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Planed Douglas fir rafter
Planed 4-sided solid Douglas fir rafter and joist, untreated, sections 55x35mm and 55x75mm, lengths 200cm and 240cm, sold individually. Smooth 4-sided finish, class 3: natural durability outdoors not in contact with the ground, regular surface suited to neat installations without additional sanding — to be protected according to exposure after fixing. Decking, lightweight framing, flooring — excluding roof structures; section to be chosen according to the span between supports and the loads to be carried.
Joist / Planed Douglas Fir Rafter - Decking and Framing, Sections 55x35mm and 55x75mm
This planed solid Douglas fir rafter / joist is a rectangular-section piece intended for decking joists, floor supports, light interior framing — excluding roof structure. Available in two sections — 55x35mm for light structures and 55x75mm for more demanding spans — and in two lengths: 200cm and 240cm. The piece is delivered rough from sawing and untreated.
The 4-sided planed finish gives a smooth surface on all faces — dimensions are slightly smaller than rough-sawn (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 visible decking or exposed structures. The term "rafter" refers to use in roofing or an inclined structure, "joist" to horizontal use as a support for decking or flooring boards — the piece itself is identical, only the use changes. For interior and exterior 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 durability, exterior above ground |
| Treatment | Untreated - to be protected according to use |
| Use | Decking, framing, flooring - excluding roof structure |
| Sawing | Sold individually |
Key points
- ✓ 2 sections - 55x35mm (light structures) and 55x75mm (more demanding spans)
- ✓ 2 lengths - 200cm and 240cm, on-site cutting possible
- ✓ Planed solid Douglas fir - class 3 - natural durability, exterior above ground
- ✓ Dual use joist / rafter - decking, flooring, light framing excluding roofing
- ✓ Untreated - to be protected according to final exposure
- ✓ Sold individually - order the exact quantity required
Uses and applications
As decking joist: the primary use for these sections. The 55x35mm section is suitable for spacings of 40cm maximum under a 21mm board (DTU 51.4). The 55x75mm section is suited to longer spans between supports (up to 60cm) or thicker boards. For a roof structure, larger sections are required in accordance with DTU 31.1 — consult a carpenter.
As decking joist: horizontal support to which decking boards are fixed. The 55x35mm section is suitable for spacings of 40cm maximum under a 21mm board (DTU 51.4), the 55x75mm section for bearing spans up to 60cm. These sections are suited to joists and light framing — not sized for roof structure (DTU 31.1 required). Allow a gap of 5 to 8mm between joists to allow ventilation under the deck.
For interior framing: partition studs, counter-wall batten, interior cladding support or ground 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 in accordance with best practice. Outdoors, apply a fungicide-insecticide treatment before installation if the species is not naturally durable.
Maintenance and protection
Same recommendations as for rough-sawn outdoors — apply a wood saturator or exterior wood oil before installation, renewed every 2–3 years. Planed surfaces absorb finishing products slightly better than rough-sawn. Greying remains natural without treatment but is more uniform across all 4 faces.
WoodUp tip
For a planed Douglas fir 55x35mm decking joist, maintain a maximum spacing of 40cm under a 21mm board (DTU 51.4). The 55x75mm section is suited to bearing spans up to 60cm. These sections are suited to joists and light framing — not sized for roof structure (DTU 31.1 required). For further guidance on sizing, consult the structural guide.
Before buying: The 4-sided planed piece has dimensions slightly smaller than rough-sawn (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 suitable for light structures: decking joists at spacing ≤ 40cm, light roofing rafters at span ≤ 1.5m, framing 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 consultant.
What is the difference between a rafter and a joist?
It is the same piece of timber — the difference lies solely in the use. As a rafter, the piece is laid at an inclined bearing under a roof covering. As a joist, it is laid flat, horizontally, as a support for decking or flooring boards. The cross-section and length are identical in both cases.
Does this Douglas fir joist need to 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 weather or contact with a damp substrate, 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 protective coat to the cut ends 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
Rough-sawn Douglas fir rafter - same species, sawn dimensions
Planed fir rafter - interior alternative
- Species/material
- Douglas
- Delivery label
- Framework
CHEVDOURAB55752000
976 Products
3420610005382
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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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