Understanding your result
The headline is the field pieces with waste. The stats show the pieces per square, the bundles including starter, the hip and ridge pieces and the weight. The breakdown shows how the count per square was found, then the starter, the ridge and the bundles.
The number to remember is the pieces per square, the figure suppliers quote and the one that changes from brand to brand. The weight is the reason most people choose these products: compare it with natural slate on the slate calculator or with the roof you are replacing on the roof weight calculator.
For the real-wood equivalent of composite shake, the cedar shake calculator uses the Cedar Shake and Shingle Bureau’s bundle rules.
How we calculate this
The geometry is the same as for slate: each piece shows its width times its exposure, and a square is 14,400 square inches. What differs from natural slate is that the exposure is set by the product, not by a headlap rule, so the data sheet is always the better source. The standard sizes table lists common piece sizes, and the material weights table puts composite products beside the materials they imitate.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Piece width and exposure | 12 in at 7½ in | A common single-width synthetic slate; check your product |
| Pieces per bundle | 20 | Typical; brands range from about 10 to 25 |
| Weight | 300 lb per square | Composite slate and shake makers’ data (250 to 400 lb) |
| Hip and ridge | 2 pieces per foot | Typical 6-in exposure on caps |
| Nails | 2 per piece | Manufacturers’ installation manuals |
| Waste | 10% | Estimating practice |
Assumptions last reviewed October 7, 2026.
The calculator does not count the underlayment (some products want ice and water membrane over the whole deck), the flashings or the vent pieces, and it does not know the brand’s minimum pitch. The guide on tile and slate weight explains where weight becomes a structural question.
Two worked examples
A gable roof from piece size
2,000 square feet, 12-inch pieces at 7½ inches, 20 per bundle, 300 pounds per square, 80 feet of eaves, 40 feet of ridge, 10% waste.
- Per square: 14,400 ÷ 90 = 160
- Field: 20 × 160 × 1.10 = 3,520 pieces
- Starter: 84; ridge: 84
- Bundles: (3,520 + 84) ÷ 20 = 181; weight: 3 tons
Three tons against seven for natural slate on the same roof.
A hip roof from the data sheet
2,600 square feet, 110 pieces per square, 12 per bundle, 400 pounds per square, 100 feet of eaves, 90 feet of hips and ridge, 15% waste.
- Field: 26 × 110 × 1.15 = 3,289 pieces
- Starter: 105; hip and ridge: 189
- Bundles: 3,394 ÷ 12 = 283; weight: 5.2 tons
The hips cost almost 200 cap pieces, a tenth of a bundle per foot.
Where to find your inputs
Roof area. The sloped area from the roof area calculator.
Piece size, exposure and pieces per square. On the product’s data sheet or installation manual.
Pieces per bundle and weight. On the bundle label or the data sheet.
Common mistakes
- Using natural slate counts. Composite products have their own exposure. Use the data sheet.
- Mixing brands. Starter, field and caps from different lines do not match and void the warranty.
- Skipping the starter. The first course needs its own pieces.
- Wrong nails. Use the fastener the manual calls for, usually stainless or hot-dipped galvanized.
- Below the minimum pitch. Most products need 4/12.
- Nailing too tight. Composite pieces expand in heat; the manual sets the nail depth.
Questions people ask
- How many synthetic slates per square?
- Divide 14,400 square inches by the area each piece exposes, its width times its exposure. A 12-inch piece at a 7½-inch exposure exposes 90 square inches, so a square takes 160 pieces; mixed-width products use the average width. Data sheets give the exact count, often 100 to 170 per square depending on the product.
- How much does synthetic slate weigh?
- Most composite slate and shake products weigh 250 to 400 pounds per square, close to architectural asphalt shingles and a third or less of natural slate (700 to 1,400) or concrete tile (900 to 1,100). That is why they are chosen for roofs that cannot carry the real thing; confirm with the maker for your structure.
- How many bundles of synthetic slate do I need?
- Pieces divided by pieces per bundle: 3,604 field and starter pieces at 20 per bundle is 181 bundles. Bundle counts vary widely by brand, from about 10 to 25 pieces, so read the label. Hip and ridge pieces are usually sold in separate bundles.
- Do synthetic slates need starter and ridge pieces?
- Yes. A starter course along the eaves, usually cut or purpose-made pieces, and hip and ridge caps from the same product line at about two per foot. Using the maker's own accessories keeps the warranty.
- What pitch do synthetic slates need?
- Most products are rated for 4/12 and steeper with standard underlayment, some down to 3/12 with ice and water membrane over the whole deck. The installation manual governs, and the warranty depends on it.
- How long does synthetic slate last?
- Manufacturers warrant most composite slate for 40 to 50 years, and some offer lifetime limited warranties. Real-world life depends on UV exposure and installation. The lifespan calculator compares cost per year with natural slate, tile and asphalt.