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Single, double, multiple timber roofs; king post truss

Short answer

Single roofs use only common rafters (span ≤5 m). Double roofs add purlins (5–9 m). Multiple roofs use trusses like a king post (central post, tie beam, struts) for >9 m, with a king‑post truss spanning 5–8 m.

Define single/double/multiple timber roofs. Draw neat sketch of king post truss labeling with corresponding elements.

Single, double and multiple timber roofs

  • Single roof: has only common rafters (with or without a tie or collar) and no purlins; used for spans up to about 5 m. Types: lean-to, couple, couple-close and collar beam roofs.
  • Double roof: has purlins in addition to the common rafters; the purlins reduce the span of the rafters. Used for spans of about 5-9 m.
  • Multiple (triple / framed) roof: has trusses (principal rafters) carrying purlins, which carry the common rafters; used for spans over about 9 m. Types of trusses: king post, queen post, mansard and combined trusses.

King post truss

A king post truss is a triangular truss for spans of about 5-8 m, in which a single vertical king post is the central tension member, hanging from the ridge and holding up the tie beam.

                      ridge
                       /\
                      /||\
                     / || \     principal rafter
                    /  ||  \
      purlin o     /   ||   \     o purlin
                  / \  ||  / \
           strut /   \ || /   \ strut
                /     \||/     \
       wall    +=======++=======+  tie beam
       plate
               || = king post
ElementFunction
Tie beamHorizontal member at the base; resists the outward thrust of the rafters (tension)
Principal raftersInclined members that carry the purlins/common rafters (compression)
King postVertical member at the centre; holds up the tie beam from the ridge (tension)
StrutsInclined members from the foot of the king post to the rafters; reduce bending of the rafters (compression)
PurlinsHorizontal members over the principal rafters; carry the common rafters
Common raftersSupport the battens/boarding and transfer the load to the purlins
CleatsSmall blocks that stop the purlins from sliding
Wall plateDistributes the truss load over the wall
Straps/stirrups (iron)Metal straps connecting the king post and tie beam

The king post truss is suitable for spans of 5-8 m.

✓ Answer checked by Claude Opus 5.5 (high effort)

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