What is a steekspant?
A steekspant (Dutch) is a simple roof frame made of two sloping rafters, a collar beam and a short vertical post at the ridge (Dutch makelaar). It is used for small spans, such as sheds, barns and the lofts of smaller Dutch houses. There is no exact English equivalent; structurally it is close to a simple collar-tied rafter pair with a post at the apex.
How it works
Purlins rest on the frames and carry the roof boarding. The frame counts as simple because it is built with basic means, the span is short and there is little usable space underneath: there is no knee wall, the collar sits low and the rafters are fairly shallow.
The sloping rafters push outwards at their feet (horizontal thrust). In a steekspant the collar and the joists of the loft floor take this thrust. The floor joists act as a tie between the rafter feet.
Application
- Leave the collar in place. To gain headroom it is tempting to remove or raise it. Without a replacement structure the rafters can sag and push the walls out.
- Be careful with openings in the floor. A stair opening through the loft joists breaks the tie locally. Have a structural engineer decide how to compensate.
- Check the rafter feet. Rot or loose joints between rafter and joist are weak points, especially after leaks at the eaves.
- Get calculations for alterations. A dormer, a heavier roof covering or a loft conversion changes the loads. A structural engineer should check this.
Common mistake: shortening or cutting through collars during a loft conversion to fit boards more easily. That weakens the whole roof structure.
Related terms
- Collar beam
- Rafter
- Purlin
- Horizontal thrust
- Roof truss
- Stacking truss
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