How many roof tiles do I need per square metre?
Tiles per m2 = 1 / (gauge x cover width), with both in metres. Plain tiles come to about 60 per m2. Large interlocking concrete tiles come to about 10 per m2, and slates to about 13 to 19 per m2 depending on size and lap. Work from the area of the slope, not the plan, then add the eaves, verge and ridge units and about 5 percent for waste.
The sum
Each tile covers a rectangle on the roof. The rectangle is as wide as the tile's cover width, and as tall as the gauge. The gauge is the distance between the tops of one course of battens and the next.
- tiles per m2 = 1 / (gauge x cover width), with both in metres
- battens per m2 = 1 / gauge, which gives metres of batten per m2
The gauge comes from the tile length and the headlap (the overlap of one course over the one two below). The usual relationships are:
- single-lap interlocking tiles: gauge is about tile length minus headlap
- plain tiles and slates: gauge is about (length minus headlap) / 2, because each tile is covered twice
Check these against your own tile. Tile makers publish a maximum gauge and a minimum headlap, and the actual gauge is set out on the roof between those limits. A wider gauge means fewer tiles, so the maker's limit matters.
Typical figures, as published
These are typical figures from manufacturers' published data. Treat them as typical, not exact, and check the product you are buying. Tile sizes and laps vary between ranges.
- Plain tile, 267 x 165 mm (Marley), 100 mm maximum gauge, 65 mm minimum headlap: 60 per m2. Minimum pitch 35 degrees. Check: 1 / (0.100 x 0.165) = 60.6.
- Large interlocking concrete tile, 420 x 330 mm (Marley Modern), cover width 292 mm: 9.9 per m2 at 345 mm gauge (75 mm headlap), or 10.7 per m2 at 320 mm gauge (100 mm headlap). Check: 1 / (0.345 x 0.292) = 9.9. Battens: 2.9 m per m2 at 75 mm lap, 3.1 m per m2 at 100 mm.
- Smaller interlocking tile, 366 x 202 mm (Sandtoft Vauban), cover width 168 mm: 21.8 per m2 at the 273 mm maximum gauge.
- Redland Cambrian slate: 14.8 per m2 at 225 mm gauge, 13.3 per m2 at 250 mm gauge. Working back, that is a cover width of about 300 mm.
- Natural slate, 500 x 250 mm (Cupa Pizarras): 18.8 per m2 at 75 mm headlap. A 600 x 300 mm slate at 90 mm headlap: 13.1 per m2.
Cupa publishes a full table of slates per m2 by size and lap, using nominal sizes. The natural slate figures above match the sum: for a 500 x 250 mm slate at 75 mm lap, gauge is (500 - 75) / 2 = 212.5 mm, and 1 / (0.2125 x 0.250) = 18.8.
Minimum pitch depends on the headlap, the tile and the exposure. The figures differ by product, so always read the minimum pitch from the data sheet for the tile and the lap you intend to use.
Work from the slope area, not the plan area
Tiles cover the sloping surface, which is larger than the area you measure on plan. Slope area is plan area divided by the cosine of the pitch.
- 22.5 degrees: multiply plan area by about 1.08
- 30 degrees: about 1.15
- 35 degrees: about 1.22
- 40 degrees: about 1.31
- 45 degrees: about 1.41
Measure the plan to the outside of the eaves overhang if the tiles run out to it. For each slope, you can also multiply the slope length (eaves to ridge) by the length along the eaves. Take off openings such as rooflights and chimneys, and add a little for tiles cut around them.
Eaves, verge and ridge extras
Tiles per m2 covers the main field of the roof. The edges need their own units, which you count separately from the maker's range:
- Eaves: the first course along each eave. Count the eaves length divided by the cover width, rounded up. Some systems use special eaves tiles, and some need an extra under-course.
- Verge: the number of courses up each verge, which is the slope length divided by the gauge, rounded up. Dry verge systems use left-hand and right-hand units. Others use tile-and-a-half or cloaked verge tiles.
- Ridge and hips: length divided by the effective cover of one ridge tile. Marley's Modern Ridge, for example, is 457 mm long, but the effective cover is shorter than the length, so take it from the data sheet. The ridge needs to overlap the top course tiles by at least 75 mm on the systems checked.
- Top course, ends, fittings and ventilation: counted off the drawing, one by one.
Hips, valleys and abutments add cut tiles and special units. Your supplier can usually price from a marked-up drawing.
Waste
Cupa recommends adding at least 5 percent for waste on slates. That is a sensible starting point for tiles too. Raise it for lots of cutting, such as hips, valleys, small roofs and short rafters, or for brittle or hand-made tiles. Buy a little over on the first order, because colours and sizes vary slightly between batches and a late top-up may not match.
Tiles are normally delivered on pallets or in packs, so round the order up to whole packs.
Worked example: a gable roof
A house roof with two equal slopes. It measures 9 m along the eaves, and 6 m wide on plan including the overhang, so each slope is 3 m wide on plan. The pitch is 40 degrees.
- plan area: 9 x 6 = 54 m2
- slope area: 54 / cos 40 degrees = 54 / 0.766 = 70.5 m2
- slope length of each side: 3 / 0.766 = 3.92 m
Plain tiles at 60 per m2: 70.5 x 60 = 4,230 tiles, and 4,442 with 5 percent for waste. At a 100 mm gauge each slope has 3.92 / 0.1 = 39.2, so 40 courses. Each eave needs 9 / 0.165 = 54.5, so 55 tiles. Battens come to 70.5 x 10 = 705 m.
Large interlocking tiles at 10.7 per m2 (100 mm lap): 70.5 x 10.7 = 754 tiles, and 792 with 5 percent. Battens: 70.5 x 3.1 = about 219 m.
Ridge: if the ridge tile you choose has an effective cover of 400 mm (an assumed figure for this example), 9 / 0.4 = 22.5, so 23 tiles plus the end tiles. Each slope has two verge edges, so there are four verge edges of about 40 courses each. Count the verge units for each from the maker's range, for example left-hand and right-hand units for a dry verge system.
Check the total against the tile maker's own quantity sheet before you place the order.
Sources
- Marley: Modern concrete interlocking tile
- Marley: Concrete plain roof tile (building design index listing)
- NBS Source: Marley Modern Ridge
- Marley: Eden fixing guide
- Sandtoft (Wienerberger): notes on the installation of Vauban tiles
- Redland: Cambrian slate technical data
- Cupa Pizarras: slate coverage and weight
- Cupa Pizarras: how many slates you need
- Hertfordshire Building Control Technical Forum: tile and slate information (2011)