How to Use the Roofing Material Calculator

Use the Builzee roofing material calculator to estimate roof surface area, roofing squares, waste allowance and approximate asphalt shingle bundle quantities.

Start by entering each roof plane separately. For a simple gable roof with two identical sides, you can enter one roof plane and set Qty to 2.

For each section, enter:

• Roof length
• Horizontal run width
• Roof pitch
• Quantity

Then choose the roofing material and waste allowance. If you are using asphalt shingles, you can also enter the actual coverage per bundle from the product packaging.

The calculator combines all roof sections and estimates the total roofing area you should plan for.

Measure the Horizontal Roof Plan, Not the Sloped Surface

The calculator is designed so you do not have to measure directly along the slope of the roof.

Enter the horizontal dimensions of each roof plane as viewed in plan.

For a simple gable roof, the Roof Length is generally the horizontal length along the eave or ridge, while the Horizontal Run Width is the horizontal distance from the eave toward the ridge.

The calculator then uses the roof pitch to convert that projected area into actual sloped roof surface area.

Include roof overhangs in your dimensions when they are part of the roofing surface.

For complex roofs, divide the roof into manageable sections and enter each one separately.

How Roof Pitch Changes Roof Area

A steeper roof has more surface area than its horizontal footprint.

For example, a roof plane measuring 40 feet × 15 feet horizontally has a projected area of:

40 × 15 = 600 square feet

At a 4:12 pitch, the sloped surface is slightly larger than 600 square feet.

The calculator automatically applies the correct pitch multiplier based on the selected roof pitch.

It also displays the approximate roof angle and multiplier so you can see how pitch affects the calculation.

What Does a 4:12 Roof Pitch Mean?

Roof pitch describes how much the roof rises vertically for every 12 units of horizontal run.

A 4:12 roof pitch means the roof rises 4 inches for every 12 inches of horizontal distance.

Common residential roof pitches include:

• 3:12
• 4:12
• 5:12
• 6:12
• 8:12
• 10:12
• 12:12

The calculator also includes a Custom Pitch option for roof slopes that do not match a standard selection.

If you are unsure of the existing roof pitch, verify it before relying on the material estimate.

What Is a Roofing Square?

Roofing materials in the United States are commonly measured in roofing squares.

One roofing square equals:

100 square feet of roof surface

For example:

1,500 sq ft ÷ 100 = 15 roofing squares

The calculator shows both:

Roofing Squares Before Waste
and
Roofing Squares Including Waste

The second figure is generally the more useful number when planning how much roofing material to purchase.

How Much Roofing Waste Should You Allow?

Roofing rarely uses every square foot of material without cuts or leftover pieces.

Waste can come from:

• valleys
• hips
• rakes
• roof penetrations
• starter courses
• pattern alignment
• damaged pieces
• trimming and cutting

A simple roof may require less extra material than a roof with several valleys, dormers or intersecting roof planes.

The calculator uses 10% waste as the standard setting, but you can select a different percentage when appropriate.

For complicated roofs, it may be reasonable to use a larger allowance.

Estimating Asphalt Shingle Bundles

When Asphalt Shingles are selected, the calculator provides an approximate bundle quantity.

If Bundle Coverage is left blank, the calculator uses a common planning assumption of:

3 bundles per roofing square

That is equivalent to roughly:

33.33 square feet per bundle

However, actual bundle coverage depends on the specific shingle product.

Some heavier, specialty or premium shingles require more bundles per roofing square.

Whenever possible, check the manufacturer’s stated coverage and enter the actual Bundle Coverage into the calculator.

The calculator always rounds the required number of bundles up to the next whole bundle.

Roofing Materials Other Than Asphalt Shingles

The calculator can also be used to estimate overall roof area for:

• metal roofing
• clay or concrete tile
• wood shakes and shingles
• slate
• synthetic or composite roofing
• other roofing systems

For these materials, the calculator reports roof area and roofing squares but does not attempt to estimate the number of panels, tiles or pieces.

Those quantities depend on product dimensions, exposure, side laps, head laps, fastening patterns and manufacturer requirements.

Use the calculated roof surface area as the starting point, then apply the actual coverage information for the roofing product you plan to use.

Underlayment Planning Area

The calculator reports the actual sloped roof surface area as the underlayment planning area.

This gives you a useful starting point for determining how much roof underlayment is needed.

Do not simply divide this number by the advertised roll size without checking the product specifications.

Actual usable coverage may be reduced by:

• side laps
• end laps
• manufacturer-required overlaps
• valleys
• penetrations
• installation layout

Use the manufacturer’s net coverage per roll when determining the final number of underlayment rolls.

Ridge and Hip Length

The optional Total Ridge / Hip Length field lets you record the combined linear length of roof ridges and hips.

This can help when planning ridge-cap shingles, metal ridge caps or other ridge and hip materials.

The calculator reports the entered length but does not automatically convert it into bundles or pieces.

Coverage varies significantly by roofing product, so check the manufacturer’s stated linear coverage when determining final quantities.

Calculating Multiple Roof Sections

Many homes have more than one roof shape or pitch.

Use Add Another Roof Section when the roof contains sections with different:

• dimensions
• pitches
• quantities

For example, a main gable roof may have a 4:12 pitch while a porch roof has a 2:12 pitch.

Enter them as separate roof sections so each surface receives the correct pitch adjustment.

For identical planes, such as the two matching sides of a simple gable roof, use the Qty field instead of entering the same dimensions twice.

Before You Order Roofing Materials

Verify roof measurements before ordering materials.

Roofing dimensions can differ from building dimensions because of overhangs, additions, dormers, valleys and other roof geometry.

Also confirm:

• roof pitch
• actual roofing product coverage
• bundle or panel packaging
• manufacturer waste recommendations
• underlayment net coverage
• ridge and hip requirements

The Builzee Roofing Material Calculator is intended as a planning tool. Final quantities should be checked against actual roof conditions, project drawings and roofing manufacturer requirements.

For help deciding whether a roof needs repair or full replacement, see Builzee’s Roof Repair vs. Replacement guide.

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