Roof Load Distribution

Roof uniform load distribution these examples use an assumed load of 40 psf.

Roof load distribution.

A clay tiled roof may have a dead load. The load increases with the use of heavier roofing material. This means that one is a fixed node and the other is a rolling node. The load bearing capacity of a roof should be adequate to support not only the roof cladding materials such as tiles or shingles but also objects that sit on top of the roof like antennae satellite dishes air conditioning units solar hot water systems and solar photovoltaic panels.

Remember these loads are distributed uniformly over the entire surface of the roof. You will be designing with two sets of loads. However in my practice i typically use the horizontal run of the roof for both types of load. The live load appropriate to your locale is specified by your building official.

In order for a roof truss load to be stable you need to assign two of your nodes on each truss to be support nodes. To do this i use conservative too heavy dead loads and full snow loads regardless of pitch. Here each square foot of roof system delivers 50 pounds of live load and 15 pounds of dead load 65 psf total to the structural support system. A fixed node will provide support in both directions down the length of the roof truss members often called the x and y directions.

Technically you should use the actual rafter length when adding up the weight of roofing materials.