Slate Roofing Calculator

Enter slate size and head lap - the gauge and coverage are calculated automatically

Roof Slopes

Slate Size

Slate Length (mm)

Slate Width (mm)

Head Lap (mm) 75 mm for pitches ≥ 30° · 90 mm at 25-30° · 110 mm at 20-25°

Calculated Gauge & Coverage

- enter slate size above

Waste Factor (%)

10%

10% for straightforward roofs; 15% for complex shapes or cut slates around features

Ridge & Hip Measure the ridge and any hip lengths. Leave blank if not applicable.

Ridge Length (m)

Ridge Unit Length (mm) Lead soakers, dry ridge units or mortar-bedded ridge tiles

Total Hip Length (m) Add all hip rafter lengths together

Felt Underlay

Felt Roll Width (m)

Felt Roll Length (m)

Felt Overlap (mm) Minimum 150 mm side lap

Job Costing (optional)

Cost per slate

Cost per ridge unit

Results

Total Roof Area
-
m²

Slates to Order

inc. waste

Batten Gauge

mm centres

Battens

linear metres

Slates per m²

coverage rate

Nails

approx (2 per slate)

Ridge Units

inc. 10% waste

Felt Rolls

rolls needed

Slope Breakdown

Slope	Eaves Length	Rafter Length	Area	Slates	Battens (lm)

Head Lap by Pitch

≥ 30° - 75 mm minimum head lap
25-30° - 90 mm minimum
20-25° - 110 mm minimum
Below 20° - slate not recommended

Head lap is the vertical overlap between two courses. Increasing it reduces the gauge (closer batten spacing) and increases the slate count.

Common Slate Types

Welsh slate - Blue-grey, the most prestigious natural slate. Very long-lasting, tight grain.
Spanish slate - Black, widely used in the UK. Good value, consistent quality.
Brazilian slate - Green-grey tones. Lower cost but variable quality - check carefully.
Westmorland green - Distinctive green colour, irregular shapes. Used on traditional Lake District buildings.
Fibre cement - Uniform artificial slate. Lighter, cheaper, consistent size. Good for budget projects.
Reclaimed - Add 20-25% waste allowance for sizing and breakage.

Measuring Tips

Eaves length - Horizontal measurement along the guttering line
Rafter length - Measured along the slope from eaves to ridge (not horizontal)
The gauge (batten spacing) is calculated automatically from slate length and head lap
Nails should be copper or stainless steel - galvanised nails corrode and stain slates
Allow 2 nails per slate for standard nailing; 3 nails (head and tail) for exposed locations

Slate Roofing Calculator