The Purlins for a Gambrel Roof calculator computes the length and number of rows of purlins needed for a gambrel roof based on the dimensions. 
INSTRUCTIONS: Choose units and enter the following:
- (L) Ridge Length
- (S) Span of Roof
- (P1) Lower Pitch
- (P2) Upper Pitch
- (AH) Attic Height
- (oH) Overhang
- (PoC) Purlin Spacing
Gambrel Roof Purlins (nP): The calculator returns length and number of rows of purlins, and the total board length of purlins for a gambrel roof. The lengths are returned in feet can be automatically converted to compatible units via the pull down menu.
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.