Pipe runs through floors and notching joists

Hot and cold pipes, heating pipes and wastes have to get from one place to another, and in most houses that means under floors, through joists and up through walls. How those pipes are routed matters. A careless notch can weaken a floor. A pipe left unsupported can knock and creak. A pipe laid where a floorboard nail will go through it can leak for months before anyone notices.

This page explains the principles behind good pipe runs in timber floors, in general terms. It does not give notch or hole sizes, and it is not a substitute for the official guidance or a structural engineer.

Where pipe routes are decided

Pipe routes are part of first fix: the stage when pipes go in before floors and walls are closed. Changing a route later usually means lifting boards or opening a ceiling. In a refit, the order of trades matters, and bathroom refit order explains where plumbing sits.

Good questions to settle early:

• Which way do the joists run, and can the pipe run between them rather than across them?
• Where will the pipe cross joists, and can that be near a wall rather than mid-span?
• Will a waste pipe, which needs a fall, fit within the floor depth?
• Where are the electrical cables, so pipes and cables are kept apart?
• How will someone get to joints and valves later?

Running with the joists

The simplest route is parallel to the joists, in the gap between them. Nothing is cut. The pipe is clipped to the side of a joist or supported on battens. Plumbers plan routes to make the most of this, even if it means a slightly longer run.

Crossing joists: notches and holes

When a pipe has to cross a joist, it goes either through a notch cut in the top edge or through a hole drilled through the joist. Both take material out of a structural timber.

Notching and drilling joists is controlled by the Building Regulations. In England, structural matters are covered by Approved Document A, published on GOV.UK. Guidance on where in a joist a notch or hole may go, and how big it may be, is set out in official and industry guidance relating to those rules. We do not repeat figures here, because the right answer depends on the joist size, its span, the load and the type of joist, and because getting it wrong can weaken a floor.

What we can say in general:

• Cut as little as possible. Fewer, smaller cuts are better.
• Where cuts are made along a joist matters as much as how big they are.
• Notches and holes should not be close together or close to other cuts.
• Engineered joists, such as I-joists and metal-web joists, have their own rules from the maker. Many cannot be notched at all, or only in marked places.
• If a joist already looks heavily cut, stop and get advice before cutting it again.

If you are not sure, ask a structural engineer, building control or an experienced plumber before cutting. Wales, Scotland and Northern Ireland have their own building rules and guidance.

Supporting and protecting pipes

A pipe that is properly supported is quieter and lasts longer. In general:

• Clip pipes at regular intervals so they cannot sag or move. The spacing depends on the pipe and is set out by the pipe maker.
• Plastic pipe needs closer support than copper and may need support along its length.
• Leave room for hot pipes to expand. A hot pipe clamped tight in a notch can tick and creak as it heats and cools.
• Use the maker's sleeves or grommets where plastic pipe passes through a hole, if they specify them.
• Protect pipes in a notch from screws and nails, for example with a metal plate, so the pipe cannot be pierced when the floor goes back down.
• Mark or photograph pipe routes before boarding over, so future fixings avoid them.

Pipe types: copper and plastic covers the differences that affect support. Knocking and ticking pipes are covered in noisy pipes and water hammer.

Pipes in concrete floors

Solid floors are different. Pipes may run in a screed, in ducts or round the edge of a room. Pipes buried in a floor are hard to reach and repair, so they are usually sleeved or ducted and joints are avoided. Plan this with whoever is laying the floor.

Testing before boarding

Pressure-test new pipework before floors and walls are closed. A leak found now costs minutes. A leak found after tiling costs a ceiling. First fix and second fix plumbing covers testing in more detail.

Gas and electrics

Gas pipes are for a Gas Safe registered engineer only, including routing them through floors. Electrical cables are for a qualified electrician. We do not give wiring instructions. Keep water pipes and cables apart, and agree routes between trades before anyone cuts.

When to call someone

Call a plumber or builder if a joist is already badly cut, if the floor feels springy, if you find engineered joists and do not know their rules, or if a route needs a structural change. Hiring a plumber covers what to agree in writing. Building control can tell you whether work needs approval.

How deep can I notch a joist for a pipe?

We do not give a figure. It depends on the joist and its span, and limits are set out in guidance related to the Building Regulations. Check Approved Document A and ask a structural engineer or building control if in doubt.

Is it better to notch or drill a joist for pipes?

It depends on the joist, the pipe and where the cut falls. Both have rules. Engineered joists have their own instructions from the maker.

Why do my pipes creak under the floor?

Often hot pipes are expanding and rubbing in tight notches or clips. A plumber can ease the notches, add clips or fit sleeves.