Example 1
- Rise: 2.8 m
- Half-span: 4.0 m
Roof angle: 34.99°
Slope: 70.0%
This is effectively a 35° roof, which makes it a clear example of the relationship between rise and half-span.
Roof pitch angle calculator
Type half-span and rise to the ridge in the header unit. The calculator computes arctan(rise/half-span) and slope percent. A 2.8 m roof on 4.0 m is 34.99° and 70.0%. A gentler 1.5 m on 3.5 m is 23.20° and 42.86%. A taller 3.2 m on 5.0 m is 32.62° and 64.0%.
A roof pitch angle calculator lets you move quickly from basic roof dimensions to an angle in degrees. It is a useful tool for roof geometry analysis and comparing different pitch options. Related: ridge height, slope percent. Article: roof pitch.
Results
Half-span (m) and Rise / height to ridge (m). The result shows up here.
How the results are calculated
Roof pitch joins rise and half the building width. The formula is arctan(rise divided by half-span). Slope % is that same ratio times 100. Optional rafter length is √(half-span² + rise²). A 4 m half-span means a building about 8 m at a symmetric gable.
A classic 2.8 m rise on a 4.0 m half-span is 34.99° and 70.0%, close to the 35° you paste into ridge height. A gentler 1.5 m on 3.5 m is 23.20° and 42.86%. A taller 3.2 m on 5.0 m is 32.62° and 64.0%. Ridge height at 8 m, 35°, and a 2.8 m wall comes back as 5.60 m.
Type half-span and rise to the ridge, both in the header unit. That is the roof rise alone, not the whole house height with the wall. A comma in 2.8 works like 2,8.
Ridge height from an angle is on the neighbouring page. A 0.6 on 6 m slope is 10% and 5.71°, a deck scale, not a 70% roof. 3-4-5 checks a right angle in plan.
Half-span is half the span, not the full width. Typing 8 instead of 4 with a 2.8 m rise would give about 19.3° instead of 35°. Rafters come out longer than half-span because they run on the slope.
Type 4.0 and 2.8, click Calculate, and match 34.99° and 70.0%. Then 3.5 and 1.5: 23.20° and 42.86°.
How to use this calculator
- Type half-span in the header unit, half the building width, for example 4.0 on an 8 m house.
- Enter rise to the ridge in the header unit, the roof rise alone, for example 2.8.
- Click Calculate. 2.8 on 4.0 m is 34.99° and 70.0%. 1.5 on 3.5 m is 23.20°.
- Ridge height from this angle lives on ridge height: 8 m, 35°, and a 2.8 m wall is 5.60 m.
- A 10% deck slope is on slope percent. This page stays roof pitch.
arctan of a 2.8 m rise on 4.0 m
The calculator computes arctan(rise/half-span) and slope percent. A 2.8 m roof on 4.0 m is 34.99° and 70.0%.
- arctan
- The angle from rise over half-span. 2.8 / 4.0 is 34.99°. A gentler 1.5 on 3.5 m is 23.20°.
- %
- Slope: the same ratio times 100. 2.8 / 4.0 = 70.0%. Not the 10% drain grade on the slope-percent card.
Example scenarios
Example 2
- Rise: 1.5 m
- Half-span: 3.5 m
Roof angle: 23.20°
Slope: 42.86%
A lower rise with a similar base quickly reduces the angle and changes the overall roof proportions.
Example 3
- Rise: 3.2 m
- Half-span: 5.0 m
Roof angle: 32.62°
Slope: 64.0%
This is a good example of why intuition is not always enough and why calculating the angle makes comparing options easier.
Related calculators
FAQ
What angle at 2.8 m rise and 4.0 m half-span?
34.99° and 70.0% slope. Close to the 35° you paste into ridge height.
What angle at 1.5 m on 3.5 m?
23.20° and 42.86% slope. Gentler than the 2.8 on 4.0 m roof, which is about 35°.
What angle at 3.2 m on 5.0 m?
32.62° and 64.0% slope. A taller rise on a wider half-span than the 35° sketch.
Is half-span the full house width?
It is half. An 8 m gable house has a 4 m half-span.
What ridge at 8 m, 35°, and a 2.8 m wall?
5.60 m on the ridge height page. Here you only compute the angle from rise.
How is a 70% roof different from a 10% deck?
70% is about 35°. A 0.6 on 6 m deck is 10% and 5.71° on slope percent.
How do I get rafter length?
√(half-span² + rise²). At 4.0 and 2.8 that is √(16+7.84) ≈ 4.88 m.
Does 3-4-5 give roof pitch?
3-4-5 looks for 90° in plan. Roof pitch is arctan(rise/half-span).
What if rise is empty?
There is no ratio to compute. Both half-span and rise need a number.
Should I round 34.99° to 35° on the ridge page?
Yes, for a sketch. Typing 35° at 8 m and a 2.8 m wall still gives 5.60 m.
Knowledge sources
Volume and waste come from the sizes you type. Below are volume and NIST SI units.
Page updated in 2026.