How Many Composite Slates Do I Need?

How We Calculated This

For a typical UK gable roof with a 7m ridge and 5m rafters (eaves to ridge), pitched at 35°, the sums work like this:

• Roof area: 7m × 5m × 2 slopes = 70m²
• Coverage rate: at pitches above 31°, a 445 × 295mm composite slate is laid at a 190mm batten gauge, giving 18 slates per m²(manufacturer’s pitch table)
• Slates needed: 70 × 18 = 1,260 slates
• With 10% wastage: 1,260 × 1.10 = 1,386 slates

This is exactly how our roof tile calculatorworks it out — pick the custom tile option, enter your slates-per-m² figure from the table below, and it applies the wastage on top.

The Cover Width × Gauge Formula

Every slating quantity comes from two numbers: the cover width (how much width each slate contributes to a course) and the gauge (the batten spacing, which sets the exposed length of each course). The formula is:

• Slates per m² = (1,000 ÷ cover width in mm) × (1,000 ÷ gauge in mm)
• Example: (1,000 ÷ 295) × (1,000 ÷ 190) = 3.39 × 5.26 ≈ 17.8, which manufacturers round to 18 slates per m²(each slate covers 0.295m × 0.190m ≈ 0.06m² of roof)

Because composite slates are laid to a pitch-dependent gauge, the coverage changes with the roof pitch. The published table for a standard 445 × 295mm cover-width composite slate (Britmet LiteSlate — TapcoSlate Classic publishes the same 22/20/19/18 coverage using imperial exposure markings of 6″, 6.5″, 7″ and 7.5″) is:

• 12–24° pitch: 152mm gauge — 22 slates per m²
• 25–27° pitch: 165mm gauge — 20 slates per m²
• 28–30° pitch: 178mm gauge — 19 slates per m²
• Above 31° pitch: 190mm gauge — 18 slates per m²

A low-pitch roof therefore needs 22 ÷ 18 ≈ 22% more slatesthan a steep roof of the same area — always confirm the pitch before ordering.

Headlap and BS 5534

BS 5534:2014+A2:2018 (the British Standard code of practice for slating and tiling) works on a simple principle: the lower the pitch, the greater the lap must be, because shallow roofs shed water more slowly and are more vulnerable to driven rain. On composite slates the pitch-banded gauge does that job. With a 445mm-long slate laid broken-bond, each slate laps the slate two courses below by the slate length minus twice the gauge:

• Steep roof (above 31°): 445 − (2 × 190) = 65mm effective headlap
• Low pitch (12–24°): 445 − (2 × 152) = 141mm effective headlap

Proprietary composite slates sit outside BS 5534’s natural-slate tables, so the manufacturer’s fixing specification is the governing document. The two leading UK brands both specify a 12° minimum pitch, 50 × 25mm treated battens, and two fixings per slate (minimum 30mm long with a 10mm-diameter head).

Composite vs Natural Slate Quantities

Our roof tile calculator’s natural-slate preset uses 500 × 250mm slates at a 100mm headlap. Double-lap slate gauge = (length − headlap) ÷ 2 = (500 − 100) ÷ 2 = 200mm, so slates per m² = (1,000 ÷ 250) × (1,000 ÷ 200) = 4 × 5 = 20 per m². On a steep roof a 445 × 295mm composite slate needs slightly fewer pieces (18 per m²), and at roughly 12kg per m² as laid(12–14.5kg/m² depending on brand and gauge) it is light enough for structures that could never carry stone or concrete.

Tips for Ordering

Allow 5–10% wastageon top of the calculated quantity — 10% is the sensible default for slates because of cuts at verges, hips and valleys, and because the eaves/starter course uses slates cut down (LiteSlate starter slates are cut at 305mm). Ridge and hip cappings are separate accessories — allow about 5.5 ridge pieces per linear metre of ridge.

Composite slates are commonly sold in packs of 22–25, so round up to whole packs (1,386 slates = 63 packs of 22). On price, treat anything you read as indicative only: mid-2026 merchant list prices sit around £2.50–£3.50 per slate(for example, packs of 25 from £66.96 ex VAT at national online merchants), but this varies by brand, colour, region and VAT status — always get a current quote. Finally, order all the slates for one roof in a single batch so the colour blend matches across every pack.