C24 Treated Timber

What C24 treated timber is

C24 treated timber is structural softwood that has been strength graded to the C24 class and then pressure treated with a wood preservative. The grade tells you what a piece can carry, and the treatment tells you how long it will last where you put it. A deck frame or a sole plate on a slab needs both, and this range gives you both.

Every piece is kiln dried to below 22% moisture content before it goes into the treatment vessel. Preservative is then driven in under vacuum pressure, so in pressure treated timber the protection runs into the section instead of sitting on the face like a brushed coat. It protects against fungal decay and insect attack. It does not make the timber stronger, stop it getting wet or stop it greying.

Treated timber is the wrong choice for dry work inside a heated building. Joists, rafters and studs behind plasterboard gain nothing from preservative, and you would be paying for it and then waiting for it to dry.

C24 treated timber sizes

Treated C24 comes in 13 sizes, the same as our sawn range, and each is still sold by its old imperial name. A 3×2 is roughly 3 inches by 2 inches, and what you receive is 75×44 mm, because the name is a rounded trade size rather than a measurement. Pick the size by the job first.

Deck joists, sole plates and studs

The 2 inch sections, 44 mm thick, do the structural work outdoors and in damp positions. Depth is what carries a span, so each step up in width lets a joist reach further between supports.

• 3×2 (75×44 mm): sole plates on slabs, deck bearers close to the ground and noggins, see 3×2 treated timber
• 4×2 (100×44 mm): external stud walls, garage framing and sub-floor framing
• 6×2 (150×44 mm): the section most decks in Ireland are built on, usually at 400 mm centres, see 6×2 treated deck joists
• 7×2 (175×44 mm): raised decks, longer deck spans and floors over unheated voids
• 9×2 (225×44 mm): the longest spans and wide deck frames, where one piece across the span saves a splice

Posts, beams and ledgers

At 75 mm thick, the 3 inch sections work as posts and beams. The extra thickness resists the twisting an exposed member suffers as it wets and dries through the year. If you need a square section instead, see our timber posts and beams.

• 4×3 (100×75 mm): pergola and carport legs, gate posts and corner posts on garden structures, see 4×3 treated posts
• 6×3 (150×75 mm): pergola and carport beams, deck ledgers and exposed purlins
• 7×3 (175×75 mm) and 9×3 (225×75 mm): main deck bearers, ridge beams and long carport spans

Battens, rails and boards

The 1 inch and 1.5 inch sections, 22 mm and 35 mm thick, are the treated battens and boards that sit on the wet side of a build. On a long rail or batten run, a longer piece means fewer joints to support and seal.

• 4×1.5 (100×35 mm) and 4.5×1.5 (115×35 mm): shed, workshop and garden room stud walls
• 4×1 (100×22 mm) and 9×1 (225×22 mm): fascia and soffit grounds, gable ladders, outbuilding linings and reusable formwork

C24 treated timber vs sawn C24

Both ranges are graded to C24 under EN 338, the European standard for timber strength, and treatment happens after grading without changing the structural properties. A treated 6×2 spans exactly what an untreated 6×2 spans, so when a drawing calls for C24 in a damp position, you are not trading strength for protection.

The choice comes down to whether the timber will ever stay damp long enough for decay to start. Choose treated for:

• Outdoor structures: deck frames, pergolas, carports and fence rails
• Masonry and ground contact: sole plates and sill plates on a slab or blockwork, where damp travels up into the timber
• Unheated buildings: sheds, garages, workshops and garden rooms, where condensation forms on the inside of the sheathing
• Behind cladding and roof coverings: counter battens, tile battens and fascia grounds

For joists, rafters and studs inside a heated, dry house, untreated C24 timber is the right specification and the cheaper one. Where a member runs from inside to outside, treat the whole length as external.

Working with treated timber

Pressure treatment protects the body of the timber, but site habits decide whether it lasts. Most failures start at a cut end, a corroded fixing or a post standing in water.

Cut ends, holes and masonry

Preservative goes in from the outside, so every saw cut, notch and drilled hole exposes timber the treatment never reached. Brush every cut end with an end grain preservative before fixing, especially ends in a post shoe or against masonry. Treat the inside of each hole, and drill rather than notch where services cross a joist. Keep a DPC (damp proof course) between the timber and any concrete or blockwork, because standing water will beat a preservative given long enough. Stand posts on a galvanised post base rather than setting them straight into concrete, which traps water around the foot.

Fixings for treated timber

The preservatives used on treated timber are copper based, and copper corrodes plain and zinc plated steel. Use hot dip galvanised fixings as the minimum, and stainless steel on anything permanently wet or holding cladding you do not intend to remove. Joist hangers, straps and post bases need to match the timber they are fixed into.

Drying, storing and finishing

Freshly treated timber can arrive damp and heavier than the same section dry, and it may shrink a little as it dries. Stack it off the ground on bearers with air moving through the pack. The green tint fades through the first months outdoors. Treatment stops rot, not weathering, so exposed timber will still grey: once it has dried, a coat from our exterior wood oils and stains keeps it looking right.

C24 treated timber in Dublin and across Ireland

Every size on this page is held in our North Dublin warehouse, ready for collection or delivery across Ireland. Pick the section from the grid, choose the length on the product page and add it to your cart. Treated C24 sits alongside the sawn range in our construction timber collection.