Thermal Movement ~ the coefficient of linear expansion for lead is
0.0000297 (2.97¾ 10 †6) for every degree Kelvin.
Eg. If the exposure temperature range throughout a year is from
†10°Cto35°C (45 K), then a 2.000 m length of sheet lead could
increase by: 0.0000297¾ (^45) ¾2 = 0.00267 m, or 2.67 mm.
Over time this movement will cause fatigue stress, manifesting in
cracking. To prevent fracture, a smooth surface underlay should be
used and the areas of lead sheet limited with provision of joints
designed to accommodate movement.
Underlay ~ placed over plywood or similar smooth surface decking,
or over rigid insulation boards. Bitumen impregnated felt or
waterproof building paper have been the established underlay, but
for new work a non-woven, needle punched polyester textile is now
generally preferred.
Fixings ~ clips, screws and nails of copper, brass or stainless steel.
Jointing ~ for small areas such as door canopies and dormers
where there is little opportunity for thermal movement, a simply
formed welt can be used if the depth of rainwater is unlikely to
exceed the welt depth.
25mm
copper clips at 500mm
spacing, nailed to decking
and folded within welt
Welted joint
underlay
decking
Jointing to absorb movement ~
- Wood cored rolls in the direction of the roof slope (see next
page). - Drips at right angles to and across the roof slope (see next
page).
Timber Flat Roof Coverings -- Lead Sheet