The Purlins for a Roof function computes the total number and length of purlins for a simple gable roof based on the ridge length of the roof, the roof pitch and the span of the roof.
INSTRUCTIONS: Choose units and enter the following:
- (L) Ridge Length (Note: this should include rake overhang)
- (S) Roof Span
- (P) Roof Pitch
- (oH) Overhang
- (PoC) Purlin On-center spacing (default: 24")
Purlins for a Roof (RP): The function returns the number and length of purlin rows for the roof. To calculate the number of boards needed for purlins, divide the length of the purlin rows by the length of boards you are using (e.g. two boards of length eight feet for each 16 foot long purlin row).
Purlins and Girts
Purlins are rows of boards nailed horizontally to a roof's frame to act as a base for surface materials such as a metal roof panels. Purlins are usually nailed but sometimes screwed to rafters or trusses and spaced in horizonal rows. The metal roofing panels are then screwed onto the purlins vertically forming the roof up to the ridge cap and down to the drip edge of the overhang.
Girts are like purlins and used on walls to be the base of metal panels for siding. Girts are horizontal boards nailed or screwed to vertical posts or studs, and they are used as the support structure to vertical wall panels such as metal siding. Girts are often 2x6 boards.
Purlin and Girt Calculators
- Purlins for a Gabled Roof
- Purlins for a Gambrel Roof
- Purlins for a Hip Roof
- Purlins for a Pyramid Roof
- Purlins for a Shed Roof
- Purlins and Girts for a Shed
- Girts for a Building
The Math / Science
A gable roof is the most recognized shape of a roof. It has a ridge down the middle and two equal area sloped sides. Roofing material is needed for the sides to prevent water from entering into the building.
The Purlins for a Roof calculator is meant to help with material estimates for a metal roof. Purlins are typically 1"x4"x12' boards, sometimes called furring strips, that are used under metal roofs. They are typically untreated wood since moisture should be blocked by the roofing material. In all cases, confirm your estimates with a professional estimate and adhering to local building codes.
The calculator uses the ridge length for the length of the purlins. The spacing is used with the total rise and run to compute the number of purlins.
Roof Types
Roofing Calculators
Building Calculators
- Area of a Roof
- Pitch of a Roof
- Number of Trusses Needed for a Roof
- 4x8s needed for a roof
- Shingles needed for a roof
- Number and Length of Metal Roof Panels
- Number and Length of Purlins
- Number and Length of Rafters
- Number and Length of Rafter and Collar ties
