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Drilling Through Joists and Rafters – The right Way 

By  cordy

In this article we are going to explain the best and safest way of drilling through either rafters or joists. As most of you may know there will be times when we need to do this.

Usually it is to run electrical cables, pipes of some kind for central heating, or for other jobs such as hiding telephone cables or other types of cable.

In this guide we answer many of the commonly asked questions and also offer you some tips of what drill bits to use for quick and effective results.

Difference Between a Rafter, a Truss and a Joist?

If you are new to all of this type of building construction then it's a good idea to get used to the terminology. Just below you will see a diagram which makes the construction of a house easier to understand

There are a few common terms used and we have explained each of these just below.

joists vs rafters

What is a Rafter?

Rafters run from the ridge board to the wall plate on the external wall. They are laid side by side as a base to support the roof. In most cases two rafters will lean against each other and they are then tied to the ridge board at the top and the wall plate at the bottom.

Battens are then laid across these rafters so as a roof covering such as tiles can be laid on top of those.

What is a Purlin?

To make the roof construction more solid, purlins run perpendicular to rafters and offer extra stiffness. That also means that rafters don't have to be thick and heavy, as the purlins make the whole construction more solid.

What are Ceiling Joists?

Ceiling joists run horizontally between the rafter feet, and these then support the ceiling. With rafters, purlins and ceiling joists all connected together, the setup is very sound for support and safety.

What is a Truss?

Most builders will make the rafters onsite from boards. A truss is a triangular construction, usually made off site and then brought to the site to make the roof. In other words it is factory built and delivered to the site.

There are a lot of rules and regulations about load bearing, span size etc. We don't really need to understand all of this when it comes to drilling through joists or rafters, but it does help to understand their basic construction, so as do not weaken it by drilling through either rafters or joists.

Drilling Through Rafters

This is not permitted and there is never really a need to drill through rafters. Rafters as you can see from the diagram below, are there for the structural importance of supporting the roof.

There are a couple of exceptions to this and some trussed rafters do allow for some types of drilling. For the vast majority of homes though you are not allowed to nor should you drill through rafters.

Drilling Through Joists

Drilling through joists is allowed but it does come with certain building regulations. Building regulations in the UK are fairly standard, but you should check with your own council as they do vary slightly from council to council.

If you are doing some pipe work or cabling, you may either have to drill through or cut notches in ceiling or floor joists. The concern for most home owners is that this could weaken the floor and potentially make it structurally unsafe.

However you can do it safely if you follow all the rules. You can find those in Eurocode 5.  We have summarised those below in a table for your convenience, but we would highly recommend reading the regulations in full.

You need to measure the length of the joist, and also the depth of the joist. Joists can be anything between 3-4.5 metres long.

Joist Drilling Table -Lengths

Span of timber

Hole locations between 0.25 of span

Hole locations between 0.4 of span

2000 mm

500

800

2500 mm

625

1000

3000 mm

750

1200

3500 mm

875

1400

4000 mm

1000

1600

4500 mm

1125

1800

Joist Drilling Table -Depths

Depth of timber

Max hole diameter 0.25 of depth

100 mm

25

125 mm

31.3

150 mm

37.5

175 mm

43.8

200 mm

50

225 mm

56.3

250 mm

62.5

275 mm

65

300 mm

65

Drilling Joists to Run Cables

Drilling Joists to Run Pipes

The most common plumbing pipes are 15mm and 22mm. Any holes you drill should be at least 4mm bigger than that. So for 15 mm pipe a 19mm hole needs to be drilled and for 22 mm pipe a 26 mm hole should be drilled.

This allows for easy fitting of the pipes and also allows for contraction and expansion which will happen with hot and cold pipes.

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cordy


I am a retired construction worker and I also happen to love power tools. I have used hundreds over the years and offer my opinions on them here.

Enda McLarnon

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