Roofing Defects: Lap of Tile not Suited to Roof Pitch


Roofing defects: 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.


Example 1

Sometimes contractors will lay tiles at the maximum gauge (usually 345mm) without properly setting out the roof before hand. This can lead to problems if the top course of tiles does not quite meet the ridge.

I have seen this issue first hand as shown in the picture below. The tiles had all been placed to maximum gauge, when the contractor reached the top of the roof they must have realised that the tiles would not meet the ridge as expected. They then proceeded to stretch the gauge of the top five courses so that they would meet 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 only way this type of problem can be rectified is by stripping out the tiles and introducing a new course of tiles so that all of the courses are 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.

Example 2

There was a newly built extension in Ayrshire that was constructed with a low pitched roof. The original roofing contractor made quite a mess of the project by failing on a number of details. The roof was leaking in a number of places and the client decided to fully strip the roof and replace the roofing contractor.

One of the problems revolved around the inadequate selection of tile based on the pitch of the roof. The pitch was around 12°. The roof covering chosen for this project was a smooth Marley Modern tile. The minimum pitch a Marley Modern tile can be laid is 17.5° with a 100mm headlap. So therefore this particular tile is inadequate for this project.

One problem with using a tile which is insufficient for the pitch is that the manufacturers warranty will be null and void if you do not follow the guidelines for installation. You will then be reliant on the contractors warranty, the defects liability on most small projects are usually around 12 months. This is most likely how long it would take for the membrane to fail if you have roof tiles which are allowing water to pass through them.


This issue as well as the defect in example 1 could have been prevented with a clear design and the use of competent contractors. For the surveyor, both of these issues may not be picked up on a first glance. In most cases you will not know if the lap of a slate or tile is incorrect without physically inspecting the roof. If the roof is defective and it turns out that the lap is inadequate or the product used does not suit the pitch, nothing can be advised except an entire re-roof which will be the last thing any client will want to hear.

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