Felt and Batten Calculator — Roofing Felt & Battens

It depends on the product. A modern breathable membrane (such as DuPont Tyvek Supro or Cromar Vent3) is sold as a 1m × 50m roll = 50m² per roll. After the BS 5534 head-lap between courses (min 150mm) and the 100mm side-laps between rolls, effective coverage is roughly 42m² per roll, so one 50m² roll covers about 42m² of roof. Traditional type 1F bitumen felt comes on a smaller 15m × 1m = 15m² roll, covering roughly 12m² after laps. Always allow about 10% extra for wastage and cuts.

50mm × 25mm treated softwood is the common all-purpose tile batten, sold in 4.8m lengths from builders' merchants. BS 5534 actually grades batten section by support spacing: 38mm × 25mm is acceptable for double-lap clay/concrete tiles and for single-lap tiles on rafters up to 450mm centres, while 50mm × 25mm is required for single-lap tiles on 600mm spans and for natural slate. All graded battens must be marked and have minimum 25mm depth (±3mm width tolerance at 20% moisture). Counter battens, used over the membrane on fully ventilated roofs, are typically 25mm × 38mm.

Batten gauge (the spacing between battens) depends on your tile type and is set by the manufacturer, not by a single BS 5534 figure. The maximum gauge equals the tile's covering length minus its minimum head-lap. A 420mm concrete interlocking tile (e.g. Marley Modern) with 75mm head-lap gives a 345mm maximum gauge. A 265×165mm plain clay tile has a maximum gauge of 100mm with a minimum 65mm head-lap; this calculator uses a conservative 88mm (about 90mm lap), which orders slightly more battens than the maximum spacing. Natural slate gauge = (slate length − head-lap) ÷ 2, around 200mm for a 500×250 slate with ~100mm lap. Large format concrete tiles are around 375mm. Always confirm the gauge against your tile's data sheet. Use our Roof Tile Calculator to estimate tile quantities once you know the gauge.

Yes. If you already know the total sloping area (from plans, a surveyor's report, or our Roof Area Calculator), enter it in the Roof Area field and it takes priority over the length × rafter inputs. For underlay this is exact for any roof shape, however many planes it has, because rolls are ordered per square metre: measure each plane, add the areas and enter the total. Battens are shape-dependent though: their row count comes from rafter length and ridge length, so if you enter only an area the calculator estimates battens by assuming a typical gable proportion (ridge about twice the rafter length). On a complex or multi-plane roof you will get a more accurate batten figure by also filling in the real roof length and rafter length, or by running the calculator once per pair of matching slopes and adding the results.

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.

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.