How to Calculate Roof Waste: A Simple Step-by-Step Guide

When you are ordering materials for a new roof, the most common mistake is ordering the exact amount of shingles that match the area of your roof. In the roofing world, you will always have waste because of cuts, overlaps, and the shape of your roof.

If you run out of shingles in the middle of a job, it costs you extra time and money. Here is a simple, step-by-step guide to calculating roof waste like a pro.

1. What is "Roof Waste"?

Roof waste is the extra material you need to buy to cover:

• Overlaps: Shingles must overlap to keep water out.
• Cuts: When a shingle reaches the edge or a corner (valley), you have to cut it. The leftover piece is often too small to use elsewhere.
• Start/Finish: The very first row and the very top row need extra material for proper sealing.

2. Common Waste Percentages

Depending on how complex your roof is, you will add a certain percentage to your total area.

3. How to Calculate (The Simple Way)

To find out how many shingles to buy, follow this basic formula:

$\text{Total Area}\times \text{Waste Factor}=\text{Total Material Needed}$

Step-by-Step Example:

Let’s say your roof area is 2,000 square feet and you have a Hip Roof (which needs 15% waste).

Step 1: Convert the percentage to a decimal.

• 10% = 1.10
• 15% = 1.15
• 20% = 1.20

Step 2: Multiply.

$2,000\times 1.15=2,300\text{ square feet}$

Step 3: Convert to "Squares"

Roofers measure in "Squares." One square equals 100 square feet.

• $2,300÷100=23\text{ Squares}$

Step 4: Convert to Bundles

Usually, 3 bundles make 1 square.

• $23\times 3=69\text{ Bundles}$

4. Why some roofs waste more than others?

The "Valley" Factor

A valley is where two roof sections meet to form a "V" shape. This is where most waste happens. You have to cut shingles at an angle to fit into the valley. You cannot use the "off-cut" (the piece you cut off) on the other side because the pattern won't match.

The "Starter Strip"

Don't forget that the very bottom edge of your roof needs a "Starter Shingle." Many people forget to calculate this. It is better to buy a dedicated bundle of starter strips rather than cutting up your expensive shingles to make them.

Summary Checklist for Beginners

1. Measure your roof area (Length x Width of each section).
2. Identify your roof type (Gable = 10%, Hip = 15%).
3. Multiply your area by the waste factor (e.g., 1.15).
4. Divide by 100 to get the number of squares.
5. Round UP. If you need 22.2 squares, buy 23. It is better to have 3 extra bundles than to be 3 shingles short.