A bottom rail is a horizontal beam that serves as a supporting structure beneath deck boards, floor panels, or other horizontal cladding. On a deck, the bottom rails run perpendicular to the deck boards and form the frame onto which the boards are screwed. The term is also used for the bottom horizontal member in a timber frame wall.
Bottom rail on a deck
The bottom rails form the supporting frame of a deck:
1. Beams — The main members running from support to support
2. Bottom rails — The cross members on the beams, on which the deck boards are laid
3. Deck boards — The visible boards you walk on
The bottom rails sit on the beams and are screwed through them. The deck boards sit on the bottom rails.
Dimensions and spacing
| Deck board thickness | Bottom rail c/c | Bottom rail dimension |
|---|---|---|
| 21 mm | Max. 40 cm | 40 x 60 mm |
| 25 mm | Max. 50 cm | 45 x 70 mm |
| 28 mm | Max. 50-60 cm | 45 x 70 mm |
Timber species
The bottom rail is not visible and has permanent contact with the structure. Choose:
- Pressure-treated softwood — Most commonly used, good value for money
- Hardwood — Longest lifespan, but more expensive
- Aluminium profiles — Maintenance-free, no rot
> Never use untreated softwood as a bottom rail outdoors — it will rot within a few years.
Bottom rail in timber frame construction
In timber frame construction, the bottom rail is the lowest horizontal member of the wall frame:
- Fixed to the foundation or floor
- The studs stand on it
- Must be separated from concrete with a waterproof barrier (lead strip or DPC membrane)
Related terms
- Deck board
- Beam
- Centre-to-centre spacing
- Timber frame construction