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Slate Manual – Page 9

Design Considerations

The roof covering must be weatherproof, and so constructed as not to allow rain or snow into any parts of the building structure which would be adversely affected by water. Underlay beneath the roof covering serves as an additional safeguard. Vulnerable points are always at the valley, where the pitch is less than that of the common rafters; at the eaves, where sprockets may reduce the effective pitch of the roof slating; and at abutments or verges. Care should be taken to follow the recommended procedures at these parts of a roof.

Selection of the correct size of slate for the roof pitch is most important. The overall height of the individual slate has also to be considered in conjunction with the width and the lap. Additional consideration must be given to the degree of exposure of the building.

Water tends to run downhill by the most direct path, but when it enters the narrow vertical joints between slates, it tends to creep in a fanwise direction. This creeping, or spreading, is caused by capillary attraction between the smooth surfaces of the slates. The extent of the spreading is known as the ‘Angle of Creep’ and is relatively small on steep pitched roofs, but increases quite considerably on flatter pitches where water runs away more slowly.

The slate nail hole should always be formed as close to the side of the slate as practicable, which is then covered by the next course of slates. Therefore the bond (or width), together with the lap of the slate, have to be related to the roof pitch and ‘Angle of Creep’.

The basic rules regarding selection of the size of roof slate and lap are as follows:

  1. The bond of the slate should never be less than the lap.
  2. Small size (lightweight) slates should be used on steep pitch roofs and large (heavy) slates on flat pitches.
  3. Wide slates with increased lap should be lain on flat pitched roofs.
  4. In exposed positions the lap should be increased.