A roof truss is an engineered panel made up of triangular parts. Set out below are the main structural components and fixing points in a standard 'A' type truss.
- Apex
- Apex plate
- Top chord
- Heel plate
- 1/3 point plate
- Bottom chord
- Slice plate
- Heel
- 1/4 point plate
- Web
- Nominal span
- Overhang
Top chord
The top chord performs the job of a rafter in a conventional roof. It carries the tile battens or sheet roofing battens, and is set at the pitch or angle of the roof. In the example above, there are two top chords which meet at the apex.
Bottom chord
The bottom chord is connected at each end to the top chords using heel plates. It main structural function is to stop the top chords from spreading apart when they are under a load. It also serves as aceiling joist, allowing the ceiling lining to be fixed to the underside.
Webs
The webs are the internal members that run between the top and bottom chords. They help to give the truss its strength and rigidity by transferring the stresses in the chords throughout the structure. This is what enables a truss to span the full width of a building using small cross-sectional members.
Joints
The chords and webs are joined at the panel points by nail plates. The plates are galvanised steel sheets that have spikes protruding on one side. When they are pressed into the timber, with one plate on each side of the join, they form a solid fixing that is very strong when the truss is in an upright position.