In the UK, slates have been mined in Wales, the Lake District and Cornwall and these sources have proved themselves to be extremely durable and good value.
However, in recent years, particularly the last 20 years or so, we have seen an increase in the use of imported slate from Spain and China, and also from mines in Canada and Brazil. We have also seen a failure of these imported slates where they don’t appear to last as long as UK sourced materials.
When applying for Listed Building Consent, the use of natural slate will be expected on a roof that is already slate but rarely does the question about the source of the slate crop up. The use of man-made slate-effect tiles is unlikely to be a viable option.
While imported slates may be cheaper than UK sourced slates, and that itself is a mystery, the fact remains that we have seen roofs covered with imported slates that have failed prematurely and need to be done again. This could be after as little as 10-20 years before delamination becomes a problem. Delamination is where the layers of the slate separate and the slate becomes porous and likely to fracture.
When it is necessary to undertake repairs or re-roofing a slate roof we would only recommend the use of UK sourced slates.