Calculating roofing materials starts with the actual roof surface area, not simply the floor area of the building. Once the roof area is known, you can estimate the number of roofing sheets, shingles, tiles or other covering materials by dividing the roof area by the coverage provided by one unit. You then account for overlaps, cuts and reasonable waste.
Quick formula:
Roofing units required = Roof area divided by coverage per unit
For sheet roofing, the coverage should be based on the sheet's effective coverage width, not its full manufactured width, because adjacent sheets usually overlap.
For a more direct calculation, use the BuildersMeasures Roofing Calculator. If you first need to work out the roof surface area, see How to Calculate Roof Area.
Table of Contents
- Start With the Roof Area
- Understand Effective Coverage
- How to Calculate Roofing Sheets
- Roofing Sheet Worked Example
- How to Calculate Shingles
- How to Calculate Roofing Tiles
- How Much Roofing Waste Should You Allow?
- How Roof Pitch Changes Material Requirements
- Calculating Materials for Complex Roofs
- Don't Forget Roofing Accessories
- Common Roofing Material Calculation Mistakes
- Roofing Material Calculation Summary
- Frequently Asked Questions
Start With the Roof Area
The first number you need is the area covered by the roofing material. A simple rectangular building footprint is not always the same as the roof surface area because a pitched roof has sloping planes.
For a simple gable roof, calculate the area of each sloping side and add them together. For a hip roof, calculate each roof plane separately. Overhangs should also be included when they extend beyond the wall line and will receive roofing material.
If the roof dimensions are measured horizontally rather than along the slope, the roof pitch must be considered. The Roof Pitch Calculator can help determine pitch, while the Slope Calculator can be used when working with slope measurements.
A simple gable roof
Suppose a building is 10 metres long and 6 metres wide. The roof has two equal sloping sides. If the horizontal run from the ridge to the eave is 3 metres and the sloping length is 3.606 metres, each roof plane is:
Because there are two roof planes:
The approximate roof surface area is therefore 72.12 square metres.
Understand Effective Coverage
One of the most common mistakes when estimating roofing materials is using the manufacturer's full product dimensions as the coverage area.
A roofing sheet may be physically wider than the area it covers after installation because one sheet overlaps the next. Shingles and tiles also have installation requirements that affect their actual coverage.
The important number is therefore the effective coverage stated for the particular product and installation method.
Check the product specification before calculating. Roofing products with similar physical dimensions can have different effective coverage because of side laps, end laps, profile shape and installation requirements.
How to Calculate Roofing Sheets
For profiled metal sheets, the basic calculation is straightforward once you know the effective coverage of one sheet.
If you are calculating each roof plane separately, it can be better to calculate the sheets required for each plane rather than dividing the entire roof area by an average coverage value. This helps account for sheet length, layout and the direction of installation.
Sheet coverage from width
If the sheet length follows the slope and the effective width is known, the coverage can be calculated as:
For example, a sheet that is 3.6 metres long with an effective coverage width of 0.95 metres covers approximately:
This is an illustrative calculation. Always use the actual effective coverage supplied for the roofing product being installed.
Roofing Sheet Worked Example
Example: Assume a roof has an estimated surface area of 72.12 m². Suppose each installed sheet provides 3.42 m² of effective coverage.
Sheets before allowance:
Because you cannot order 0.09 of a sheet, the result is rounded up to 22 sheets before considering waste, cuts and project-specific requirements.
In practice, the final quantity may need to be higher depending on the roof layout. A roof with several valleys, hips, ridges, penetrations or unusual dimensions can require more cutting than a simple rectangular roof.
Do not treat the example as a purchasing quantity. Sheet length, profile, effective width, roof geometry, overlap requirements and installation details should be confirmed before ordering materials.
How to Calculate Shingles
Shingles are normally estimated using the manufacturer's stated coverage rather than treating each shingle as a simple rectangle.
Many asphalt shingles are sold by the bundle, and the number of bundles required depends on the coverage specified by the manufacturer. Some products may also require additional material for starter courses, hips and ridges.
For example, if a product specification states that one bundle covers 3 square metres and the roof area is 72 square metres:
The actual order may need additional bundles for cuts, waste, roof complexity and installation details.
How to Calculate Roofing Tiles
Roofing tiles are usually calculated from the manufacturer's coverage rate. The number of tiles per square metre can vary according to tile dimensions, overlap and the required roof pitch.
For example, if a particular tile installation requires 11 tiles per square metre and the roof area is 72.12 m²:
The calculated quantity would be rounded up to at least 794 tiles before adding an allowance for cuts, breakage and other losses.
The tile manufacturer's installation specification should take precedence over a generic tile-per-square-metre estimate.
How Much Roofing Waste Should You Allow?
Roofing waste is not a fixed number for every project. A simple roof with long uninterrupted slopes may have fewer cuts than a roof containing multiple hips, valleys, dormers, skylights and penetrations.
A small allowance is often added to the calculated material quantity, but the appropriate amount depends on the material and roof design.
| Roof situation | Why additional material may be needed |
|---|---|
| Simple gable roof | Fewer cuts and straightforward installation |
| Hip roof | More cuts around hips and roof edges |
| Roof with valleys | More cutting and detailing around intersections |
| Multiple roof sections | More complex layouts and offcuts |
| Many penetrations | Extra cutting around vents, chimneys and skylights |
Instead of choosing a waste percentage blindly, look at the roof layout and the product installation instructions. The person supplying or installing the roofing material can also help determine an appropriate allowance.
How Roof Pitch Changes Material Requirements
Roof pitch affects the relationship between the building footprint and the actual roof surface. As a roof becomes steeper, the sloping surface becomes longer even though the horizontal footprint stays the same.
For example, two buildings can have the same 10 metre by 6 metre footprint but different roof pitches. Their actual roof surface areas can therefore be different.
Pitch can be expressed as a ratio such as 6:12. The first number represents the rise while the second represents the horizontal run. For more detail, see How to Calculate Roof Pitch and Roof Pitch vs Roof Slope.
The slope factor can then be used with the horizontal roof area to estimate the sloping roof surface for a simple roof geometry.
Calculating Materials for Complex Roofs
Complex roofs are easier to estimate by breaking them into smaller sections instead of trying to use one formula for the entire structure.
Divide the roof into identifiable planes, calculate the surface area of each plane, and then add the results.
- Measure each roof section.
- Determine the slope or sloping dimensions where required.
- Calculate the surface area of each section.
- Add all roof sections together.
- Use the effective coverage of the selected roofing product.
- Account for overlaps, cuts and reasonable waste.
- Calculate accessories separately.
This approach is particularly useful for hip roofs, L-shaped buildings, roofs with extensions and structures containing different roof pitches.
Don't Forget Roofing Accessories
The roof covering is only part of the material order. Depending on the roofing system, the project may also require components such as ridge caps, hip caps, valley materials, flashing, underlayment, fasteners, sealants and ventilation products.
The quantities for these items should not simply be derived from total roof area. Many are measured by length, number of locations or manufacturer-specific installation requirements.
For example: ridge-cap quantity is normally based on ridge length, while flashing is often based on the length of the roof edge or penetration being protected.
Common Roofing Material Calculation Mistakes
Using the building footprint as the roof area
The footprint may be smaller than the actual sloping roof surface. Pitch and overhangs can increase the amount of roofing material required.
Ignoring overlaps
Sheet roofing rarely covers the full physical width of the sheet after installation. Side laps and end laps reduce effective coverage.
Using generic coverage instead of the product specification
Different roofing products have different installation requirements. The manufacturer's coverage information is more useful than a generic estimate based only on product dimensions.
Forgetting roof complexity
Hips, valleys, dormers, skylights, chimneys and other interruptions can increase cutting and waste.
Rounding too early
Keep useful decimal precision during the calculation and round the final purchasing quantity upward where the material is sold as whole units.
Calculating only the main covering
Roofing projects can also require ridge, hip, valley, flashing, underlayment, fasteners and other components. These should be estimated separately.
Roofing Material Calculation Summary
The general process is simple, but accurate material ordering depends on the actual roof design and product specifications.
| Step | What to calculate |
|---|---|
| 1 | Measure the roof dimensions |
| 2 | Calculate the actual roof surface area |
| 3 | Check the product's effective coverage |
| 4 | Divide roof area by effective coverage |
| 5 | Round the material quantity appropriately |
| 6 | Allow for cuts, overlaps and project-specific waste |
| 7 | Estimate roofing accessories separately |
Frequently Asked Questions
How do I calculate how many roofing sheets I need?
Calculate the total roof surface area, determine the effective coverage of one sheet after overlaps, then divide the roof area by that coverage. Round the result appropriately and account for cuts and waste.
Does roof pitch affect the number of roofing sheets?
Yes. A steeper roof generally has a larger surface area than its horizontal footprint. The actual sheet quantity also depends on sheet dimensions, effective coverage, roof layout and installation requirements.
Should I calculate roof area from the floor area?
Not by itself. Floor or building footprint area does not account for roof pitch, overhangs or complex roof geometry. The actual roof surface should be calculated for a material estimate.
How much roofing waste should I add?
There is no single waste percentage that applies to every roof. A simple roof may require less additional material than a complex roof with hips, valleys, penetrations and many cuts. Follow the roofing product guidance and consider the roof layout.
How do I calculate roofing shingles?
Use the roof surface area and the manufacturer's stated coverage per bundle or package. Divide the roof area by the stated coverage, then account for the product's installation requirements and additional material for cuts and waste.
Do roofing materials include ridge caps and flashing?
They are normally estimated separately from the main roof covering. Ridge caps are generally based on ridge length, while flashing and other accessories depend on the edges, valleys, penetrations and installation details of the roof.
Important: Roofing calculations are useful for estimating and planning, but the final material order should be checked against the actual roof measurements and the installation requirements for the selected roofing product. For important construction work, verify measurements before purchasing materials or beginning installation.