Notching and Drilling Joists: the Numbers You Have to Know

This is one of the very few things at Level 1 with real numbers attached, which means it is one of the few where an approximate answer earns nothing.

The short answer

A notch may be up to one eighth of the joist depth, in the zone from 0.07 to 0.25 of the span from the support.

A hole may be up to one quarter of the joist depth, drilled on the centre line, in the zone from 0.25 to 0.4 of the span, with at least three diameters between holes.

On a 200 mm joist that is a 25 mm notch and a 50 mm hole. Never both at the same point.

Why a notch and a hole get different rules

This is the bit that makes the numbers stick, because otherwise they are just two fractions to swap round under pressure.

A loaded joist bends. The top of it is squeezed — in compression. The bottom is stretched — in tension. In between there is a line where the timber is neither being squeezed nor stretched, and it is doing almost no work at all. That is the neutral axis, and it runs along the centre.

So a hole drilled on the centre line takes material from the least important part of the joist, which is why a hole is allowed to be twice as deep as a notch. A notch is cut in the top edge — the compression face, the part working hardest — so it is limited far more tightly. Cut a deep notch in the top of a joist and you have created a hinge in the exact place the timber was strongest.

Why the zones are where they are

The same logic explains the zones. Bending is greatest in the middle of the span and least near the supports, so neither a notch nor a hole is allowed in the middle third.

Shear — the sliding force that tries to tear the joist vertically — is greatest right at the supports, which is why the notch zone starts at 0.07 of the span rather than at the wall. Neither zone is arbitrary. Each one is the stretch of joist where that particular kind of cut does the least harm.

And it explains the last rule. A notch and a hole at the same point remove material from the top and the middle together, and the two allowances were each calculated on the assumption that the other one was not there.

Doing it on site

Measure the joist depth first — 200 mm is common in older houses but nothing is guaranteed, and the fraction is of the actual depth, not a remembered one. Measure the span, work out the zone, and mark it before you pick up a drill. It takes two minutes and it is checkable.

Where a pipe simply will not fit inside the allowances, the answer is not a bigger notch. It is to run the pipe a different way: along the joist rather than across it, under the joists in a ceiling void with a boxing, or on the surface. Plastic pipe has made this easier because it will go through a hole in a long continuous run without a joint in the floor.

And never leave a joint buried in a floor void. Nobody can see it, nobody can reach it, and it is the leak that damages a ceiling for three months before anyone notices.

Engineered joists are a different set of rules entirely

Posi joists, I-joists and metal web joists are increasingly what a new build has, and none of these fractions apply to them. An I-joist has a thin plywood web and a small solid flange top and bottom — cut the flange and you have effectively cut the joist through.

They come with pre-formed service holes or knock-outs, and with manufacturer's instructions saying exactly what may be cut and where. That is the document you follow, and it is another instance of the same rule from the last lesson: the manufacturer’s instructions take priority.

What comes next

Level 2 puts these numbers alongside pipe support spacings and the routing decisions that go with them. The structural reasoning is the same one that governs where you are allowed to chase a wall.