If you’re planning on building your garden room with a timber frame, then it’s important to make sure you get the right timber for the job.
Most constructional timber is referred to in the trade using imperial measurements, and is typically (approximately) 2 inches thick. The height of the timber will be determined by what the timber will need to support and, in the case of joists1, by the gap (or ‘span’) you are trying to cover. An '8x2' (8" high by 2" thick) joist will be able to span a greater distance and/or support a greater weight than a '4x2'.
The imperial sizes tend to be 'nominal' sizes, meaning their rough sawn size before they are ‘regularised’, which basically means tidied up with the long edges rounded off, to make them more comfortable to handle on site. Depending where you buy your timber from you may find differences in the ‘actual’ sizes. In my own experience of the timber I bought, 4x2s ranged from 94mm to 98mm in height, while the 8x2s varied even more widely, from 192mm to 207mm in height. The width typically lies somewhere between 44mm and 48mm.
With such variety, it makes sense to get your timber at the same time and from the same supplier (at least for each section of the build) which should result in less variance between each length. But also, you ideally want to factor it into your planning and design, especially if you’re keen to grab every mm under whatever overall height constraint you’re working to.
The exact dimensions of the timber also impact the span it can comfortably cover and the load it can support. If you look at the span tables referenced in the Tools and Resources section you’ll see different numbers depending on if your timber is 44m thick or 47mm thick.
While the size of your timber for the base and roof will vary depending on the size of your garden room and how you construct it, wall framing tends to be a bit more straightforward, with 4x2s used most commonly. This typically allows plenty of space for up to 90mm of solid insulation, plus room for cables and wiring. There are exceptions. Walls that will have significant gaps for windows and doors may warrant the use of something sturdier, and for internal studwork where the need for insulation is less you can get away with something less substantial.
When buying your timber you’ll probably see two different types on offer – C16 and C24. These refer to the type of wood (C stands for ‘conifer’ which can include woods such as pine, fir or spruce), and the quality (with the higher number (24) being the stronger, with less knots, etc.). Generally the price difference between the two isn’t massive so I always tend to go with C24 for a belt and braces approach. But that’s not to say C16 shouldn’t be used. As long as it meets the span and load requirements of your garden room then you’re good to go.
It’s worth noting that you’re probably likely to get better quality timber (regardless of whether it is C16 or C24) and better prices at your local timber merchant than the typical DIY chain names such as B&Q and Wickes. You don’t normally have to be in the trade to use timber yards or builders’ merchants, and they are normally delighted to gain new business so find our where yours is and give them a call. They will often include free delivery and may even throw in a discount if you are placing a significant enough order with them. And while it’s not required, most of them will let you go down and pick out the lengths of timber you want if you really want to double check the actual sizes and quality of the stock they have.
1Joists are lengths of timber that support floors or ceilings, being laid horizontally. Lengths of timber that go from floor to ceiling vertically are known as studs.