Check out the estimation recommendations below and some additional roofing resources if you’ve made the decision to handle the project on your own:


You need to know the total square footage of your roof’s surface in order to determine how much roofing material, such as bundles of architectural shingles or rolls of synthetic roofing underlayment, you’ll need.

How to Determine Your Roof’s Total Square Footage

To determine the total square footage of your roof:

  • The length times the breadth of each plane of the roof, including dormers, should be multiplied.
  • The combined square footage of all the planes.

For instance, a shed roof with a single roof plane

This shed roof, for instance, has a single roof plane.

  • Add width (B) to the product of length (A):

The overall square footage of the roof is A x B = 40′ x 30′ = 1200 sq. ft.

Gable Roof with Two Roof Planes, Example #2

There are two planes to this gable roof.

Therefore, in order to calculate the roof’s total square footage, you would:

  1. Get the square footage for each plane by multiplying length (A) by width (B) and length (A) by width (C).
  2. The two planes are added together.

For instance:

  • Plane 1: 40′ x 30′ = 1200 sq. ft. (A x B).
  • 40′ x 30′ = 1200 square feet on Plane 2 (A x C).
  • The overall square footage of the roof is 2400 square feet (planes 1 and 2).


“Squares” are used to measure roof surfaces. 100 square feet of roof area equals one roofing square.

Divide the 2400 square feet of the gable roof example in this post by 100 to get the number of squares (2400 100 = 24).

In order to cover that roof, 24 squares of shingles would be required. Make sure to include a 10%–15% trim allowance (waste factor) in all of your material totals.

PRO TIP: Do you want to avoid doing some math? To examine the size of your roof in squares and the pitch of your roof, order a FREE RoofScopeX aerial roof report.

Keep the report handy when speaking with roofers in the future or when comparing quotes for your subsequent roof replacement or repair.


A bundle is a group of roofing shingles.

How many shingles are there in each square?

One of the most popular kinds of roofing shingles, laminate or architectural shingles, are often presented in three bundles per square:

1 roofing square equals 3 bundles.

You would therefore require 72 bundles of shingles for the 2400 square foot gable roof example (24 squares x 3 bundles per square = 72 bundles).

How about the issue of waste? Should I get additional roofing supplies?

In order to account for waste, you need to order extra material. Waste factors can differ.

A good guideline is 10%–15%, however, your outcomes may vary. Because there are typically more chopped shingles at corners, walls, and edges on more difficult roofs, the waste factor will be larger.

Having a few extra shingles is acceptable. They can be stored in case there is further roof damage or if repairs are required.

Therefore, for the example of a 24-square roof, you would need to add 2.4 squares, which is equivalent to 7 or 8 additional bundles.


The same amount of underlayment would be required to cover the roof deck if you were also installing roofing underlayment.

Therefore, 24 squares of roofing underlayment would be required for the 2400 square-foot gable roof example in this essay.

In how many squares does underlayment come in rolls?

The roll size is determined by the underlayment product type. Regular #15 felt is available on 4 square rolls whereas synthetic underlayment is available on 10 square rolls.

Therefore, 2.4 rolls of synthetic underlayment or 6 rolls of standard #15 felt would be required for the example of a 24-square roof.

  • 10 squares per roll divided by 24 squares is 2.4 rolls of synthetic underlayment.
  • 4 squares per roll divided by 24 squares equals 6 rolls of normal #15 felt.

Remember to include a factor of 10%–15% for underlayment waste.

Last, but not least, consult a roofing contractor in your region if you have any queries regarding your estimate. Most people will be glad to provide you with a free estimate.


Additionally, you’ll need to be aware of your deck’s slope.

Measure your deck’s vertical increase in inches over a 12′′ horizontal space to find out. Your roof slope is 4 in 12 if this rise is 4 inches.

When expressing roof slopes, the vertical rise is always stated first, followed by the horizontal run (12′′).


Use one of the alternative techniques listed below to gauge a steep roof:

Method 1: 

To determine the length of the roof, measure the outer walls and the overhang along the ridge of the house.

Next, fling a rope across the ridge, marking its intersection with each eave as you go. This will provide the width dimension for your area calculation. Each roof portion with a horizontal ridge needs to have this done.

Method 2: 

Apply a mathematical method to calculate the roof area while taking into account the length, span, and pitch of the roof:

  1. Use a pitch gauge (often sold at home improvement stores) or a smartphone app (free in any app store) to calculate your roof’s pitch.
  2. The length of the roof surface, including overhangs, should be measured.
  3. Calculate the roof’s span, taking into account overhangs.
  4. Add the length and the breadth together. The area of the plane will be the outcome. Calculate the total roof area using the chart below.
  5. Plane Area x Correction Factor = Roof Area, which will give you the entire roof area.


In general, you should use four nails per shingle in the roof’s field and five nails per shingle in the beginning row. This would take 320 nails per square for field shingles for standard three-tab shingles. To calculate the number of nails needed to fasten the starter course, multiply the number of starter shingles needed by 5.

Six nails per shingle, or 480 nails per square, are needed in strong wind zones or when installing shingles on a mansard. This is based on a square with 80 shingles.

Other shingle designs may have more or less than 80 shingles per square and may require more or fewer nails per shingle. To determine the proper nailing pattern, refer to the application instructions on your shingle wrappers.

Always confirm the requirements for fasteners with your local building code. Request from your dealer the appropriate number of nails in the length you want for the size of your roof.