One of the most important factors to consider is the water absorption of a slate which increases over time and will eventually render the slate useless as a weatherproofing material. Newly quarried Bangor Blue slate has a water absorption rate of around 0.1% (i.e. it is effectively non porous and the rain simply washes of the slate. Over time, however, the slates will absorb water and, after 100 years or so of constant bombardment by the elements, the water absorption will increase to 0.3% (i.e. 3 times more porous than when it was originally fitted). In this case, the slate will obviously not last another 100 years, but rather maybe a further 20-30 years.
Another common issue with second hand slates is determining their original origin and whether or not they are all from the same building. In many cases, salvage yards will gather up a range of second hand slates from a cross section of different jobs and re-palletize them into one bunch. So you could have one batch of slates which are from a church in the countryside with relatively low pollution and are only 50 years old (probably in good condition). On the other extreme, you can have slates that have been up for 150 years + in heavily polluted industrial cities like Liverpool or Manchester (probably in bad condition and these slates may have 5 or possibly 10 years life left in at best them before they eventually fail). Frequently, these 2 different slates from different buildings will be next to each other in the same pallet!
So, it is important to bear in mind that all slates, including Penrhyn Blue Bangors, will not last forever and have a finite lifespan. So, determining the age of a slate is an important factor to consider if considering a second hand slate but this is not always a straightforward process. Even if you know the age of the building, how do you know this has been the first time the slate has been used? We frequently salvage second hand slates from roofs which have been holed 2 or 3 times (this means that the slates have been fitted and re-fitted 4 times in their lifetime). In such cases, there only be 5- 10 years of useful life left in the slate.
In saying all this, second hand slates do have a market and the following is a checklist of things to look at if you are considering a second hand slate:
- All reclaimed slates need to be examined to ensure that no excessive chipping or fragmentation has occurred around the nail holes.
- There should be no obvious signs of delamination (splitting or flaking) on the slate.
- If the back of the slate appears powdery the slate should be disposed of.
- Multiple nail holes are an indication that the slates have already been refixed in their lifetime – making reuse unadvisable.
- Most importantly does the slate “Ring True” – the slate should give a clear ring when tapped – a dull thud is an indication that it has deteriorated.
If re-holing of slates is necessary the new holes should NEVER be below the existing holes or weather tightness will be compromised. Slates should also be sorted with the thickest being used at the eaves working up to the thinner slates at the ridge.
In the event that the slates are holed on more than one occasion, it is important to check the LBS technical guide to ensure the slates will function properly and keep the rain out of the nail hole (another thing which can lead to roof failure).
If you are intending to use reclaimed slates on your roof and have any questions please do not hesitate to contact us and our expert technical team will be on hand to answer any specific questions you may have.