What is a decking joist (onderregel)?
A decking joist is a horizontal timber that acts as a support structure beneath decking boards, floor planks or other horizontal cladding. In a deck, the joists run perpendicular to the decking boards and form the frame onto which the boards are screwed. The term is also used for the bottom horizontal rail in a timber frame wall.
Decking joists in a deck structure
The decking joists form the support framework of a deck:
- Bearers — the main beams running from support point to support point
- Decking joists — the cross timbers on the bearers, on which the decking boards rest
- Decking boards — the visible boards that you walk on
The decking joists lie across the bearers and are fixed through them. The decking boards sit on top of the joists.
Dimensions and spacing
| Decking board thickness | Joist spacing (c/c) | Joist size |
|---|---|---|
| 21 mm | Max. 400 mm | 40 × 60 mm |
| 25 mm | Max. 500 mm | 45 × 70 mm |
| 28 mm | Max. 500–600 mm | 45 × 70 mm |
Timber species
The decking joist is hidden and in permanent contact with the structure. Choose:
- Pressure-treated softwood — most widely used, good value for money
- Hardwood — longest service life, but more expensive
- Aluminium profiles — maintenance-free, no rot
> Never use untreated softwood as a decking joist outside — it will rot within a few years.
Decking joists in timber frame construction
In timber frame construction the bottom rail is the lower horizontal member of the wall frame:
- Fixed to the foundation or floor
- The studs stand on top of it
- Must be separated from concrete by a damp-proof strip (lead flashing or DPC membrane)
Related terms
- Decking board
- Bearer / joist
- On-centre (o.c.)
- Timber frame construction
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