How much roofing felt do I need per m²?

A standard roll of roofing felt (breathable membrane) is 15m × 1m giving 15m² per roll. However, with the required 150mm horizontal overlaps, effective coverage drops to approximately 12.75m² per roll. Always allow 10% extra for wastage and vertical overlaps at joins.

What size battens are used for roofing?

Standard UK roofing battens are 25mm × 50mm (1" × 2") treated softwood, compliant with BS 5534. Counter battens (used over the membrane on fully ventilated roofs) are typically 25mm × 38mm. Battens are usually sold in 4.8m lengths from builders' merchants.

What is the correct batten gauge for my tile?

Batten gauge (the spacing between battens) depends on your tile type. Concrete interlocking tiles typically need a 345mm gauge, plain clay tiles need 100mm (due to double-lap), natural slates around 200mm, and large format concrete tiles around 375mm. Always check the manufacturer's specifications for your specific tile.

Do I need breathable membrane or traditional felt?

Modern UK practice strongly favours breathable roofing membranes (such as Tyvek or Cromar Vent3) over traditional bitumen felt. Breathable membranes allow moisture vapour to escape from the roof space while remaining waterproof, reducing the risk of condensation. They also eliminate the need for eaves ventilation in most cases. BS 5250 covers condensation control in buildings.

How do I calculate battens for a hip roof?

For a hip roof, the batten lengths are shorter on the hip ends, reducing the total amount of batten needed. However, there is more cutting waste at the hips. As a rule of thumb, calculate battens as if the roof were a simple gable (full-length rows) and the wastage allowance will cover the difference. For accuracy, increase wastage to 12-15%. Use our Roof Area Calculator to convert plan dimensions to actual sloping area.

Felt and Batten Calculator — Roofing Felt Rolls and Batten Lengths

Felt and Batten Calculator — Roofing Felt & Battens

Calculate roofing felt rolls and batten lengths for any UK roof. Includes correct batten gauge for your tile type.

Roof Area (if known)

Leave as 0 to use length × rafter below

Tile Type (for batten gauge)

Roof Length

Length along the ridge

Rafter Length

Eaves to ridge (sloping distance)

Include wastage allowance

Adds 10% extra for overlaps and cuts

Wastage Percentage

10% standard, 15% for complex roofs

Price Per Felt Roll (optional)

Per 15m × 1m roll

Price Per Batten Length (optional)

Per 4.8m length (25×50mm)

How We Calculate This

Our felt and batten calculator determines the number of roofing felt rolls and batten lengths needed based on your roof area, rafter length and tile type. It accounts for felt overlaps and correct batten gauges.

Felt calculation

Rolls = Roof area ÷ 12.75 m² (effective coverage per roll)

Standard rolls are 15m × 1m = 15m². With the required 150mm horizontal overlap, effective coverage is 15m × 0.85m = 12.75m² per roll.

Batten calculation

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

Standard batten gauges

Concrete Interlocking: 345mm gauge
Plain Clay: 100mm gauge (double-lap)
Slate: 200mm gauge
Large Format Concrete: 375mm gauge

Battens are standard 25×50mm treated softwood sold in 4.8m lengths. We recommend 10% wastage to cover joins and cuts at hips and verges.

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Felt and Batten Calculator — Roofing Felt & Battens

How We Calculate This

Felt calculation

Batten calculation

Standard batten gauges

Frequently Asked Questions

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