Nail Sickness

Terms like “nail sickness” are evocative. Whenever a slate slips off a roof, there is a suspicion that the roof is suffering from this mysterious affliction that brought along by old age. We see our own health decline in old age and assume that buildings do the same.

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Slipped slate on traditional highland roof inspected by MAAC Studio conservation architects

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Expressions like “Nail Sickness” are catchy, and people latch on them, but they are not very helpful. If you ae not careful, you will end up paying for a lot of expensive roof work that is entirely unnecessary or could even do more damage.

So lets breakdown the issue, why would a slate slip off your roof.?

The slates are held in place by nails, so perhaps the nails have failed. If the slates are still held in place by the original iron nails, when the iron corrodes the shank and head of the nail are sheered off. The slate is no longer held in position and it slips. The question to ask is, why did the nail corrode.? Corrosion requires moisture and it may come from many sources, such as a defective flashing or badly installed insulation causing condensation within the roof. Is it the nail that has failed or is the timber board onto which it was fixed rotten?

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Inspecting damaged slates in highlands MAAC studio conservation architect

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Traditional roofs were often finished with a lead ridge, as rainwater runs of the lead and down the roof it kills moss. Where there is a ridge made from galvanised steel, stone or concrete tiles, moss is unaffected and can take hold. Moss retains moisture and allows it to penetrate into the roof structure.

Each of these examples is a symptom of another problem elsewhere which needs to be fixed before you consider re-slating, otherwise it will just continue to cause damage.

Slate can also be affected by weathering, especially if they have been in place for a long time. You might hear it said that slates go soft with age. Again, this is an over-simplification. The impact of weathering depends on three things; the minerology of the slate (some slates are very stable and durable others less so), air pollution that increases the acidic nature of rainfall (an issue in urban areas, not normally an issue in the Highlands), and the area around the nail fixing getting wet regularly (this shouldn’t happen in a well-maintained roof.)

A good clue as to what is going on, is to look at the slates that have come off the roof. Are there still nail holes visible and intact at the head of the slate, are the holes worn, or has the slate fractured at the nail hole, leaving the nails in place on the roof.

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Inspecting broken slates in the Highlands MAAC Studio conservation architects

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Sometimes the cause can be even simpler, have you recently had work done in the attic or on the roof. A careless, heavy handed workman can do a lot of damage, ether laying ladders on the roof outside, or by hammering new plasterboard onto the rafters inside.

Lessons to take away

  • Don’t assume that your roof is a right off just because a couple of slates have slipped.
  • Whenever someone uses an expressions like “Nail Sickness” be careful, they probably don’t know what is going on with your roof, get it checked by a conservation professional.
  • Employ workmen who know what they are doing and are going to show a little bit of care and attention to your building.

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MAAC studio are accredited conservation architects and principal designers working with traditional buildings throughout Scotland. 

If you need independent advice give us a call ?