|---|---|
| Hot and cold water | Copper or PEX for most installations — PEX is faster and more freeze-tolerant, copper is more robust and takes heat without question. |
| Central heating | Copper, or a barrier-grade plastic pipe — non-barrier plastic lets oxygen into the system and corrodes it from the inside. |
| Boiler condensate | Plastic, never copper, at a generous diameter with continuous fall — and insulate any external run, because frozen condensate is the classic winter breakdown. |
| Gas | Steel, copper of the right grade, or yellow MDPE underground — and in most jurisdictions only a registered installer may do the work. |
| An underground water supply | Blue MDPE for potable water, in one continuous length at the required depth, with no joints under a building. |
| Compressed air | Copper, steel, stainless or a purpose-rated aluminium or HDPE air system — never PVC, which fails explosively. |
| Irrigation | Low-density polyethylene tubing with barbed fittings for drip lines, and PVC or MDPE for buried mains. |
Common questions
- Which pipe should I use for waste and drainage?
- Solvent-weld or push-fit PVC-u or polypropylene, at the right diameter and fall — and ABS and PVC cements are not interchangeable.
- What should I avoid?
- Mixing ABS and PVC with either cement Excessive fall, which strands solids Connecting a new appliance into a stack without checking trap seal protection and venting
Related pages
- Which pipe should I use for hot and cold water? Copper or PEX for most installations — PEX is faster and more freeze-tolerant, copper is more robust and takes heat without question.
- Which pipe should I use for central heating? Copper, or a barrier-grade plastic pipe — non-barrier plastic lets oxygen into the system and corrodes it from the inside.
- Which pipe should I use for a boiler condensate drain? Plastic, never copper, at a generous diameter with continuous fall — and insulate any external run, because frozen condensate is the classic winter breakdown.
- Which pipe is used for gas? Steel, copper of the right grade, or yellow MDPE underground — and in most jurisdictions only a registered installer may do the work.
- Which pipe should I use for a buried water supply? Blue MDPE for potable water, in one continuous length at the required depth, with no joints under a building.
- Which pipe should I use for compressed air? Copper, steel, stainless or a purpose-rated aluminium or HDPE air system — never PVC, which fails explosively.
- Which pipe should I use for irrigation? Low-density polyethylene tubing with barbed fittings for drip lines, and PVC or MDPE for buried mains.
Sources
- Typical range compiled from manufacturer specifications — At One Place