A ridge purlin is the horizontal beam that runs along the highest point of a roof — the ridge — and connects the rafters or trusses at the top. It is the “backbone” of the roof structure: all sloping rafters meet here.
Function
The ridge purlin has two tasks:
1. Absorb forces — The rafters lean against the ridge purlin and transfer part of their load to it
2. Connection — The ridge purlin connects all trusses at the top and keeps them at the correct spacing
In a trussed roof, the ridge purlin rests on the trusses. In a rafter roof (without trusses), the ridge purlin itself must span between the gable walls or intermediate walls.
Ridge purlin vs. wall plate
| Ridge purlin | Wall plate | |
|---|---|---|
| Position | Top (ridge) | Bottom (on the wall) |
| Rafters go | Downward from the ridge purlin | Up to the wall plate |
| Fixing | On trusses or wall supports | On ring beam or wall |
Dimensions
The ridge purlin must be heavier than an intermediate purlin, because forces from two roof surfaces converge here:
| Span (between supports) | Dimension (indicative) |
|---|---|
| Up to 2.5 m | 63 x 175 mm |
| 2.5 – 3.5 m | 75 x 200 mm |
| 3.5 – 5.0 m | 75 x 225 mm |
| Above 5.0 m | 100 x 250 mm or steel |
In self-build projects
For a shed or garage with a gable roof, the ridge purlin is one of the first components you install:
1. Install trusses or gable supports — The ridge purlin rests on these
2. Place the ridge purlin — Check it is level
3. Mount the rafters — On both sides against the ridge purlin
For a garden building or small outbuilding, the ridge purlin is sometimes supported by the two end gables (no intermediate supports needed for short spans).
Related terms
- Purlins
- Purlin
- Wall plate
- Truss
- Rafter
- Ridge capping