Joists are an integral (albeit hidden) part of your deck’s framework; joists are the beams that support the deck boards and give the structure its shape. When building decking frames, the most important factor to consider is what type of timber to use for the joists. The most essential step when selecting your decking frame materials is to verify local building codes; a suitable frame is imperative to the durability and sturdiness of your project.
Best timber for deck frame
Choosing timber for your decking project comes down to a trifecta of importance: budget, strength, and quality.
Most people are happy to use softwood-treated timber but some people get creative with their deck framing, seeing as there as so many species to choose from – cedar, redwood, douglas fir and spruce, to name a few. Whilst pressure-treated softwood tends to be the favoured material to build decking, if your budget allows, you can digress from the softwood division and opt for hardwood such as oak for your decking areas.
As a rule of thumb, though, pressure-treated wood is the way to go. We advise against using untreated softwood timber varieties in decking frames; untreated softwoods won’t get you far in terms of longevity and the overall preservation of your deck.
Using pressure-treated timber for your decking frame
Pressure-treated timber is a popular choice for timber frames due to its affordability and resistance to insect attack, moisture and rot. Unlike untreated wood and most softwood timbers (that lack natural resistance to the elements), it is infused with chemicals that endow it with impeccable rot and moisture resistance.
On the topic of pressure-treated timber, it should be noted that while most UK websites might throw around Americanisms like ‘pressure-treated lumber’, here in the UK we simply refer to it as ‘timber’ or ‘wood’. The irony is that some species are indeed imported from America.
Best size timber for decking joists
Once you have decided on the type of timber for decking joists, it is time to select the size.
Commonly used joist sizes are 4-by-2, 6-by-2 and 8-by-2, the size that you use will depend on the width of your deck area and how often you can support the joists down to the floor. Depending on your construction you should use a guide like this to calculate the clear span of the timber you are using and the number of support posts required.
There is a general guide for the span of each joist to support your decking. The following indicates the distance between the joist centres to provide suitable support for your decking.
| Decking board size (Thickness x width mm) | Joist span centres |
| 20mm x 95mm | 300mm |
| 20mm x 120mm | 400mm |
| 27mm x 144mm | 500mm |
| 32mm x 120mm | 600mm |
Installing additional decking boards to your joists may allow you to select a smaller size of timber, saving on space and cost.
Once laid, you can get to backfilling around the frame with gravel or pea shingle.
We would advise that you always consult a professional before undertaking any construction project to ensure that your deck is built safely and correctly.
DIY decking
If you’re looking to construct your own decking area then my full guide will help you with the construction. Generally, C24 timber is the highest grade timber for your frame, which might not be required if you are building on a flat surface. C24 timber is normally free of knots and other imperfections and will be stronger than C16 timber.
Decking can transform your outdoor space but should be maintained regularly to ensure it lasts. Here is the ultimate decking maintenance guide to help you understand the importance of looking after your decking wood and treating it properly. For just an hour a year, you can add years to the life of your decking.