Roofing Formula’s Summary
Rise per / m run of common rafter:
Tan 30° = rise per m run
Place your roof pitch here
True length per / m
√ rise per / m² + 1.0²
Centre line length of common rafter
True length per / m
(above answer)
x ½ Span
Order length of common rafter
True length per / m X ( ½ span + eave width + any brick & cavity allowance)
=
_________
=
_________
then add 100mm for plumb cut allowance
Then round up to orderable lengths (increments of 300mm)
Number of Rafters (same for gable & hipped roofs)
Total length of roof
Spacing’s
+1
x 2 sides at end of calculation
Length of Ridge (gable)
Total length of roof including verge overhang
Length of Ridge (Hip)
Length of building – width of building + one rafter thickness
True length of hip per / m run of common rafter
√ rise per / m² + plan length hip² or
√ rise per / m² + 1.414²
Centre Line length of hip
True length of hip / m
(above answer)
x half span
Order Length of Hip
True length of hip / m x (half span + eaves overhang + any allowance for brick & cavity)
=
_________
=
_________
then add 150mm for plumb cut allowance
Then round up to orderable lengths (increments of 300mm)
Number of Hips
Count the number of external corners on plan
Purlins (hip roof)
Length to long side =
plate length + ridge length
2
x 2 sides once you have length
Length of short side =
plate length + ridge length
2
x 2 sides once you have length
Alternatively:
the length of the building x 2 will give you the total lineal / m for purlins