Understanding your result
The headline is the slates to order with breakage. The stats show the slates per square, the exposure, the nails and the installed weight. The breakdown goes from the slate size and headlap to the exposure, the coverage of one slate, the slates per square and the total, and ends with the headlap the National Slate Association recommends at your pitch.
The number to remember is the slates per square: it is how slate is priced and how a slater checks an order, and it changes with the size and the headlap. The weight is the number to give the engineer; the roof weight calculator compares it with the roof coming off.
For a lighter roof with the same look, the synthetic slate calculator counts composite slates at a quarter of the weight, and the tile calculator covers the other heavy materials.
How we calculate this
The exposure formula is what makes slate different. Every point on a slate roof is covered by at least two thicknesses of slate and the headlap is a third; each slate’s exposure is therefore half of what is left of its length after the headlap. A longer slate exposes more per piece, so long slates mean fewer pieces per square, but they cost more each.
The headlap check compares your figure with the National Slate Association’s guidance by pitch, which the pitch calculator gives you. The material weights table lists slate by thickness alongside the other materials.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Slate size | 18 × 10 in | A common size; slates run 10 to 24 in long |
| Headlap | 3 in at 8/12 and steeper; 4 in from 4/12 to 8/12; 2 in over 20/12 | National Slate Association guidance (Slate Roofs: Design and Installation Manual) |
| Weight per square | 700 lb (3/16 in), 950 lb (1/4 in), 1,400 lb (3/8 in) | NSA and quarry data |
| Nails | 2 per slate, copper or stainless | NSA guidance |
| Breakage | 8% | Estimating practice: 5 to 10%, more with hips and valleys |
| Minimum pitch | 4/12 | NSA and common practice |
Assumptions last reviewed October 7, 2026.
The calculator does not count starter slates at the eaves (a doubled course), hip and ridge treatments (saddle, mitred or copper ridge), copper flashings or the underlayment, and it does not assess the structure. The guide on tile and slate weight covers when an engineer is needed and what they check.
Two worked examples
Standard slate on an 8/12 roof
2,000 square feet (20 squares), 18 × 10 inch slate, 3/16 inch thick, 3-inch headlap at 8/12, 8% breakage.
- Exposure: (18 − 3) ÷ 2 = 7.5 in
- Per slate: 10 × 7.5 ÷ 144 = 0.52 sq ft; per square: 192
- Slates: 20 × 192 × 1.08 = 4,148
- Nails: 8,296 copper; weight: 7 tons
The headlap matches the National Slate Association’s figure for 8/12, so the count stands.
Larger slate on a 6/12 roof
1,500 square feet (15 squares), 20 × 12 inch slate, ¼ inch thick, 4-inch headlap at 6/12, 8% breakage.
- Exposure: (20 − 4) ÷ 2 = 8 in
- Per slate: 12 × 8 ÷ 144 = 0.67 sq ft; per square: 150
- Slates: 15 × 150 × 1.08 = 2,430
- Nails: 4,860; weight: 7.13 tons
Bigger slates mean 150 per square instead of 192, but the thicker slate makes this smaller roof as heavy as the first one.
Where to find your inputs
Slate size. From the quarry or supplier, or by measuring an existing slate: length top to bottom, width side to side.
Pitch. From the pitch calculator; it sets the headlap.
Headlap. From the National Slate Association table for your pitch, or the slater’s specification.
Thickness. From the supplier; 3/16 inch is standard, thicker “graduated” roofs use heavier slates at the eaves.
Common mistakes
- Using the slate area instead of the exposed area. A slate covers less than its size because of the laps. Count by exposure.
- The same headlap at every pitch. Shallower roofs need more headlap; the count goes up.
- Galvanized nails. They rust long before the slate fails. Copper or stainless only.
- Nailing tight. Slates hang on their nails. Driven tight they crack and cannot be replaced.
- Skipping the structure check. Seven tons on a roof built for two needs an engineer.
- Walking on the slate. Slate cracks under feet. Roofers use ladders and hooks.
Questions people ask
- How many slates per square?
- Divide 100 square feet by the area each slate exposes: its width times its exposure, where the exposure is (length − headlap) ÷ 2. An 18 by 10 inch slate with a 3-inch headlap exposes 10 × 7.5 = 75 square inches, 0.52 square feet, so a square takes 192 slates. A 24 by 12 slate at the same headlap takes 114.
- What headlap should slate have?
- The National Slate Association recommends a 3-inch headlap for pitches from 8/12 to 20/12, 4 inches from 4/12 to 8/12, and 2 inches on very steep roofs over 20/12. The headlap is how far each slate overlaps the slate two courses below, and it is what keeps water out on a shallower roof.
- How much does a slate roof weigh?
- Standard 3/16-inch slate weighs about 650 to 800 pounds per square installed, ¼-inch about 950 and ⅜-inch about 1,400. A 20-square roof of standard slate is about 7 tons, three times the weight of asphalt shingles. The structure must be checked before slate replaces a lighter roof.
- What nails are used for slate?
- Copper or stainless steel slating nails, two per slate, long enough to go through the slate and at least ¾ inch into the deck: typically 1¼ to 1½ inches for standard slate. The nails are not driven tight; the slate hangs on them, which lets one be replaced with a slate hook without disturbing its neighbours. Galvanized nails rust out decades before the slate fails.
- How long does a slate roof last?
- Hard slates from the best quarries last 75 to over 100 years; softer slates 50 to 80. The roof usually fails at its nails, flashings or underlayment before the slate itself, which is why copper flashing and copper nails are standard on slate. The lifespan calculator compares the cost per year with other materials.
- Can slate go on a low-pitch roof?
- Not below about 4/12 by normal practice; at lower pitches water driven up the laps reaches the nail holes. Some specialists use special underlay details down to 3/12, but most simply say slate needs a steep roof. The calculator warns below 4/12.