The Purlins for a Gable 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.
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.
Gable Roof Calculators
- Gable Roof Area: This computes the surface area of a gable roof based on dimensions and pitch.
- Trusses Needed for Roof: This computes the number trusses need for a roof including gabled roof.
- 4x8 Sheathing for Gable Roof: This computes the total surface area of a roof, the number of 4x8s and framing nails needed for a gabled roof based on the length, span, pitch and overhang.
- Gable Roof Sheathing Cost: This computes the number of 4x8 sheets (32 ft² each) needed to cover a gabled roof based on the ridge length, roof span, overhang and the pitch and applies the unit cost of a 4x8.
- Nails for Gable Roof Sheathing: This computes the approximate number of nails needed to nail 4x8 sheets to trusses or rafters on a roof based on the dimensions of the roof, spacing of nails on the edges and field of the 4x8, and whether blocking or clips are use.
- Purlins for Gable Roof: This 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.
- Metal for Gable Roof: This estimates the metal panels and trim needed for a metal roof for a simple gabled roof based on the roof dimensions.
- Metal Screws for Gable Roof: This computes the number of special screws needed for a metal roof.
- Shingles for Gable Roof: This computes the number of shingles (bundles), underlayment, ridge shingles, starter strip, and roofing nails for a gable roof.
- Gable Roof Shingle Cost: This estimates to shingle a basic gable roof based on dimensions (length, span, pitch and overhang) and the unit price of a bundle of shingles.
- Underlayment for Gable Roof: This estimates the cost of underlayment to cover a basic gable roof based on the area of a roof computed from the dimensions (length, span, pitch and overhang) and the unit price underlayment material.
- Roofing Nails for Gable Roof: This estimates the number of roofing nails needed for a common gabled roof based on roof dimensions and wind conditions.
- Rafters for Gable Roof: This computes the total number of rafter boards (roof joists) for a gabled roof based on the ridge length, pitch and span. It also returns the length of the ridge board, and the length and number of rafter ties and collar ties.
- Rafter Length: This computes the length of the rafters based on the span, pitch and overhang.
- Gable Roof Geometries: This computes the length of rafters, pitch and cut angles, soffit and facia base widths of a gable roof.
- Weight of Snow on Roof: This computes the weight of snow on a gable roof based on roof dimensions, depth and type of snow.
Roof Types
Roofing Calculators
Pole Barn Calculator
The Pole Barn Calculator contains functions to compute the construction materials for a pole barn with concrete piers, a slab, wood framing, roof trusses for a normal gabled roof and metal panels and trim for the roof and siding.
- Gable Roof Metal: This estimates the materials needed for a metal roof for a simple gabled roof based on the roof dimensions.
- Purlins for a Roof: This 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.
- Trusses Needed for Roof: This computes the number of trusses needed for a roof based on the ridge length and spacing.
- Metal Siding for Gable Ends: This computes the metal siding materials (vertical panels) for a wall under a gable roof end based on the dimensions.
- Metal Siding for Gable End with Door: This computes the metal siding materials for a wall under a gable roof end based on the dimensions.
- Girts for a Building: This computes the number of rows of girts and their lengths for a building.
- Wall Beams for a Building: This computes the lengths beams for the outer wall of a building.
- Posts for a Building: This computes the number of vertical posts for a building based on dimensions and spacing.
- Rebar and Concrete in a Slab: This computes the total length and weight of rebar and the volume of concrete needed for a slab.
- Piers for a Building: This computes the number of piers for a building and the amount of concrete needed.
- Pole Barn Materials: This computes the top to bottom framing and roofing materials needed for a pole barn (see picture) based on a few dimension.
- Pole Barn Metal Roof and Siding Cost: This uses an estimated the linear feet of metal panels needed for a pole barn with a door way, and provides an estimated cost for the metal panels for the roof and siding based on a the dimension and cost per linear foot of panels.
- Corrugated Metal Paint or Coating Volume: This estimates the volume of paint or other coating (e.g., spray insulation) is needed for a corrugated surface.
IMPORTANT NOTE: Confirm all calculations with authoritative sources such as architectural drawings before risking health or wealth on the results of any calculator, and ALWAYS adhere to government approved building codes.