What this calculator works out
This calculator multiplies roof area by tiles per square metre and adds a waste allowance, returning the number of tiles to order.
The tiles-per-square-metre figure is specific to the tile and to the lap it is laid at. It comes from the manufacturer, not from a general rule, and using a figure from a different tile will give a badly wrong order.
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Before you order
- Roof area means the sloping area, not the plan area of the building. A 26.6° roof has about 12% more surface than its footprint.
- Take tiles per square metre from the manufacturer's data for the lap you are using. It typically ranges from about 10 for large interlocking tiles to 60 or more for plain tiles.
- Allow 5% waste for a simple rectangular roof and 10% or more where there are hips, valleys, dormers or rooflights.
- Order ridge, hip, verge and eaves tiles separately. They are not included in the field tile count.
Increasing the headlap to suit a lower pitch reduces the exposed length of each tile, which increases the number needed per square metre. The manufacturer publishes a table of gauge and coverage against pitch — using the figure for the wrong pitch can change the order by 10% or more.
How this calculator works
Area, rate and allowance:
Tiles = roof area × tiles per m² × (1 + waste ÷ 100), rounded upThe calculation covers field tiles only. Battens, underlay, nails, clips, ridge and hip tiles, dry verge systems and eaves protection are all separate items and together often cost as much as the tiles.
Worked example: a 60 m² roof
Using the default figures — 60 m² at 10 tiles per m² with 10% waste:
- Before waste: 600 tiles
- Total to order: 660 tiles
Ten tiles per square metre indicates a large interlocking tile. Switch to plain tiles at 60 per m² and the same roof needs 3,960 — six times as many, at a very different weight. Tile weight matters structurally: a plain tiled roof can weigh more than twice an interlocking one, which is why re-covering a roof in a heavier tile is not a like-for-like swap and may need the structure checking.
Common mistakes
- Using plan area instead of sloping area. The slope is always larger.
- Taking a coverage figure from a different tile. It is specific to the product and the lap.
- Forgetting ridge, hip and verge tiles. They are ordered separately.
- Re-covering in a heavier tile without checking the structure. Weight per square metre varies enormously.
- Ordering from different batches. Colour variation shows badly on a roof.
Frequently asked questions
How do I work out the sloping roof area?
Multiply the plan area by 1 ÷ cos(pitch). At 26.6° that factor is about 1.12, so a 10 by 5 metre footprint has roughly 56 m² of roof per side rather than 50. Alternatively measure the rafter length directly and multiply by the roof length.
How many spare tiles should I keep?
A dozen or so, stored somewhere dry and accessible. Tiles get broken by ladders, aerials and weather, and matching a discontinued tile years later is difficult and expensive. Keeping the offcuts from the waste allowance costs nothing.
Do I need to fix every tile?
It depends on pitch, exposure and the tile. Fixing specifications have tightened considerably and the current standard requires far more mechanical fixing than older practice — in many cases every tile in exposed locations. Dry fixing for ridges and verges has largely replaced mortar bedding for the same reason.
Is roofing a DIY job?
Repairs and shed roofs, sometimes. A house roof is work at height with real fall risk, and the detailing around valleys, abutments and penetrations is where roofs leak. Falls from height remain one of the leading causes of workplace death in the UK, and roof work is a substantial part of that.
Is what I enter stored?
No. Areas and rates are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — Approved Document guidance on roof construction and weather resistance
- Health and Safety Executive — roof work, fall prevention and working at height regulations
Sources are checked at publication and can change — how I choose and check references.