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Roof pitch calculator

Pitch in degrees from the rise and run, or the rise from a pitch you have been given. Then the common rafter length with the eave, the stock length to buy and how many rafters the roof takes.

Roof

What do you know?

mm

mm

mm

m

mm

The answer

Roof pitch 21.8 degrees
Rise 1,200 mm
Rafter length, including the eave 3,716 mm
Stock length to buy 3,900 mm
Rafters 34 rafters
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Three roofs, worked out

Pitch = inverse tangent of rise over run. Rafter = the slope length, scaled out to include the eave. Stock length rounds the rafter up to the next 300 mm, the step structural timber is sold in. Rafter spacing is from your span tables or the truss and frame design, not from this page.

A gable roof from its rise

1,250 mm rise over a 3,000 mm run, 450 mm eaves
Roof Gable
What do you know? The rise
Rise 1250 mm
Run (half the span, or all of it on a skillion) 3000 mm
Eave overhang, measured level 450 mm
Length of the roof along the ridge 10 m
Rafter spacing 600 mm
Rise 1,250 mm
Rafter length, including the eave 3,738 mm
Stock length to buy 3,900 mm
Rafters 34 rafters
Roof pitch 22.6 degrees

A gable roof from its pitch

22.5 degrees over a 4,200 mm run, 600 mm eaves
Roof Gable
What do you know? The pitch
Pitch 22.5 degrees
Run (half the span, or all of it on a skillion) 4200 mm
Eave overhang, measured level 600 mm
Length of the roof along the ridge 14 m
Rafter spacing 600 mm
Rise 1,740 mm
Rafter length, including the eave 5,195 mm
Stock length to buy 5,400 mm
Rafters 48 rafters
Roof pitch 22.5 degrees

Working back from the pitch on the plans to the rise you set out.

A low skillion

300 mm fall over a 3,600 mm span, rafters at 900 mm
Roof Skillion
What do you know? The rise
Rise 300 mm
Run (half the span, or all of it on a skillion) 3600 mm
Eave overhang, measured level 300 mm
Length of the roof along the ridge 6 m
Rafter spacing 900 mm
Rise 300 mm
Rafter length, including the eave 3,914 mm
Stock length to buy 4,200 mm
Rafters 7 rafters
Roof pitch 4.8 degrees

Under 15 degrees the page asks you to check the roofing maker's minimum pitch.

Most roof tiles need a steeper pitch than this. Check the tile or sheet maker's minimum pitch before you build.

Roof pitch questions#

How do I work out roof pitch?
Divide the rise by the run and take the inverse tangent. The run is half the span, measured level. A 1,200 mm rise over a 3,000 mm run is 1,200 / 3,000 = 0.4, and the inverse tangent of 0.4 is 21.8 degrees.
How long is a rafter?
The slope length is the square root of rise squared plus run squared. Add the eave by carrying its level overhang up the same slope. The calculator does both and gives the next 300 mm stock length to buy.
What pitch is a normal Australian roof?
Pitched tiled roofs are commonly between about 20 and 25 degrees, and metal roofs go much lower. Tiles and sheeting each have a minimum pitch set by their maker, so check that before you settle on one.
Is the run half the span?
For a gable roof, yes: the run is from the outside of the wall plate to the centre of the ridge. For a skillion, the run is the whole span.
How many rafters do I need?
Divide the roof length by the rafter spacing, round down, add one for the end, and double it for the two sides of a gable. A 10 m roof at 600 mm centres takes 17 a side, 34 in all. A skillion has one side, so it takes 17.