What is a girder?
A girder (or beam) is a horizontal beam that supports floor boards, beams, or other structural elements and transfers the loads to the support points (walls or columns). The difference from an ordinary beam is subtle: a girder often carries other beams or a floor, while “beam” is a more general term.
Girder in practice
A girder is best understood as a main beam: the element that spans the distance and on which other elements rest.
Examples:
- The floor girder in a shed supports the floor joists
- The main girder of a pergola runs from post to post and supports the purlins
- A steel girder above a wall opening takes over the function of the removed wall
Types of girders
Simply supported girder
Rests on two support points (for example, two walls). This is the most common form in DIY projects.
Continuous girder
Runs over multiple support points (for example, three columns). A continuous girder is more efficient: at the same dimensions, it can span more than two separate girders.
Built-in (fixed) girder
Firmly built into or welded to the support points. Stiffer than a simply supported girder, but more complex to construct.
Materials and dimensions
Wood
| Application | Dimensions (mm) | Max. span |
|---|---|---|
| Decking joist | 50 x 150 | ~2.5 m |
| Shed floor girder | 75 x 200 | ~4.0 m |
| Main girder pergola | 75 x 225 | ~5.0 m |
| Main girder large structure | 100 x 250 | ~6.0 m |
Steel
For spans above 5-6 metres or heavy loads, a steel girder (IPE or HEA profile) is often used. Advantage: a steel girder is much slimmer than a timber beam at the same load-bearing capacity.
Glulam (laminated timber)
Glued together from multiple layers of wood. Stronger than solid wood, available in large dimensions. Suitable for exposed beams in pergolas and verandas.
Girder vs. purlin vs. beam
| Term | Supports… | Direction |
|---|---|---|
| Girder | Floor, other beams | Usually the long span |
| Purlin | Roofing, rafters | Parallel to the ridge |
| Beam | General term | Horizontal |
| Lintel | Masonry above an opening | Above door/window |
Girder in DIY projects
- Decking — The girders form the frame on which the deck boards rest. Usually 50×150 or 63×175 mm at a maximum of 50 cm centre-to-centre.
- Pergola/carport — The main girder runs from post to post and supports the purlins or roof panels
- Shed — Floor girders support the timber floor, roof girders form the base of the roof structure
Common mistakes
- Bearing too short — A girder must rest at least 5-10 cm on each support. For wood on a wall: use a joist hanger or steel bracket.
- Deflection not checked — Even if a girder is strong enough, the deflection may be too great. Rule of thumb: maximum deflection is 1/300 of the span (for 3 metres, that means a maximum of 10 mm).
Related terms
- Beam
- Purlins
- Column
- Centre-to-centre (c/c)
- Bottom rail