How to Calculate Roof Truss Loads

A wooden roof truss secured to a wooden housing frame.
What You'll Need
Roof truss members
Construction design software program (optional)

A roof truss is a triangular wood structure that is engineered to hold up much of the weight of the rest of the roof. The straight lengths of wood, known as members, that roof trusses built with are connected with intersections that distribute the weight evenly down the length of each member. Roof trusses are created by attaching the ends of members to joints known as nodes. The load on your roof trusses can be calculated based on the number of members and the number of nodes in the structure.

Sketch Out Truss Structure

This step is recommended to give you a better idea of how all the pieces fit together for the type of truss structure you are building. Note the lengths of your roof truss members on your sketch, and mark where each node will be placed as well.

Designate Node Types

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. This means that one is a fixed node and the other is a rolling node. A fixed node will provide support in both directions down the length of the roof truss members, often called the X and Y-directions. A rolling node is assigned to provide support in only one direction, often the Y-direction of a truss member. The remaining third node of each triangle is known as the load-bearing node.

Calculate Truss Load

The formula for truss loads states that the number of truss members plus three must equal the twice the number of nodes. If the number of members is labeled M and the number of nodes is labeled N, this can be written as M+3=2*N. Both sides of the equation should be equal in order to end up with a stable and secure roof structure.

Use this truss load equation while constructing your roof. You can add or remove nodes and members at any time in order to get the numbers to balance out, similar in concept to balancing both sides of a scale. This step can take some time and patience, but it is worth arriving at a stable roof truss structure in order to avoid integrity problems and costly repairs in the future.

Create a Truss Load Diagram

It is a good idea to fill in the resulting numbers from the truss load calculations on your roof truss sketch from the beginning. This will help you keep track of them while installing each triangular truss and it can be a handy reference for which nodes you have assigned as load-bearing, fixed, and rolling. Alternately, there are now computer software programs that will both calculate your roof truss load and render a diagram of what the end result should be. This is based on the number of members and nodes you enter. The programs will even notify you if needed numbers or elements are missing or do not meet the requirements for your structure.