The Purlins for a Shed Roof calculator computes the total number and length of purlins for a single sloped roof based on the span, ridge length and the front and back overhangs.
INSTRUCTIONS: Choose units and enter the following:
- (L) Ridge Length (include any rakes)
- (S) Span
- (P) Pitch (choose: flat, 2/12, 4/12, 6/12, 8/12, 10/12, 12/12, 14/12)
- (oH) Overhang (default: 12")
- (PoC) Purlin On-center spacing (default: 24")
Purlins for Shed Roof (RP): The calculator 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). Note: a safety margin of 3" is added to the length of the purlins.
The Math / Science
The Purlins for a Shed Roof calculator is meant to help with material estimates for a metal roof and siding. Purlins are typically 2"x4"x12' boards used under metal roofs. They are typically untreated wood since moisture should be blocked by the roofing material.
The length of the roof and back wall purlins are length of the building. The length of the side purlins are width of the building. The number of the roof purlins are the rafter length divided by purlin spacing plus one. The rafter length uses the pitch (rise over run) and the span, which is the width plus the two overhangs (front and back).
In all cases, confirm your estimates with a professional estimate and adhering to local building codes.
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.