Roof Pitch and Rafter Length Calculator: Rise, Slope Length, Roof Area and Pitch Converter

Get the length of the rafters, the rise and the roof area from the width of the building and the pitch.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose a gable or a skillion roof and enter the width and length of the building, the pitch and the overhang of the eaves.

The calculator finds the run, the rise to the ridge, the length of the rafter along the slope to the end of the eave, and the area of the roof to be covered.

It also gives the pitch in degrees, as a rise in 12, as a ratio and as a percent, and the factor that turns the plan area into the roof area.

Formulas used

  • h = r\tan\theta
  • \ell = \frac{r + o}{\cos\theta}
  • A_{roof} = \frac{A_{plan}}{\cos\theta}

Worked example

With the default numbers in the calculator above:

  1. Run: the horizontal distance from the wall to the ridge: 4.00 m
  2. Rise from the top of the wall to the ridge: 1.66 m
  3. Rafter length along the slope, from the ridge to the end of the eave: 4.82 m
  4. Roof area to be covered (including the eave overhang on all sides): 124.27 m2
  5. Roof area compared with the plan area: 1.08 times the area in plan (the roof is bigger than the floor under it)
  6. The pitch in other forms: 22.50 degrees; 4.97 in 12; ratio 1 : 2.41; 41.42 percent

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

Why is the roof area bigger than the floor area?

The roof slopes, so it is longer than its horizontal plan. The area is the plan area divided by the cosine of the pitch.

What is a 4 in 12 roof?

A roof that rises 4 units for each 12 units of run, which is about 18.4 degrees.

What is the minimum pitch for a metal roof?

It depends on the profile and the laps, from about 2 degrees for long-run sheets to 5 degrees or more for corrugated. Check the manufacturer.

Does it include hips and valleys?

No. Those need their own measurements, with the hip and valley lengths longer than the common rafter.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: AS 1684 Residential timber-framed construction; AS 1562.1 Design and installation of sheet roof and wall cladding.

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