A middle purlin is a horizontal beam in a roof structure that runs about halfway up the roof slope and supports the rafters. It sits between the wall plate (at the bottom) and the ridge purlin (at the top). It is therefore an intermediate purlin that reduces the span of the rafters.
Function
A roof with long rafters would deflect under its self-weight plus snow and wind load. The middle purlin helps to prevent this:
- Shortens rafter span — less deflection
- Distributes loads — onto supporting trusses below
- Provides stiffness — to the roof plane
- Stabilises rafters — prevents lateral movement
Position in the roof
A traditional roof has three horizontal members in the slope:
| Member | Position | Function |
|---|---|---|
| Wall plate | At eaves, on external wall | Lowest rafter bearing |
| Middle purlin | About midway | Intermediate support |
| Ridge purlin | At the top, under ridge tile | Upper rafter bearing |
On larger roofs there may be two middle purlins — a lower one (just above the wall plate) and an upper one (just below the ridge).
Construction
The middle purlin itself is supported by:
- Trusses — bay trusses or roof trusses at fixed centres
- Tenon and mortise — traditional timber joints
- Halving or dovetail joints — in modern carpentry
- Steel brackets — in renovation or new build with steel
The size of a middle purlin depends on span and loading. Rule of thumb for softwood in a traditional roof:
| Span | Section of middle purlin |
|---|---|
| Up to 3 m | 75 × 125 mm |
| 3–4 m | 75 × 175 mm |
| 4–5 m | 75 × 225 mm |
| > 5 m | calculation required |
Application
Middle purlins are mostly used in:
- Pitched roofs — tiled, slate or thatched roofs
- Farm buildings — large barns with timber frames
- Listed roof structures — churches, castles, manors
- Renovation — when converting attics or extending the roof
- Timber-frame construction — modern houses with traditional roofs
Difference from other roof members
- Middle purlin — intermediate horizontal beam in the slope
- Ridge purlin — uppermost purlin, under the ridge
- Wall plate — lowest horizontal member, on the wall
- Rafter — sloping member in the roof plane
- Collar beam — horizontal tie between two opposing rafters
In old timber-frame barns the middle purlin sometimes also carries a ceiling joist layer, giving it a double function: purlin and floor beam.
Related terms
- Purlin
- Wall plate
- Ridge
- Rafter
- Roof structure
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