How to measure a roof and calculate square feet.

DIY Roofing

To estimate how many shingles you’ll need, first estimate the total square footage of your roof’s surface. To do this, measure the length and width of each plane on the roof, including dormers. Then, multiply length x width to get the square footage of each plane.

Why learn How to Estimate Your Roof?

Let’s face it, making even a relatively small mistake when performing a roofing estimate could put a huge financial hole in your wallet! For instance, being off by just a few roofing squares in the already expensive metal roofing job could cost you thousands. What is even more shocking is that there are many so called professionals in the field, who frequently make mistakes that cost consumers a lot of money. Consider this as one of the many countless examples of roof measuring mistakes made by the roof consultants giving estimates for a new roof to unsuspecting homeowners. Recently, we were providing a roofing estimate for a homeowner who was interested in installing a metal roof on his ranch house. During our conversation we were shocked to find out that a salesman from a competing company was off by as many as seven roofing squares, when he provided a roofing estimate for this homeowner.

Roof measuring mistakes may cost you thousands

It is an undeniable truth that a Roofing project can be a great expense for the home owner. Now, in metal roofing, which typically costs three to four times what the regular roof may cost, a small mistake in estimating roof size can cost us 3 times more than a similar mistake in an estimate for a regular roof. No matter what type of roof you are considering, a precision and accuracy in measuring and estimating your roof size can go along way and save you a lot of money. Let’s go back to the example in previous paragraph, which illustrates the point of making a roof measuring mistake in a metal roofing estimate; Being off by seven roof squares could easily mean that homeowner would pay $7000 extra for his new roof. This could easily be the case, if we have not told him of the true size of his roof. This goes to show how easy a homeowner can be mislead by the unscrupulous quote on quote roof estimating professionals.

Learning Objectives

This easy step “do it yourself roof estimation guide” will show you how to figure out the number of roofing squares on your house, and teach you some basic pricing considerations for the common roof types.

How do I know how much to buy?

The first step to figuring out material estimates is to calculate the dimensions of your roof.  This means measuring the surface area/square footage and finding the slope. To calculate the square footage, multiply the roof’s length by the roof’s height.

If you take this number and divide it by 100, that will give you the number of “squares” on your roof. The same measurements apply when estimating and purchasing underlayment.

For each “square”, it takes three bundles of standard 3-tab asphalt shingles to cover that area.  One standard underlayment (#15 is the most common) will cover four of these “squares”.  Be prepared by adding 10% material to your estimate to factor in waste and other variables.

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For drip edges, just measure the entire length and then again add 10% when purchasing. In the case of corners and cap materials (such as ridge caps), simply measure the length of the ridges and grooves that will need to be covered.

Always get 1-inch nails for normal asphalt roofing.  On average you should use four nails per shingle (more if in a windy area).  This comes out to 320 nails per square foot.