If you are trying to measure a roof area of your house and it has a simple gable roof, then you can simply multiply the distance between ridge line and eaves line by the distance of either ridge or eaves line. However, if you have more than two roof planes and if your roof has valleys, dormers and/or covered porches, calculating your roof becomes more complicated.

In this article, we cover the basics of taking reliable roof measurements to ensure that you order a correct amount of metal roofing material.

It is important to remember that your roof area is not the same as the living area of your house, as the roof is sloped, usually has 18”-24” overhangs and sometimes attached garage, which has to be taken into account.

Steps to Measure a Roof

Draw An Aerial View Of Your Roof

The first step is to draw an aerial view of your roof – what a bird might see if it flew over your house.

It doesn’t have to be anything complicated – something as simple as this will suffice:

Take All The Necessary Measurements

You’ll notice that we’ve outlined four key measurements here. You can make as many as you like, but you need enough to characterize your roof’s layout, so you don’t have to go back and take more measurements later.

Notice that the distance to the top of the roof is 5 feet from the side, but only 8 feet when measured all the way across. That’s because the roof is sloped. We’ve picked an arbitrary slope to make the numbers round and easy to deal with.

Calculate The Area Of Each Section

We’ve divided the roof into four sections. The next task is to calculate their areas.

Let’s take the two triangles at the top and bottom first – coloured red. The formula for the area of a triangle is half base times height. The base is 8’, and the height is 9. Plugging that into the formula gives 0.5 x 8’ x 9’ = 36 square feet.

We have two triangles, so the total area is 36 sq ft x 2 = 72 sq ft.

Now for the other two sections – the trapezoids coloured in red on the roof diagram below.

This is relatively easy too. We start with the two triangles indicated by the dashed lines.

Again, we use the formula for triangles: half base times height.

The base of the triangle is half of 16’-1’, which is 7.5’. The area of the triangle, therefore, is 0.5 x 7.5’ x 5’ = 18.75 sq ft.

We’ve got two triangles, so we multiply by two to give 37.5 sq ft. We then just add the remaining square area, which is 1’ x 5’ = 5 sq ft.

The area of the trapezoid is, therefore, 37.5 + 5= 42.5 sq ft. We have two of them, so we have a total of 85 sq ft.

We can now calculate the entire area of the roof.

  1. Triangular roof slopes =  72 sq ft
  2. Trapezoidal roof slopes = 85 sq ft
  3. Total = 157 sq ft

You will need to measure your roof in both feet and inches to get an accurate measurement of your total roof area. Multiplying with inches is difficult, so you may find it easier to simply convert to feet with decimal place, when you perform the calculations. 

Measure Additional Structures

You can follow the general principles we discussed above to measure any additional structures around your home, like your garage or shed.


Measuring a roof for metal roofing isn’t necessarily simple, especially if you have more than two roof planes. If you’re having trouble, get in touch with our team at AMT Metal Roofing. We have years of experience installing metal roofing on residential and commercial properties, and we are here to help you. We aim for 100 percent customer satisfaction on every job. Try us today.