How Much Felt and Batten for a Roof?
Quick Answer
4 rolls of breathable membrane + ~145 linear metres of batten
Based on: 50m² roof area, 15m² effective coverage per felt roll (150mm laps), battens at 345mm gauge for concrete interlocking tiles
Roofing Felt / Breathable Membrane
Modern UK roofs use breathable roofing membrane (BRM) rather than traditional bituminous felt. A standard roll is 1m wide x 50m long = 50m² gross area. However, with 150mm horizontal overlaps and 300mm end laps, the effective coverage is approximately 15m² per roll in practice (accounting for overlaps and waste around edges).
More precisely:
- Gross area per roll: 50m²
- Effective width after overlap: ~0.85m (1m minus 150mm lap)
- Effective coverage per roll (with end laps and waste): ~12–15m² depending on rafter length
- For 50m² roof: 50 ÷ ~13 = 3.8 rolls, round up to 4 rolls
Roofing Battens
Battens are horizontal timbers fixed across the rafters at a set spacing (gauge) to support the tiles. Standard batten size is 25mm x 50mm(or 25mm x 38mm for slating). Battens are sold in 3.6m or 4.8m lengths.
Calculating Batten Quantity
The batten gauge (spacing between battens) depends on the tile type. For concrete interlocking tiles, the standard gauge is 345mm:
- Number of batten rows: roof rafter length ÷ gauge. For a 5m rafter: 5 ÷ 0.345 = ~14.5, round to 15 rows
- Assuming a 10m ridge length (two 5m-deep slopes = 50m² each side): 15 rows x 10m = 150 linear metres per slope
- For a simple gable roof (50m² total, two 25m² slopes): approximately 145 linear metres total
In practice, for 50m² of roof area you need approximately 2.9 linear metres of batten per m². So: 50 x 2.9 = 145m.
Batten Gauge by Tile Type
- Concrete interlocking (e.g. Marley Ludlow Plus): 345mm gauge
- Plain clay tiles: ~100mm gauge (double-lap tiles)
- Natural slate (500x250mm): ~190–210mm gauge
- Large format interlocking: ~330–345mm gauge
A shorter gauge means more rows and more batten per m². Plain clay tiles at 100mm gauge need roughly 10 linear metres of batten per m².
Counter-Battens
If using a breathable membrane, counter-battens (vertical battens over the rafters) are not always required but are recommended for ventilation. Counter-battens are typically 25mm x 38mm, one per rafter at 600mm centres. For a 50m² roof: roughly 17 rafters x 5m = 85 linear metresof counter-batten.
Fixings
- Batten nails: 65mm x 3.35mm galvanised. Approximately 2 nails per batten intersection = ~250 nails for this roof
- Membrane staples or cap nails: as needed to hold membrane before battening
Tips
Always use BS 5534-compliant graded battens (blue-striped). Untreated battens from timber merchants are not suitable for roofing. Lay membrane from eaves upward with the print facing out. Allow a 25mm drape between rafters for drainage.