How to Calculate Roof Area

In this article, we’ll explain how to correctly calculate roof area.

These calculations are needed not only for proper roofing work but also for preparing a cost estimate for construction materials. Currently, the most popular roofing materials are asbestos cement sheets, onduline, tiles, metal profile, and metal roofing. The last two types are now the strongest among roofing materials.

Metals roofing belongs to the most durable roofing materials. If the surface layer is not damaged during installation, it can last reliably for 45–55 years. Its weight is significantly lighter than that of asbestos cement sheets. The only drawback of tiles is increased noise during rain or hail. Overall, any of the listed coverings can be chosen for roofing — all these materials are manufactured according to GOST standards, so there's no need to worry about their reliability.

Calculating Area of a Four-Slope Roof

Most articles on roof calculation are overly complicated, as they include sines and cosines. However, with relatively simple roof geometry, you can avoid such complexities, like needing to find sin 30 or 45 degrees during calculations. That’s why we’ve simplified the process as much as possible.

Roofs are either four-slope or two-slope; you must know all dimensions accurately. To make it clear, break down the roof into simple shapes. If it's a four-slope roof, it typically consists of 2 triangles and 2 trapezoids (hip roof) or 4 triangles (pyramidal roof).

The area of an equilateral triangle is calculated using the formula:

S = 0.5ah,

where a is the base of the triangle and h is the height.

We now have the actual area of one side of the roof, S. If it’s a pyramidal roof with equal sides, multiply the area of one side by 4:

S_total = 4S

Rarely do all sides of the building, and thus the lower parts of the roof, match exactly. Therefore, most often you need to calculate each slope using the above formula and then sum up the areas of all triangles:

S_total = S1 + S2 + S3 + S4

Now let’s find the area of a hip roof. It consists of 2 trapezoids and 2 triangles. The area of a trapezoid is:

S = h(a + b)/2,

where b is the base length, a is the top length, and h is the height.

Similarly, calculate the triangle area as in the first method.

To find the total roof area, sum the areas of triangles and trapezoids:

S_total = 2S_triangle + 2S_trapezoid.

In rare cases, you must calculate each triangle and trapezoid separately and then sum the results.

Calculating Area of a Two-Slope Roof

A two-slope roof consists of 2 parallelograms. The simplest way to find its area is:

S_total = 2ab = ab + ab,

where a is the height of the parallelogram and b is its width.

We’ve shown you how simple it is to calculate the actual roof area. However, remember: more material is needed for installation than the formula suggests, because profile sheets overlap. Additionally, a four-slope roof generates more waste.

Some stores have staff who calculate the required amount of metal roofing or other materials. It’s better to trust the calculation to professionals than to return later for additional orders. This service is usually free. All you need to do is provide accurate roof dimensions.