Truss Design: The Core Principles
Every roof truss, regardless of shape, is built on one engineering principle: triangulation. A triangle is the only polygon that cannot change shape without changing the length of a side, which is why every truss — from a simple king post to a complex Fan truss — is built from a network of triangles.
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Triangulation, Load Path, and Member Force
A truss carries roof load down to its two end supports through a defined load path: the top chord picks up roofing weight and transfers it to the nearest joint, web members carry that force diagonally across the truss in tension or compression, and the bottom chord ties the whole assembly together, resisting the outward thrust that would otherwise push the walls apart.
Every member in a well-designed truss carries either pure tension (being pulled apart) or pure compression (being pushed together) — almost never bending — which is what makes a truss so material-efficient compared to a solid beam of the same span. This is also why the joints matter as much as the members: a gusset plate must transfer the full force between every member meeting at that point.
Different truss families (King Post, Fink, Howe, Pratt, Fan — see the full truss types guide) are really just different solutions to the same problem: how to triangulate a given span and load with the fewest, most efficient members. Longer spans and heavier loads generally need more triangulation (more web members), which is why truss complexity increases with span.
Frequently Asked Questions
Why are trusses made of triangles?
A triangle is geometrically rigid — it cannot change shape unless a side changes length — while a square or rectangle can rack into a parallelogram under load. Building a truss entirely from triangles keeps the whole structure rigid without needing every member to resist bending.
What is a load path in truss design?
A load path is the route roof load takes from where it lands (the top chord) down through web members to the two end supports. Tracing the load path is how an engineer determines whether each individual member is in tension or compression.