Leadwork
Lead flat roof or gutter bays: drips and rolls from LSA maximum sizes
- where
- bays down the fall
- roof length in the direction of fall (m)
- maximum spacing of drips (mm)
- bays across the fall
- roof width across the fall (m)
- maximum spacing of rolls or seams (mm)
- bay length between drips (m)
- bay width between rolls (m)
- total number of bays
Explain this equation
Divides a lead flat roof or gutter into bays so that no piece is bigger than the Lead Sheet Training Academy maximum for the code. Use it when setting out drips and rolls.
Lead moves a lot with temperature, so each piece is limited in size. D_m is the maximum length between drips (down the fall) and R_m the maximum width between rolls (across it). Dividing the roof dimensions (in millimetres) by those limits and rounding up gives the fewest bays that stay within them; then the roof is split equally. The total number of bays is the product.
Rounding up the number of bays, rather than the bay size, guarantees every bay is at or under the maximum.
Sense check: a 4 m fall with 2.25 m maximum drip spacing gives 2 bays of 2 m; a 3 m width with 675 mm rolls gives 5 bays of 600 mm, so 10 bays.
Watch out: every drip needs a step in the deck, typically 50 to 60 mm depending on the detail, so check the deck has enough depth for the number of drips.
Splits a lead flat or gutter into equal bays no bigger than the Lead Sheet Association maximum for the code, to limit thermal movement.
Enter your values to see the result.
Good to know
Enter the maximum sizes for your code from the LSA tables. For flat roofs the published maxima rise with code, from 2000 mm between drips and 600 mm between rolls up to 3000 mm and 750 mm; 2250 mm and 675 mm are the figures generally used for code 6. Each bay size here is the finished size: add the roll undercloak and overcloak and the drip turn-downs to get the lead size to cut. Lay the roof to fall, at no flatter than 1:80 finished.
Common mistake
Laying long bays in a light code to save drips. Oversized bays fatigue and crack from thermal movement.
Limits
Rectangular area with one direction of fall. It gives finished bay sizes only, not cut sizes, and does not check drip heights or the extra fall the drips need.
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.