What is a stacking truss (stapelspant)?
A stacking truss (Dutch stapelspant) is a roof frame of truss-like construction that exerts no outward thrust on the walls. The frame sits on the walls as a single unit and passes on only vertical loads. The walls therefore do not need to resist sideways pressure, and no tie rod is needed to hold them together. The downside is that the structure takes up a lot of room: the usable space under the roof is small.
A well-known variant is the Japanese-style stacked roof frame, in which horizontal beams and short posts are stacked layer by layer into a pitched roof shape. This gives a little more usable space and a distinctive, open timber look. Metal stacking trusses also exist.
Why no thrust matters
In a simple couple roof, the sloping rafters push outwards at the bottom. That thrust must be taken by a tie beam, a collar at the right height or heavy walls. Without it, the roof pushes the walls out of plumb. A stacking truss avoids this, which makes it suitable for buildings with light or slender walls.
Application
- Choose it for light walls. On timber frame, slender walls or a storey added on top of an existing building, it prevents the walls being pushed outwards.
- Expect little loft space. If you want to use the roof space, another truss type with a tie beam or portal frame is usually more practical.
- Have it designed. Member sizes, connections and timber grade follow from span, roof load and wind load. A structural engineer or truss supplier designs this.
- Provide longitudinal bracing. A stacking truss also needs purlins and bracing so the trusses do not fall over like dominoes.
Common mistake: removing or shortening members to gain headroom. Each member has a role in the whole; without calculation the truss may sag or spread.
Related terms
- Couple roof
- Collar beam
- Purlins
- Horizontal thrust
- Roof truss
- Portal frame
Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.