Roof slate and batten calculator

From the roof area, the slate size and the headlap: the batten gauge, slates per m², slates to order, metres of batten and the nails.

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Measure the roof off the drawing with Take-off, and the slates, battens, membrane and nails go into a material order or a quote. Save it to the job.

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Worked example

40 m² of roof slope at 35°, in 500 × 250 mm slates with a 75 mm headlap and 4 mm gaps:

779 slates to order

Batten gauge 212.5 mm, 18.5 slates per m² on 40.0 m² of slope
197.6 m of batten, 1,558 nails (about 5.1 kg)
Your 75 mm headlap meets a typical 75 mm minimum at 35.0° (check the maker's table and BS 5534)

How the sum works

Batten gauge = (slate length − headlap) ÷ 2. Each slate covers the gauge times its width plus the gap, so slates per m² = 1,000,000 ÷ (gauge × (width + gap)). The calculator multiplies by the slope area and adds the waste. Battens run the length of the roof at each gauge, plus 5 percent; nails are two per slate.

If you measured the roof on plan, the slope area is the plan area divided by the cosine of the pitch.

Headlap and exposure

The headlap you need depends on the pitch, the slate and how exposed the site is. The calculator compares yours with a typical minimum for natural slate in moderate exposure. The slate maker's table and BS 5534 have the final word, and fixings in exposed places need checking.

Guides that go with it

How do I work out roof pitch and roof area?Work out roof pitch from rise and run, the rafter length by Pythagoras, and roof area as plan area divided by cos(pitch), with overhangs, hips and valleys.How do I do a quantity take-off from drawings?How to take off quantities from drawings for a small job: scale check, set order, deductions, waste and pricing, with a worked garden room example.Work at Height Regulations 2005: what builders must doWhat the Work at Height Regulations 2005 ask of builders: no minimum height, avoid, prevent, minimise, planning, competence, inspections and fragile roofs.

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