Roofconsult Website

Identifying Broken Double Lap Slates on a Roof - by Chris Thomas

Contact Us
Industry News
Check out our web directory of the UK roofing and cladding industry

Sign up for our monthly news letter.

A good double-lap slated roof can be a good and faithful servant, looking good and keeping out the elements over a long period of time. A poorly constructed double-lap slated roof containing broken slates can be an ongoing nightmare. Identifying the broken slates can be part of the nightmare.
Access to the roof for inspection purposes must be safe and within the health and safety risk guidelines laid down for all roofing work. To do a proper inspection and repair, the roof will need to be divided into sections and every slate checked from the ridge down to the eaves, marking, removing and replacing defective slates as the inspection proceeds. Returning to the same section of roof at a later date to undertake the removal or replacement of the slates could result in further breakage.
Some broken slates are easy to see as they have either split down the length of the slate or have slid down the roof into the gutters. Others however will be much more difficult to find as they are not visible, having broken above the nail fixings.
Hidden breaks
To find the broken slates a thin blade (1-2mm thick) should be slid under the leading edge of the slate, and twisted to lift the slate. If the slate is good it will not lift. If however it does lift with very little effort, and does not slide down the roof, it is probably broken above the nail holes. If it swings sideways it probably has a diagonal break starting above one nail fixing and finishing below the other. If the slate slides down the roof it has either broken below or between the nail fixing holes, or has not been nailed at all. Picture 1
Using a thin blade to lift the leading edge of a slate will not break a good slate unless extreme force is used
Whilst un-nailed slates on a roof may sound unlikely, some roofers have been known to replace slates using mastic or silicone sealant. The mastic/sealant may stick very well to the surface of a slate, but unfortunately with natural slates the laminar structure of the slate means that the mastic/sealant may only be adhered to the outer surface flake. Resin slates should be better in this respect as they are not laminar, however to prevent them from sticking in the mould during manufacture, silicone release agents are used. Traces of the silicone remain on the surface of the slates, which will discourage the mastic from sticking to it.
Slates fixed with mastic may be easily identified by traces of the material showing around the edges. Alternatively by using the thin blade technique, the blade will be difficult to slide between the slates. Once in position the slates will ping apart before any twist is applied. If the adhesion between the mastic and the slate is impossible to break, consideration for the long-term life of the mastic relative to the design life of the roof should be considered.

Picture 2

The adhesion life of the mastic needs to be longer than the design life of the roof.
Hairline Fractures
It is possible that a slate that has been walked on has been fractured but has not failed. In this instance it may be possible to see the fracture as a hairline, especially while a roof is drying out. Water in a crack will take longer to evaporate than from the surface of a slate. The long-term effect of ice expanding in the crack will eventually cause the hairline to develop into a broken slate. Where this is not possible top see the hairline fracture, they can be detected by sliding a thin blade under the leading edge of the slate, and twisting it, the fracture will show itself, or the stress created will break the slate. A fractured slate will need very little effort to break it. Picture 3
Type of fracture
It is possible to identify whether a broken slate was caused by a natural fissure in the material or caused by an imposed load such as hitting or walking on a slate. By looking at the shape of the fractured edge a difference can be seen. If the fracture is straight it is likely to be a natural fissure. If it has a combination of a convex curve and a vertical edge, it is likely to be a stress fracture caused by an imposed load. With resin slates a natural fracture is impossible, so all breakages are likely to be stress fractures.

Picture 4

Picture 5

Having identified the broken, fractured or glued slates, and the possible cause of the problem, all the defective slates will need to be replaced to make the roof weather tight. By working systematically backwards across and down each section of roof, it can be returned to a safe long lasting condition.
Home > Articles