Roof Tile, Slate and Shake Calculator
Last reviewed How we calculate
Tile, slate and shake roofs are ordered by the piece, and how many pieces cover a square depends on how much of each one is left showing. This calculator works that out from the size of the piece and its exposure, then turns your roof area into a number to order.
Tiles, slates or shakes to order
Pieces per square and pieces to order for tile, slate or shake, worked out from the piece size and exposure.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Pieces to order Field pieces only. Ridge, hip and eave pieces are ordered separately.
- Exposure per course
- Pieces per square
- Roofing squares, before waste
- Area including waste
The count is plain geometry: each piece shows its width times its exposure, and a square is 14,400 square inches. Where a manufacturer publishes its own pieces-per-square figure for your pitch, use theirs. Tile and slate are heavy, so a roof moving to them from asphalt needs its structure checked first.
How the calculation works
A roofing square is 100 sq ft, which is 14,400 square inches. Each piece shows its width multiplied by its exposure, the depth of the course that stays visible once the next course is laid over it. Divide 14,400 by the area one piece shows and you have pieces per square.
Slate exposure is not usually quoted directly. It is the slate length minus the headlap, divided by two, because slate is double-lapped and every part of the roof is covered by at least two thicknesses.
Worked examples
An interlocking concrete tile with a covered width of 12 in and an exposure of 13.5 in shows 162 square inches, so a square takes about 89 tiles. A roof of 1,250 sq ft with 10 per cent waste needs about 1,223. A 20 in by 12 in slate laid with a 3 in headlap has an exposure of 8.5 in and needs about 141 slates a square, or about 1,942 for the same roof.
Where the numbers come from
Manufacturers publish the maximum exposure, or the minimum headlap, for each pitch. On a lower pitch the courses overlap more, which raises the count. Where a tile maker publishes its own pieces-per-square figure for your pitch, use it in preference to this. Ridge, hip, verge and eave pieces are separate products and are not included here.
Weight
These materials are several times heavier than asphalt shingles. A roof that has only ever carried shingles was not necessarily built for tile or slate, so a change of material needs the structure checked first. The roof weight calculator gives the load to discuss with an engineer.
Frequently asked questions
How many roof tiles are in a square?
It depends on the size of the tile and how much of it is left showing. Interlocking concrete tiles often come to somewhere around 80 to 100 per square; slate varies more, from about 100 to over 200 depending on size and headlap.
What is headlap on a slate roof?
Headlap is how far each slate overlaps the slate two courses below it. Together with the slate length it sets the exposure, and it has to increase as the pitch gets lower.
Can my roof take the weight of tile or slate?
Not necessarily. Tile and slate can weigh several times as much as asphalt shingles, so a roof built for shingles needs its structure checked by an engineer before it is re-covered with either.