Environmental Design Considerations

During the process of installing a slate roof it is necessary to take into account the environment of the building, of which the main factors are the site's exposure to wind and rain. This together with the pitch of the roof will affect the design of the roof.


Site Exposure

The degree of exposure to driving rain determines the minimum lap which should be specified. The anticipated degree of exposure is given in the map below.

Pitch of the roof

In general the lower the pitch of the roof, the greater should be the slate lap. This longer lap assists in resisting wind uplift and capillary action.

Slate Lap

The lap of the slate is determined by taking into account the wind uplift and exposure to rain and the pitch of the roof. The following table gives the recommended minimum laps for various roof pitches and exposures:

  Driving Rain Exposure: <56.5 l/m2 Per Spell (Moderate)
Slate size 20° 22.5° 25° 27.5° 30° 35° 40° 45°
600 x 300 - - 95 85 80 70 60 55
500 x 300 115 105 95 85 80 70 60 55
500 x 250 - - 100 85 80 70 60 55
450 x 220 - - - - 80 70 60 55
400 x 250 - - - - 80 70 60 55
400 x 220 - - - - 80 70 60 55
400 x 200 - - - - 80 70 60 55
350 x 200 - - - - 80 70 60 55
300 x 200 - - - - 80 70 60 55
  Driving Rain Exposure: >56.5 l/m2 Per Spell (Severe)
Slate size 20° 22.5° 25° 27.5° 30° 35° 40° 45°
600 x 300 - - - - 120 110 90 85
500 x 300 - 130 120 110 100 90 80 70
500 x 250 - - - - 120 105 100 95
450 x 220 - - - - 130 125 110 110
400 x 250 - - - - 100 90 80 70
400 x 220 - - - - 100 90 80 70
400 x 200 - - - - - 110 105 100
350 x 200 - - - - 100 90 80 70
300 x 200 - - - - 100 90 80 70
Notes:
A) The recommendation for laps given might not be adequate at roof pitches of less than 30 Degrees or less under the following conditions:
(i)In driving rain exposures of less than 56.5 l/m2 per spell for rafter lengths greater than 9m

(ii) In driving rain exposures of 56.5 l/m2 per spell or greater for rafter lengths greater than 6m in these circumstances please consult our technical department. B)Where abnormal weather conditions might be expected - eg; on elevated sites near the coast, in localities where heavy snow falls are commonly experienced or in conditions of severe exposure, the recommendations for lap will not always ensure full protection. The specifier should seek guidance from a competent person conversant with local conditions.

Further information can be obtained from BS 5534:2003 Code of practice for Slating and Tiling.

To download a copy of the Stoneleaf Slates Headlap Tables please consult the Technical Library.

Approximate wind driven rain map

All in all, natural slate is the most aesthetically pleasing, reliable and cost effective roof covering to use.