Portal frame or truss: choosing a roof structure
Two standard answers for a wide clear span, and what usually decides between them.
For a single-storey building with a wide clear span — a warehouse, a workshop, a hall — the structure is usually a portal frame or a truss. They behave differently and suit different requirements.
Portal frame
Columns and rafters joined rigidly, so the frame resists load by bending. Fewer pieces, quick erection, and an uncluttered interior. Economical across a wide range of everyday spans, which is why it is the default for most industrial buildings.
The rigid connections concentrate force at the haunches and at the base, so foundations take moment as well as vertical load. That cost sits underground and is easy to overlook when comparing frames.
Truss
A framework of members in tension and compression, carried on columns. More efficient over long spans and for heavy loads, and the depth of the truss gives a natural route for services and suspended plant.
The cost is in the number of pieces: more members, more connections, more fabrication, and generally a longer erection sequence.
What decides it
- Span — the longer it gets, the more a truss makes sense
- Loads — suspended plant, cranes or heavy services favour a truss
- Services — truss depth is useful where a lot has to be routed at high level
- Foundations — poor ground makes portal frame moment reactions more expensive
- Programme — fewer pieces usually means faster erection
The decision belongs to the structural engineer, but knowing what drives it makes the conversation far more productive.