The Truss Count calculator computes the number of trusses
needed for a roof based on the ridge length and spacing.
INSTRUCTIONS: Choose units and enter the following:
- (L) Ridge Length (see diagram)
- (oC) onCenter Truss Spacing
Truss or Rafter Count (TC): The calculator computes how many roof trusses are needed.
IMPORTANT NOTE: Always use trusses built to local code requirements with the proper certifications and markings.
The Math / Science
The number of trusses needed is based on the ridge length and the spacing. The formula for the number of trusses is:
TC = (L / oC) + 1
where:
- TC = Truss Count
- L = Ridge Length
- oC = Spacing between the trusses
Unless the ratio of L / oC is within 5% of an integer, the calculator rounds up. 
General Information
The user can enter the on-center figure for the spacing of the truss, but the system has put 24" as the default. An adder of one (1) is added to the truss count if the onCenter spacing does not come out close to even. This means that the spacing with the last truss may be less than the onCenter value, because engineering allows for trusses to be closer together without weakening the roof. Whereas, it is not permissible to space the trusses out further to make up the overage.
Roof Types
