Why is the roof area larger than the area of the house?

Because the slopes are inclined. The plan shows the horizontal projection; the real surface is larger, and the steeper the roof the larger the difference. The calculator applies that factor to each slope.

How do I know the pitch of my roof?

Look in the house plans. If you do not have them, measure from the attic with an inclinometer laid on a roof timber. Do not climb onto the roof to measure.

How many tiles per m² should I use?

It depends on the model. The range in the APICER and CTCV guide gives an idea; to buy, use the data sheet of the tile maker you chose.

What margin should I add?

We do not state a standard value. Cuts, flashings and breakage vary with the shape of the roof. Ask the contractor and have the margin written into the quote.

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Roof area and tile calculator

The area of a roof is not the area of the house. Because the slopes are inclined, the real area is larger than the one you see on the plan. Measure each slope, enter the pitch and see the real area and a range for the number of tiles.

Calculator

Measure from the ground or from the plans. Use the dimensions on the plans, or measure the house outside from the ground. Do not climb onto the roof. For the pitch you can use the plans, or an inclinometer laid on a roof timber from the attic.

Pitch given in

Slope 1

Width on plan (m)

From the eave to the ridge, horizontally, as in the drawing seen from above. Include the overhang beyond the wall.

Length along the eave (m)

If the slope is a trapezium or a triangle, use the average length.

Pitch (%)

Slope 2

Width on plan (m)

From the eave to the ridge, horizontally, as in the drawing seen from above. Include the overhang beyond the wall.

Length along the eave (m)

If the slope is a trapezium or a triangle, use the average length.

Pitch (%)

Tile

Tile format

Margin for cuts and breakage (%)

Leave blank to add no margin. Ask the contractor which margin they use.

How it is calculated

Each slope is an inclined rectangle. On the plan you see only its projection: width (eave to ridge) times length. The real area is that area multiplied by a factor that depends on the pitch: the square root of 1 plus the pitch squared (as a decimal). In degrees, the factor is 1 divided by the cosine of the angle.

For example, a 30% pitch gives a factor of about 1.044, and 100% (45°) gives about 1.414. The steeper the roof, the bigger the difference between the plan and the real roof.

Where the tile numbers come from

The number of tiles per m² and the weight of each tile are the average values of the Guia para Projeto e Aplicação de Telhas Cerâmicas, by APICER and CTCV (2018), the same as in our guide to tiles. They are market ranges, not one manufacturer's data. To buy, use the data sheet of the model and the batten spacing the manufacturer gives.

What the calculator does not do

It does not count ridge tiles, flashings, eave tiles or accessories, and it does not say which cutting margin to use: that depends on the roof and the contractor. It does not work out timber or insulation either. The APICER and CTCV guide calls a roof pitched when it is steeper than 8%; below that it is a flat roof, with other rules. Always confirm with whoever does the work before ordering material.

Frequently asked questions

Need a quote?

Describe the roof. We are not contractors: we send your request to someone who can give a quote.