Battens and underlay
Underlay rolls with horizontal laps
- where
- courses of underlay per slope
- slope length to cover (m)
- horizontal lap (mm)
- roll width (m)
- total underlay length (m)
- slope length along the eaves (m)
- number of slopes like this
- end lap and ridge allowance (%)
- rolls to order
- roll length (m)
Explain this equation
Works out how many horizontal courses of roofing underlay each slope needs, the total length, and the number of rolls.
Underlay runs along the roof in strips. Every course after the first loses the horizontal lap h to the one below, so each extra course adds only 1000 W − h of cover. The first course covers its full width, which is why h is taken off the slope length too. Rounding up gives whole courses. Each course is as long as the eaves E, repeated for every slope, plus an allowance v for end laps and the ridge. Dividing by the roll length gives rolls.
This is the standard laps arithmetic: n courses cover n × (width − lap) + lap.
Sense check: a 5 m slope with 1 m rolls and 150 mm laps needs 6 courses. Two 10 m slopes then need about 126 m, or 3 rolls of 50 m.
Watch out: laps get bigger at lower pitches; use the figure for your pitch from BS 5534 or the underlay maker.
Underlay is run horizontally from the eaves up, each course lapping the one below. This finds the courses per slope, the total length and the number of rolls.
Enter your values to see the result.
Good to know
Each course after the first adds one roll width less the lap. Minimum horizontal laps depend on the pitch and whether the underlay is supported: one leading underlay guide gives 150 mm above 22°, 200 mm from 15° to 22°, and 225 mm (unsupported) at 12.5° to 15°. Laps should be held under a batten. Avoid vertical joins; where unavoidable, lap over at least one rafter bay. At the eaves the underlay is dressed into the gutter over an eaves support tray.
Common mistake
Dividing the slope length by the full roll width and forgetting the laps, which under-orders by one course on most slopes.
Limits
Rectangular slopes. Hips and valleys need extra for the cut triangles and for the valley and hip strips; add to the allowance.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the roof covering manufacturer’s fixing specification, BS 5534, BS 8000-6, BS 6229, the Lead Sheet Training Academy guidance and Building Regulations. Wind uplift and structural design need a competent designer.