Lap of Slates or Tiles not suited to Pitch

In the west of Scotland we see an awful lot of rainwater, to ensure that this rainwater does not enter the building your roof must be designed and installed correctly. The west of Scotland is categorised as having a severe exposure to wind-driven rain and therefore the laps on tiles and slates need to be adjusted accordingly.  

Rainwater does not always fall vertically and run down the roof under the force of gravity. It is often driven over the plane of the roof by the wind, and up and under the overlapping units. The distance that water will travel back under the slate or tile will increase as the pitch decreases. 

The lap between successive courses of slates or tiles must therefore be large enough to prevent driven water from penetrating as far as their top edges and spilling over into the building.


It is important to make sure the tile or slate that has been chosen for the roof is appropriate for the roof pitch. For example in Marleys range the ashdowne plain tile can be laid from 35° to 90°. Whereas the Wessex interlocking tile can be laid on a pitch as low as 15°.  For a lower pitch you may need to choose a different product or material such as the artificial slates which can be installed to a pitch as low as 10°.

Common Mistakes

  • Insufficient lap for exposure and pitch of roof.
  • Battens gauged to suit length of rafter, not the required lap.
  • Small slates used at below 30° rafter pitch.
  • Lap of head-nailed slates not measured from nail hole.
  • Single lap tiles used at absolute minimum pitch.

Real life example


We have personally been on jobs where the tiles have been placed to maximum gauge and the gauge at the top of the roof needed to be stretched to reach the ridge. This resulted in the top courses of tiles being beyond the maximum gauge to the point that the nail holes were visible. This was causing vast damage to the battens and underlay and causing the tiles to slip out of place.

The way we remedied this was by stripping out the tiles and introducing a new course of tiles so that all of the courses were within their maximum gauge limit. By properly setting the roof out at the start of the job this problem would have been prevented from happening.

Annandale Roofing

By choosing Annandale roofing you can be rest assured that your roof is in good hands. We do all the design work to ensure that the roof is not only aesthetically pleasing but is suited to our climate and above all keeps all water from entering into your building.