Metal, tile & flat
Tile and slate: pieces per square, weight and what the structure must carry
Tile and slate last a lifetime and weigh three to five times as much as asphalt. Here is the count, the load and when the framing needs a second look.
Counted in pieces
Shingles are sold in bundles of a third of a square. Tile and slate are counted piece by piece, because each piece’s coverage depends on its size and on how much of it is lapped by the course above, and that lap depends on the pitch.
Concrete and clay tile hang on battens at a spacing called the gauge, which is the tile’s exposure. The maker’s data sheet gives the pieces per square for each headlap: about 86 to 95 for standard flat and S-profile concrete tile, 150 to 180 for two-piece clay barrel (pans and covers counted together).
Slate is laid so every point is covered by at least two thicknesses plus the headlap, and its exposure is (length − headlap) ÷ 2. The National Slate Association’s guidance sets the headlap: 3 inches from 8/12 to 20/12, 4 inches from 4/12 to 8/12.
Slate and tile on a 20-square roof
18 × 10 inch slate at a 3-inch headlap:
- Exposure: (18 − 3) ÷ 2 = 7.5 in; per square: 14,400 ÷ 75 = 192
- With 8% breakage: 20 × 192 × 1.08 = 4,148 slates and 8,296 copper nails
Flat concrete tile at 90 per square:
- 20 × 90 × 1.08 = 1,944 tiles, 11 pallets, plus ridge and rake tiles
From the slate calculator and the tile calculator.
The weight
| Roofing | Installed weight per square | Per square foot | Compared with architectural shingles |
|---|---|---|---|
| Architectural asphalt | 250 to 350 lb | 2.5 to 3.5 lb | 1× |
| Metal | 80 to 150 lb | about 1 lb | ⅓ to ½× |
| Synthetic slate | 250 to 400 lb | 2.5 to 4 lb | about 1× |
| Clay tile | 600 to 1,100 lb | 6 to 11 lb | 2 to 4× |
| Concrete tile | 900 to 1,100 lb | 9 to 11 lb | about 3× |
| Natural slate | 650 to 1,400 lb | 6.5 to 14 lb | 2 to 5× |
A 2,000 square foot roof of concrete tile weighs about 10 tons, against about 3 tons of shingles: 7 extra pounds on every square foot of roof, for the life of the building. The roof weight calculator computes the change for any pair of materials, and the material weights table lists the ranges.
Why the structure matters
Rafters and trusses are sized for a total load: the dead load of the roof itself, plus snow, plus a construction live load, within limits on stress and deflection. A roof designed for asphalt shingles assumed a dead load of perhaps 10 to 15 pounds per square foot including the framing and deck. Adding 7 pounds of tile takes a large share of the margin, and in snow country it can take more than all of it.
Signs that a structure is not coping include a sagging ridge, rafters that bow between supports, and cracks in ceilings below. But a structure can be overloaded without showing it until a heavy snow. That is why changing to a heavier material is a question for a structural engineer, who checks the rafter or truss sizes, spans and spacing against the code’s span tables and the local snow load, and specifies any reinforcement: sistered rafters, added purlins and struts, or new trusses. Truss roofs are especially sensitive, because trusses are designed for a stated dead load and modifying them needs the truss maker’s or an engineer’s approval.
The arithmetic of rafters is covered in rafter maths; their capacity is not something a calculator should decide.
Lighter alternatives
Lightweight concrete tile weighs about 500 to 700 pounds per square, close enough to heavy shingles that many structures carry it without change. Synthetic slate and composite shake weigh about the same as asphalt and imitate slate and cedar closely from the ground; the synthetic slate calculator counts them. Metal tile and shingle profiles weigh a third of asphalt.
Battens, underlayment and pitch
Tile hangs on battens at the gauge, which also sets the batten footage: about 92 linear feet per square at a 13-inch gauge. Slate on battens takes more, at its shorter gauge. The battens calculator counts both. Underlayment under tile is often heavier than under shingles, and below 4/12 most makers require a special low-slope detail. Slate is not recommended below 4/12 at all.
Walking, breakage and repairs
Tile and slate break under feet. Roofers working on them walk on the lower third of each tile, where it rests on the one below, or use ladders hooked over the ridge and planks to spread the load. Anyone else should stay off. A cracked tile or slate is replaced individually: slate with a slate ripper and a copper strap, tile by lifting the courses above. Keep a few spare pieces from the original order, since colours and profiles change between production runs and quarries close. The breakage allowance in the order is partly for this.
Life
The reason to carry the weight is the life: concrete tile 40 to 60 years, clay tile 50 to 100, slate 75 to 100 or more, against 25 to 30 for architectural shingles. Underlayment and flashings, not the tile or slate itself, usually set the end of a tile or slate roof’s life. How long roofs last compares them, and cedar shakes covers the other traditional material counted by exposure.