Ceiling joists are the timber members which span across the width of a property. Their purpose is to provide the structural support to the ceiling. Also in properties with upper floors, the joists will support the floorboards and everything in the rooms above them.
If you are trying to find ceiling joists, it can be useful to know how far apart the ceiling joists are. Once you know this, only need to find one joist. Following this, then the others should be easy to find.
A few reasons you might want to find joists include:
- Fixing something to the ceiling
- Fitting lights or other fixtures and fittings
- Over boarding
- Creating a loft hatch
- Etc
In the UK, the most common spacing for joists in residential properties, is 400mm centres. This means, the centre of the first joist will be 400mm from one of the walls. Every joist after this, will be spaced at 400mm.
Less frequently, you may come across joists spaced at 450mm centres. Then finally, in some instances, you will find joists spaced at 600mm centres. However, this is more common in engineered joists, such as I joists, and open web joists.
Why are joists set at 400mm centres?
There are two main reasons that joists are set at 400mm centres.
- Convenience for fixing the ceiling – Standard plasterboards are 1200mm x 2400mm. This means the edge of your plasterboard will land on the middle of a joist. Basically, it wont need cutting to make it fit to the joists. This size board would also fit nicely on 600mm centres.
If you have joists at 450mm, you can buy different boards that will fit this spacing. These boards usually have a width of 900mm. For example 1800mm x 900mm. - Regulations – The size and spacing of the joist will determine how far they can span. This takes into account the load from above and the risk of issue, such as sagging or even ceiling collapse. The same sized timbers can span further when spaced at 400mm. For example, if we take a ceiling made with 2×8 inch joists, the maximum span at 400mm centres, would be approximately 4 meters.
If your joists were spaced at 600mm centres, they would be able to span slightly less than 3.4 meters. This means, 400mm centres, will allow joists of this size to span nearly 20% further.
These regulations apply to standard timber joists. Engineered joists, such as I joists, have different regulations based on their span and depth.
See the chart below for an example of timber joist regulations by joist size.
This table relates to graded timber joists C16 and above.
| Joist size | Max span 400mm centres | Max span 450mm centres | Max span 600mm centres |
|---|---|---|---|
| 38mm x 97mm | 1.72m | 1.56m | 1.21m |
| 38mm x 122mm | 2.37m | 2.22m | 1.76m |
| 38mm x 140mm | 2.72m | 2.59m | 2.17m |
| 38mm x 147mm | 2.85m | 2.71m | 2.33m |
| 38mm x 170mm | 3.28m | 3.1m | 2.69m |
| 38mm x 184mm | 3.53m | 3.33m | 2.9m |
| 38mm x 195mm | 3.72m | 3.52m | 3.06m |
| 38mm x 220mm | 4.16m | 3.93m | 3.42m |
| 38mm x 235mm | 4.43m | 4.18m | 3.64m |
| 47mm x 97mm | 1.92m | 1.82m | 1.46m |
| 47mm x 122mm | 2.55m | 2.45m | 2.09m |
| 47mm x 147mm | 3.06m | 2.95m | 2.61m |
| 47mm x 170mm | 3.53m | 3.4m | 2.99m |
| 47mm x 195mm | 4.04m | 3.89m | 3.39m |
| 47mm x 220mm | 4.55m | 4.35m | 3.79m |
| 50mm x 97mm | 1.98m | 1.87m | 1.54m |
| 50mm x 122mm | 2.6m | 2.5m | 2.19m |
| 50mm x 147mm | 3.13m | 3.01m | 2.69m |
| 50mm x 170mm | 3.61m | 3.47m | 3.08m |
| 50mm x 195mm | 4.13m | 3.97m | 3.5m |
| 50mm x 220mm | 4.64m | 4.47m | 3.91m |
| 63mm x 97mm | 2.19m | 2.08m | 1.82m |
| 63mm x 122mm | 2.81m | 2.7m | 2.45m |
| 63mm x 147mm | 3.37m | 3.24m | 2.95m |
| 63mm x 170mm | 3.89m | 3.74m | 3.4m |
| 63mm x 195mm | 4.44m | 4.28m | 3.9m |
| 63mm x 220mm | 4.91m | 4.77m | 4.37m |
| 75mm x 122mm | 2.97m | 2.86m | 2.6m |
| 75mm x 147mm | 3.56m | 3.43m | 3.13m |
| 75mm x 170mm | 4.11m | 3.96m | 3.61m |
| 75mm x 195mm | 4.68m | 4.52m | 4.13m |
| 75mm x 220mm | 5.11m | 4.97m | 4.64m |
Finding the direction your ceiling joists run.
There are several ways to find ceiling joists. The first thing you will want to do, is determine which direction they are running in.
Generally the joist will span the narrowest distance in the room. Also they will often run in the same direction as your roof rafters.
Another thing you can do is look at your floorboards. The floorboards will run at 90° with the joists (across the joists).
You could also check your attic or basement to see which way joist are running.
Once you determine the direction, it is time to find the joist. This can be quite frustrating. However, you know the joists are most likely 400mm centres. This means, all you need to do is find the first joist.
Finding the first joist
Finding joists is not actually that difficult. Lots of people will recommend a stud finder, but I have personally never owned one and I don’t think they are required. In most cases you can just tap the ceiling to find a joist.
Most of the ceiling will sound hollow, but when you tap on a joist, there is usually a significant difference in the sound. Basically, it won’t sound hollow anymore.
Over the years, this has been my preferred way to find timbers behind plasterboard. Whether it is a stud wall, or ceiling joists, it tends to work great.
With that said, this is not going to work as well on older walls, such as lath and plaster.
Using a magnet to find the distance between joists
Another great way to find the joists, is to find the fixings. In most cases, plasterers and decorators will have done a good job. Therefore, the ceiling will be finished well, and the position of fixings won’t be obvious. However, a good little trick is to use a strong magnet.
The beauty of this technique is the fixings are in the joists, we know this is how the ceiling is fitted. If your magnet finds a fixing, you have found the joist.
Once you find the first joist, measure 400mm and check again with the magnet. If you find another, you know joists are spaced 400mm apart. Now you should have no problem finding every other joist in the ceiling.
Conclusion
In most properties you will find joists spaced at 400mm centres. This is by far the most common spacing. However, you will still find them spaced slightly different in some buildings.
Modern buildings that use engineered joists are more commonly spaced at 600mm. With that said, even engineered joists will often be spaced at 400mm much of the time.
The best way to determine the joist spacing, is to do a little investigation. As we have mentioned, there are several ways to determine the direction and spacing of joists. If you take some time, and follow the steps mentioned above, finding the joists should take no time at all.
