The roof covering is a building’s first line of defence against the weather. Scottish slate is a long-lasting roofing material, and although slate roofs have a finite life, a lifespan of up to – or beyond – 100 years is quite common.
It’s wise to inspect slate roofs regularly to spot potential problems as soon as they appear. Regular maintenance and repairs will ensure that your slate roof remains watertight and continues to perform well.
Look out for:
broken, cracked or missing slates on the roof or ground, especially after bad weather
dislodged slates at roof edges – slates here are particularly vulnerable
A single missing slate will let in only a limited amount of water. But if not put back in place, the sarking (wooden boards) beneath will decay. This will then lead to further loss of slates. Poor repairs will only make the situation worse.
‘Nail sickness’ – one of the most common causes of problems with slate roofs – occurs when the iron nails used to secure slates to the sarking rust through. Individual slipped slates can be secured again, but there may come a point where the number of slates affected means this is no longer cost effective. Instead it may be better to strip back and reslate the entire roof.
Slate hasn’t been quarried in Scotland since the 1950s and supplies for replacement purposes are very limited. So it’s best to reuse slates for maintenance and repairs wherever possible.
Many traditional Scottish buildings have roofs covered with local slate. Scottish slate has a good reputation as a long-lasting material. Recent research supports this belief: many slates are in service for 100 years or more. Regular maintenance and repairs to their fixings and support is vital in achieving longevity.
The relatively small and thick Scottish slates were made in a variety of lengths and widths. Slates were laid in diminishing courses, with the largest slates placed at the base of the roof, and smaller ones at the top. This was the most cost-effective method, as it made best use of all of the slate quarried. And it gave us the unique skylines we enjoy today.
Slate roof designs
Each roof slope bears the mark of the quarry that supplied the material. The exact pattern in which the slates are laid, and their colour and textures are all highly distinctive. This means it’s possible to spot neighbouring buildings roofed with material from the same source – and those that are not.
Scottish roofs tend to be steep, with slopes of around 40°. Steeper pitches are less prone to letting wind-driven rain or snow enter the roof space. Water quickly runs down the slope, also making it less likely to get inside. This design also offers good headroom in the attic, often used as living space.
The variable size of Scottish slates makes them well suited to steeply pitched and intricately shaped roofs. This may well have shaped the development of other typical slate roof features such as turrets and near-vertical slopes.
Regular checks let you spot and tackle problems before they become worse – and before repair costs rise.
Always inspect your slate roof at least once in the autumn, before the winter weather sets in. You should also check a slate roof after any storms or gales.
Check the roof from outside, standing at ground level. Using binoculars will help you to see the slates in more detail. Note down any concerns or signs of problems to discuss with the contractor who will carry out the repair work.
You should also check the internal roof space for signs of leaks, for example, staining of the roofing joinery. Such signs may not be obvious from the outside but may point to external problems.
Like all natural materials, some Scottish slates can undergo changes that make them more likely to break. Your roofing contractor can test for this change by holding a loose single slate at its edge and gently tapping it in the centre with a hammer. A sound slate will ring out clearly, while a porous slate will produce a dull sound. Slates that are no longer sound should be replaced.
Slates may also break due to:
being lifted by the wind
nails driven in too far – the slate is held too firmly against the sarking
nails not driven far enough into the sarking – their heads stand proud, which may damage slates in the rows above
Looking along the bottom of the slates in a single row is a good way to work out how many are damaged, missing or slipped. This process should be repeated with each row across the roof, and a note taken of the total number of slates in a row and how many are affected.
Any slates that are out of line are probably loose and in need of attention. If more than 20 to 25% of the total roof covering is affected, it usually makes more sense to reslate the whole roof than replace only individual slates.
Problems with a slate roof most commonly stem from slate sizes being selected badly, impact damage and nail sickness.
Water can enter the roof space if slates have been badly selected and laid. For example, if the coverage or overlap isn't great enough to stop wind-driven rain getting in.
For example, larger slates are needed:
at the bottom of the roof where more water flows over them
for shallower roof slopes where the water runs off more slowly, and is more likely to seep through joints
Wider slates are required at roof junctions so that they can be better secured.
Whenever the roof is accessed to undertake repair work, there’s a risk that slates will be broken. Specialist roof ladders or properly protected scaffolding should be used to reduce the risk of standing directly on the slates and breaking them.
Slates were often fixed to sarking (wooden boards) using iron or poor quality galvanised nails. ‘Nail sickness’ occurs when the nails rust through, and causes individual slates to slip.
It’s unlikely that all the nail fixings across an entire roof would fail at the same time. But as more and more nails decay, numerous slates may become loose and slip out of place. Eventually it may be more practicable and cost effective to reslate the whole roof than fix all of the affected slates individually.
Slippage may also be caused by decay of the slate around the nail hole or decay of the sarking. In such circumstances, the roof must be completely stripped and reslated.
Care should be taken to retain as many sound original slates as possible. These can often be resized and reused.
Regular maintenance of a slate roof should be relatively inexpensive. The occasional replacement of a handful of slates is usually all that’s necessary.
Slates are traditionally fixed to the sarking (wooden boards) by a single nail through a hole at the slate head. This is enough to hold the slate in place and stop it being lifted by the wind. Water is less likely to get through the nail hole if the slates are also properly overlapped.
Individual slates can be easily swung aside so that a broken slate beneath can be removed and replaced. The overlapping slates can then be swung back into their original positions over the new slate to keep the roof watertight.
Every sixth row of slates tends to be cheek nailed (double nailed). Slates in these areas can't be swung aside.
Reuse vs replacement
It’s best to reuse slates wherever possible. Slate hasn’t been quarried in Scotland since the 1950s, so supplies of slate for replacement purposes are becoming very scarce.
Any replacement slates must be selected carefully, paying attention to:
A replacement that’s slightly lighter in colour will blend in better than a darker slate if an exact match can’t be found.
Choosing a roofing contractor
It’s best to use a local roofing contractor who is familiar with local slating traditions and experienced in local techniques.
You can ask to see photographs of previous work or to speak to other clients to check that a contractor has the right background. It may even be possible to visit a site in person to help you assess their competence. But be sure to check that the same workforce will be used for your own repair.
When inspecting a roofer’s work, check that:
slates are laid evenly and sit well across the roof
rows of slates get smaller, in a regular fashion, from eaves to ridge
If you’re planning to do major reslating work, you should check whether you must first apply for planning permission or any other type of consent. Listed building consent or conservation area consent may be needed, depending on the property and its location. Contact your planning authority to find out more.
Great care should be taken when removing the existing slate from the roof. Minimising breakages will allow for more of these slates to be reused. If a large quantity of new slate is required, you should seek the advice of your planning authority on the most appropriate material to use for re-roofing.
A detailed record should be made of the existing roof covering before it’s dismantled, in case it’s not possible to source enough Scottish slate to replicate the original roof.
Slate roofs really came into their own in the 19th century. The skylines we still enjoy today feature many designs that were created at this time, often involving steep pitches, intricate shapes and even turrets.
Around 1800, improved woodcutting techniques made it possible to mechanically saw timber into thin boards to cover structural roof timbers. This 'sarking’ allowed slates of different sizes to each be secured to a roof using a single nail.
Previously, slates had been hung on timber battens using wooden pegs to create a roof covering.
Sarking had the advantages of:
increased structural stability
better draught proofing and insulation
improved resistance to water penetration
Also during the 19th century, slates were trimmed into scalloped and diamond shapes before fixing. With careful sizing, these individual pieces were combined to create complex architectural patterns across entire roof surfaces.
The Engine Shed is accredited by the Scottish Qualifications Authority (SQA) and National Open College Network (NOCN).
The Engine Shed has been supported by a grant from the National Lottery Heritage Fund. Thanks to National Lottery players, we have been able to share knowledge of traditional building materials, develop skills and raise standards in conservation for traditional buildings.