What is a slaper?
A slaper (Dutch, literally “sleeper”) is a load-bearing, usually horizontal beam in a roof structure that picks up the ends of purlins that have been cut through, typically to make room for a dormer. It is often doubled. In UK practice the closest equivalent is a trimmer: a timber or steel member installed to carry the load of members interrupted by an opening.
How it works
In a purlin roof, the rafters rest on purlins that span between trusses, gable walls or posts. When a dormer is added, part of a purlin often has to be removed. The cut ends then lose their support. The slaper runs along the side of the opening and connects the interrupted purlin to a continuous member, such as the purlin above, so the load is redirected into the structure that is still intact.
In Dutch the word has two minor meanings as well: an auxiliary stair string running under the treads, and a decorative scroll piece on the shoulder of a neck gable.
Application
If you are planning a dormer or loft conversion that involves cutting purlins:
- Get calculations first. Cutting a purlin changes how the whole roof carries load. A structural engineer should size the trimmers (often doubled timbers or steel beams) and check the existing timbers.
- Prop before cutting. Temporarily support the purlin and rafters before any saw touches them.
- Fix as designed. Use the hangers, bolts or screws specified in the calculations, not whatever is to hand.
- Check the rules. In England and Wales, Building Regulations Part A (Structure) applies to loft conversions, and building control normally expects structural calculations. Planning rules vary; listed buildings need separate consent.
Common mistake: removing a section of purlin and letting the dormer frame take the load without anyone checking that it can.
Related terms
- Purlin
- Dormer
- Trimmer joist
- Rafter
- Roof truss
- Structural engineer
- Building permit
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