A truss is a part of a roof that together with the purlins and rafters supports the strength of the roof. Truss is also the steel or wooden frame between walls and the ridge. A truss consists of a truss leg and different parts of mounts.
The maximum weight a roof has to be able to sustain can be calculated by adding the weight of snow, wind and rain by each other. This weight and the weight of the construction will be sustained by the construction below (walls).