A beam is a horizontal load-bearing element that transfers forces across a span to the supports (walls, columns or posts). Beams together with columns form the skeleton of virtually every structure — from a simple canopy to a complete building.
How does a beam work?
A beam bridges an opening. The weight resting on the beam (self-weight, floor, roof, people) is transferred through the beam to the supports. In the process:
- Compression occurs at the top (being squeezed together)
- Tension occurs at the bottom (being stretched)
- Shear force occurs at the supports
The greater the span and the heavier the load, the stronger the beam needs to be.
Beam vs. girder vs. purlin
| Term | Direction / Function |
|---|---|
| Beam | General term for a horizontal load-bearing element |
| Girder | Beam that supports a floor or other beams |
| Purlin | Beam in a roof structure that supports rafters or roof panels |
| Lintel | Beam above a wall opening |
In practice, “beam” is the umbrella term. Girder, purlin and lintel are more specific names.
Materials
Timber beam
The most common in DIY projects:
| Dimension (mm) | Max. span (indication) | Application |
|---|---|---|
| 50 x 150 | approx. 2.5 m | Light decks, small canopy |
| 63 x 175 | approx. 3.5 m | Canopy, garden building |
| 75 x 200 | approx. 4.5 m | Shed, large canopy |
| 75 x 225 | approx. 5.0 m | Garage, large shed |
| 75 x 250 | approx. 5.5 m | Large spans |
> Note: These are indications. The actual span depends on the load, wood species and centre-to-centre spacing.
Steel beam
For large spans or heavy loads:
- IPE profiles — I-shaped, most efficient form
- HEA/HEB profiles — Wider, greater load capacity
- Box sections — For a clean appearance
Steel is often used for garage doors (wide opening) or when removing load-bearing walls.
Concrete beam
In professional construction, precast or cast in situ. Not common in DIY.
Beams in DIY projects
- Canopy — The main beams (purlins) carry the roof from post to post
- Deck — The beams (joists) form the frame onto which the deck boards are screwed
- Garden building/shed — Beams in the floor, walls and roof structure
- Conservatory — Load-bearing beams from the house to the posts
Common mistakes
- Beam too thin for the span — The beam deflects visibly. Rule of thumb: the height of the beam should be at least 1/20 of the span (for a 4-metre span, at least 200 mm high).
- Beam not centred on the support — The load must land centrally on the column or wall.
- Notch in the beam — A notch on the underside (tension zone) weakens the beam enormously. If necessary, make it on the top side and a maximum of 1/3 of the beam depth.
Related terms
- Girder
- Purlins
- Column
- Post
- Centre-to-centre (c/c)