What is an improved Dutch truss?
An improved Dutch truss (Dutch: ‘verbeterd Hollands spant’) is a timber truss for a purlin roof, usually combined with a masonry knee wall, in which the roof load is transferred mainly onto the floor structure. This means the roof pushes the outer walls outward less (roof spread), and the attic is taller and more usable than with the ordinary Dutch truss. It is typical of older Dutch houses.
Structure
- Principal rafter: runs continuously from wall plate to ridge and carries the purlins.
- Raking strut (Dutch: ‘kappoot’ or ‘kreupele stijl’): an inclined post that supports the principal rafter and stands at its foot in a sole piece across the floor joists, or sometimes on a joist.
- Sole piece (Dutch: ‘slof’): spreads the load from the strut over several floor joists.
- Collar beam: resists the spreading forces and limits deflection of rafters and purlins, often made as paired boards on either side of the principal rafters.
- King post and braces (not always): tie ridge, rafters and collar together and stiffen the frame.
Difference from the ordinary Dutch truss
In the ordinary Dutch truss, a tie beam at wall-plate level takes up the outward thrust. In the improved version, much of the load goes through the strut and sole piece into the floor, with the knee wall below. There are more purlins, resting on the continuous principal rafters. The type suits fairly steep roofs. A stiffened variant runs the raking strut up between the collar boards: stronger, but with slightly less attic space.
Tips for conversions
- Do not cut members: struts, sole pieces and collars are structural. Removing them for an open loft or a dormer always needs a structural engineer’s calculation.
- The floor carries the roof: weakened or notched floor joists are especially critical here.
- Cracks in the knee wall: if the knee wall is out of plumb or the wall plate is moving outward, have it assessed by an engineer.
Related terms
- Dutch truss
- Purlin roof
- Knee wall
- Collar beam
- Wall plate
- Attic
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