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Planed Douglas fir rafter
Planed 4-face solid Douglas fir joist / rafter — sections 55x35mm and 55x75mm, lengths 200cm and 240cm. Class 3. Smooth 4-face finish. Untreated. Sold per piece. Indoor and outdoor.
Planed Douglas Fir Joist / Rafter – Decking and Framing, Sections 55x35mm and 55x75mm
This planed solid Douglas fir joist/rafter is a rectangular section piece intended for decking joists, floor supports, light interior frames — excluding roof framing. Available in two sections — 55x35mm for lighter structures and 55x75mm for more heavily loaded spans — and in two lengths: 200cm and 240cm. The piece is delivered sawn and untreated.
4-face planing gives a smooth surface on all faces — dimensions are slightly smaller than raw (55mm instead of 60mm) due to machining. This finish makes installation easier, reduces the risk of splinters and gives a neater visual result, appreciated for exposed decking or visible structures. The term "rafter" refers to use in roofing or inclined structures, "joist" to horizontal use as a support for decking or flooring boards — the piece itself is identical, only the use changes. Indoor and outdoor use.
Main characteristics
| Characteristic | Value |
|---|---|
| Species | Solid Douglas fir |
| Finish | Planed — untreated |
| Available sections | 55x35mm and 55x75mm |
| Available lengths | 200cm and 240cm |
| Use class | Class 3 — natural resistance, outdoor use excluding ground contact |
| Treatment | Untreated — to be protected according to use |
| Use | Decking, framing, flooring — excluding roof framing |
| Cut | Sold per piece |
Key points
- ✓ 2 sections — 55x35mm (lighter structures) and 55x75mm (more heavily loaded spans)
- ✓ 2 lengths — 200cm and 240cm, can be cut on site
- ✓ Planed solid Douglas fir — class 3 — natural resistance, outdoor use excluding ground contact
- ✓ Dual joist / rafter use — decking, flooring, light frame excluding roofing
- ✓ Untreated — to be protected according to final exposure
- ✓ Sold per piece — order the exact quantity required
Uses and applications
As a decking joist: the main use of these sections. The 55x35mm section is suitable for spacings of 40cm maximum under 21mm boards (DTU 51.4). The 55x75mm section is suited to longer spans between supports (up to 60cm) or thicker boards. For roof framing, larger sections are required according to DTU 31.1 — consult a carpenter.
As a decking joist: horizontal support on which decking boards are fixed. The 55x35mm section is suitable for spacings of 40cm maximum under 21mm boards (DTU 51.4), the 55x75mm section for bearing spans up to 60cm. These sections are suited to joists and light frames — not designed for roof framing (DTU 31.1 required). Allow a gap of 5 to 8mm between joists to allow ventilation under the deck.
As interior framing: partition studs, counter-wall batten, interior cladding support or low floor.
Installation advice
Store pieces flat on a ventilated support, off the ground and sheltered from rain before installation — untreated Douglas fir absorbs moisture quickly. For decking, use A4 stainless steel screws or fixing clips. For framing, prefer bolts or galvanised nails according to best practice. Outdoors, apply a fungicide-insecticide protection before installation if the species is not naturally resistant.
Maintenance and protection
Same recommendations as untreated outdoors — exterior wood saturator or oil before installation, renewed every 2-3 years. Planed surfaces absorb finishing products slightly better than raw. Natural greying occurs without treatment but is more uniform across all 4 faces.
WoodUp tip
For a planed Douglas fir decking joist 55x35mm, observe a maximum spacing of 40cm under 21mm boards (DTU 51.4). The 55x75mm section is suited to bearing spans up to 60cm. These sections are suited to joists and light frames — not designed for roof framing (DTU 31.1 required). For further guidance on sizing, consult the framing guide.
Good to know before buying: 4-face planed pieces have dimensions slightly smaller than raw (55mm instead of 60mm) — this is normal, it is the effect of machining. Take this into account in your installation calculations.
Frequently asked questions
Which section to choose between 55x35mm and 55x75mm?
The 55x35mm section is suited to lighter structures: decking joists at spacing ≤ 40cm, light roofing rafters at span ≤ 1.5m, frame battens. The 55x75mm section is suited to longer spans, heavier loads (heavy tiles, snow) or wider spacings. If in doubt about sizing, consult a carpenter or timber engineering office.
What is the difference between a rafter and a joist?
It is the same piece of timber — the difference is purely in the use. As a rafter, the piece is placed at an angle under a roof covering. As a joist, it is laid flat, horizontally, as a support for decking or flooring boards. The section and length are identical in both cases.
Should this Douglas fir joist be treated before use?
Untreated Douglas fir is naturally resistant to fungi and insects (natural class 3) — it can be used outdoors in sheltered conditions without treatment. For direct exposure to the elements or in contact with a damp surface, apply a saturator or exterior wood oil before installation and renew every 2-3 years.
Can these pieces be cut to the desired length?
Yes. Solid Douglas fir cuts cleanly with a circular saw or mitre saw. Order the length immediately above your requirement (200 or 240cm) and cut on site. Deburr the cut edges and apply a coat of protection on the cuts if used outdoors.
Which joist section for a deck with 21mm boards?
For 21mm thick boards laid on joists, the 55x35mm section with a spacing of 40cm maximum is the standard configuration. For 27mm boards or dense timbers (IPE, thick Douglas fir), the spacing can increase to 50cm. The 55x75mm section is recommended if the spacing exceeds 50cm or if you are laying thick cladding boards.
Complementary products
Raw Douglas fir rafter — same species, sawn dimensions
Planed fir rafter — interior alternative
- Species/material
- Douglas
- Delivery label
- Framework
CHEVDOURAB55752000
1000 Items
3420610005382
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Dispatch within 10 days
Dispatch within 10 days when the product is in stock *.
* Except bespoke products
Dispatched within 6-8 weeks
Dispatched within 6-8 weeks for personalised and made-to-measure products.
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Satisfied or refunded
Satisfied or refunded if returned within 14 calendar days (Sundays and public holidays included) following delivery.
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