Douglas Fir Joists

Douglas Fir Joists for Sale in Ireland

Douglas Fir is the softwood people specify when a frame has to be strong and look good at the same time. It is used around the world for structural beams, posts and joists, and its straight grain, strong knot pattern and warm reddish pink tone give it a presence that pine and spruce don’t have. That makes Douglas Fir joists a good choice wherever the frame stays on show, such as an open garden structure, a raised walkway or the underside of a high deck.

Every joist is sawn, so it has a rougher, more rustic face than a planed board.

Douglas Fir joist sizes

The range splits into standard and heavy joists, so choose by the load and the span. The standard joists are 50 mm thick. Use 70×50 mm for light framing, bearers on a firm base and small low decks, 100×50 mm as a light deck joist for smaller garden decks, and 150×50 mm, the everyday deck joist for most garden decks.

The heavy joists are 70 mm thick, for heavier frames. The 175×70 mm section is the first step up, 200×70 mm suits longer spans, raised decks and walkways, and 220×70 mm is the deepest section in the range, for the heaviest frames.

The span any joist can carry depends on its size, the spacing and the load, so have an engineer or a span table confirm the size before you build. Where you can, pick a length that runs the full width of the deck, so there are no joints to support. The calculator on each product page works out how many joists you need.

Why choose Douglas Fir joists?

With a density of around 530 kg/m³ and an excellent strength to weight ratio, Douglas Fir is the softwood structural engineers reach for when timber has to do real work where it can be seen. It is Durability Class 3, with a service life of 15 to 25 years outdoors when properly detailed, and its natural resin adds weather resistance.

Joist spacing

For a residential timber deck, space Douglas Fir joists at a maximum of 400 mm centres. For heavier loads, close them up to 300 mm centres or less. Give the frame a slight fall so water runs off.

Douglas Fir vs other timber joists

For a lighter kiln dried softwood, see Ecowood joists. For a hardwood frame, oak joists come in the same heavy 70 mm thickness, and hardwood joists come planed all over. To build the whole structure in one species, add Douglas Fir beams and posts and finish with timber decking on top.

Fixing and looking after Douglas Fir joists

A few habits keep the frame sound:

• Seal every cut and notch with end grain preservative before fitting.
• Fix with stainless steel hangers and fixings to avoid corrosion staining. See our timber connectors.
• Pre drill near the ends to stop the timber splitting.

Douglas Fir doesn’t need treating to perform, but exposed joists weather to silver grey. Where the frame is on show, most customers oil it to keep the reddish pink tone, using a UV protective oil from our exterior wood oils and refreshing it every two or three years.

See every species in our timber joists range.