1. Queen Post Truss
Queen post trusses are not common in modern construction but they are still used. The example on the right shows exposed queen post trusses. You can see the purlins above the truss - two on each slope.

2. The primary function of the truss is to support the purlins This is usually because the need for an open space below precludes the use of internal loadbearing walls and struts.
The trusses can also provide support for the ceiling joists which can run from truss to truss, rather than across the load-bearing walls.

3. Trusses typically at 2.4 to 4.0 metre centres.
Joists can span from truss to truss

4. The purlins also span from truss to truss
The purlins also span from truss to truss. Their function is to help carry the rafters. Queen post trusses normally support two purlins on each slope. Typical purlin size is 75mm wide and 175mm deep.

5. As in a simpler roof the rafter ends are supported top and bottom by a ridge board and a wall plate.

6. ridge
purlins
When all these timbers are in position the rafters can be fixed.
Wall plate