Measuring the Surface Area of a Roof

Do you know how to measure the surface area of a simple gable or hip roof? Whether it is for a renovation or just for a building inspection purpose? There is a pretty simple way to do it.

First of all, it is only applicable for a gable typed roof and a hip typed roof. The main difference between this 2 type of roof is that the hip typed roof has angles on every side of the roof (as you can see at the “top view”), whereas the gable type of roof does not. You also need to have the blueprints of the building for easier calculation.

Gable Type :

 Hip Type :

Okay, here is how you measure the surface area of the roof. Simply apply equation (i) : Base area divided by cosinus α, where α is the angle of the roof’s slope. This equation is applicable for both the gable typed and the hip typed roof.

Where :
Abase = Base area of the roof
α = angle of the roof’s slope

Where is the base area and α ? From picture below, the base area is the area in the green lines, so it’s the multiplication of x and y - equation (ii). Practically, the base area can be obtained from the building's softt drawing, or by measuring with a regular measure tape method.

Meanwhile, the data of α angle is a bit difficult to get. If you have blueprints of the building, you may find this data easily. But what if you don’t have the blueprints? Well, I guess you still have to climb the roof, to obtain data about the roof’s height, described in this picture as “z”. Afterwards, you can get the α angle with equation (iii). Always remember not to confuse between α and β. Finally, you can measure the roof's surface area using the equation described on the previous slide.

More Complex Roof

It will be a bit tricky for more complex roof like the one shown below.

There are two roof area (roof I and roof II). To measure surface area of roof I, you can apply the described equation by moving the area in the red triangle to replace the area in the green triangle, so roof I will still remain a full hip roof and you can use the same equation - equation (iv). Roof II contains two parallelogram shape, which could also be considered as a gable typed roof, if you transform the parallelogram shapes into rectangular shapes. Therefore, you can still use the same equation as roof I, or equation (v) in the above picture. Finally, you can measure the whole surface area of the roof by adding the surface area of roof I and the surface area of roof II.

Hopefully this would be useful.