Felt and Batten Calculator — Roofing Felt & Battens

Our felt and batten calculator determines the number of roofing underlay rolls and batten lengths needed based on your roof area, rafter length and tile type. It accounts for the BS 5534 underlay laps and the correct batten gauge for your covering.

Underlay calculation

Rolls = Roof area ÷ effective coverage per roll

Underlay is laid in horizontal courses running from the eaves up to the ridge. The 1m roll width runs up the slope, so the head-lap between successive courses (BS 5534 minimum 150mm unsupported, 100mm supported, at pitches ≥15°) reduces the up-slope coverage of each course, and a 100mm side-lap between adjacent rolls (over a rafter) trims a little more.

• Breathable membrane (e.g. Tyvek Supro, Cromar Vent3): 1m × 50m = 50m² per roll, ≈42m² effective after laps.
• Traditional 1F bitumen felt: 15m × 1m = 15m² per roll, ≈12m² effective after laps.

Batten calculation

Batten rows = (Rafter length ÷ Gauge) + 1 (eaves batten)
Total metres = Batten rows × Roof length × 2 sides

Batten gauges (max gauge = tile length − head-lap)

• Concrete Interlocking: 345mm gauge (420mm tile − 75mm head-lap)
• Plain Clay: 88mm conservative gauge (265×165mm tile; max gauge 100mm, min 65mm head-lap)
• Slate: ~200mm gauge ((slate length − head-lap) ÷ 2)
• Large Format Concrete: 375mm gauge

50×25mm treated softwood is the common all-purpose batten, sold in 4.8m lengths. BS 5534 permits 38×25mm for double-lap tiles and single-lap tiles on rafters up to 450mm centres, and requires 50×25mm for single-lap tiles on 600mm spans and for natural slate. We recommend 10% wastage to cover joins and cuts at hips and verges.