Slate Roof Calculator

Slate is one of the most traditional and long-lasting roofing materials in the UK. A well-laid natural slate roof can last 100 years or more. But working out how many slates you need isn't just a matter of dividing the roof area by the size of a slate, because each slate overlaps the ones below.

Our Slate Roof Calculator works out the batten gauge, slates per square metre, total slates, battens and nails for your roof. This guide explains the terms and the maths so you can plan with confidence.

Key slating terms

• Headlap: the overlap between a slate and the slate two courses below it. It keeps rain out and is usually between 65mm and 100mm, depending on roof pitch and exposure.
• Gauge: the distance between battens, which is also the amount of each slate left showing. It's worked out from the slate length and headlap.
• Course: a single horizontal row of slates.
• Pitch: the angle of the roof. Slates need a minimum pitch, often 20° to 25° or more, depending on the slate and exposure.

The batten gauge formula

For slates nailed near the top, the gauge is:

Gauge = (slate length − headlap) ÷ 2

For a 500mm slate with a 75mm headlap: (500 − 75) ÷ 2 = 212.5mm.

The number of slates per square metre is then:

Slates per m² = 1 ÷ (gauge in metres × slate width in metres)

For a 500 × 250mm slate at a 212.5mm gauge: 1 ÷ (0.2125 × 0.25) = about 18.8 slates per m².

How to use the calculator (Slate Roof Calculator)

1. Enter the roof length along the eaves.
2. Enter the rafter length from eaves to ridge.
3. Enter the number of roof slopes. A simple gable roof has two.
4. Choose your slate size.
5. Set the headlap, following the manufacturer's guidance for your pitch and exposure.
6. Add an allowance for cuts and breakages, usually around 5%.

The calculator shows the gauge, slates per m², total slates, metres of batten and nails needed.

A worked example

A gable roof 9m long with 4.5m rafters on each side has 81m² of roof. Using 500 × 250mm slates with a 75mm headlap:

• Gauge: 212.5mm
• Slates per m²: about 18.8
• Total slates with 5% extra: about 1,600
• Battens: about 400m
• Nails (two per slate): about 3,200

Try your own measurements in the Slate Roof Calculator.

Choosing a slate size

Larger slates mean fewer pieces to fix but may need a steeper minimum pitch. Natural slates vary in thickness and quality, so choose a reputable source and check they meet the relevant British and European standards.

Natural slate vs fibre cement slate

• Natural slate is quarried stone. It's very durable, attractive and often required for listed buildings and conservation areas. It costs more and is heavier.
• Fibre cement slates are manufactured, uniform and lighter, and cost less. They don't last as long as good natural slate.

The calculator's quantities work for both types, but always follow the manufacturer's fixing guidance.

Extra slates for details

You'll also need breathable roofing membrane, ridge tiles or slates, and lead or other flashings around chimneys and abutments.

Building Regulations and safety

Re-roofing more than a quarter of a roof usually counts as renovating a thermal element, so Building Regulations may require you to improve the roof's insulation where it's practical. A roofer registered with a competent person scheme can usually self-certify the work.

Roofing is one of the most dangerous jobs in construction. The Health and Safety Executive advises proper scaffolding and edge protection for roof work. If you're not experienced, use a qualified roofer.

Measuring your roof safely

You don't need to climb onto the roof to measure it. The length along the eaves can usually be measured from the ground or from the gutter line, and the rafter length can be measured in the loft from the wall plate to the ridge, adding the overhang at the eaves. Alternatively, measure the building's footprint and use the roof pitch to work out the sloping length. Many roofers also use aerial or drone surveys to measure roofs without going up.

Reclaimed slates

Reclaimed natural slates are popular for repairs and older buildings because they match existing roofs. Check that they're sound, of consistent thickness and free from cracks, and order extra to allow for breakages.