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I’m talking about the incredible water absorption figure (0.1%) that came back from the latest round of testing at the RICS accredited laboratory just round the corner from our offices in South Woodford, London.
To put that into perspective, a slate that passes the notoriously stringent French standard (NF 228) has to achieve a water absorption of 0.4% – four times higher than the level scored here.
It’s officially pretty much impermeable, and the lowest water absorption of any slate we know (indigenous or imported).
Practically speaking, once the slates are on your roof, you’re not left worrying about the credibility of the 100-year guarantee that comes with it (which incidentally SSQ issues on the basis of exactly these type of test results).
Now there is a good reason why we’re seeing this type of performance and geologically speaking it comes down to the rock type – precisely because Riverstone is a “phyllite” and not just a slate.
Riverstone has evolved to be a phyllite, because it has metamorphosed to a greater degree than would have been the case if it was just a slate.
Being subjected to heat and pressure over a longer time period has altered the mineral structure of the underlying metamorphic rock, leading to a more coarser and larger grain size. The stone is thereafter more resilient to water penetration.
As you can see from the images, we have immersed a shale (a low grade slate) and a phyllite fully in water and left them both to dry for 45 minutes. As time passes, you can see the water being absorbed by the shale, whereas the water sits on the phyllite for a lot longer. By the end of the 45 minute period, the shale has fully absorbed the water with the surface appearing completely dry, whereas water is still lying on top of the phyllite stone!
If you want to see this for yourself, ask a free Riverstone sample here, add a few drops of water to the surface and get bored watching the water just sit on top of the phyllite for what seems like forever.
When the water does eventually end up disappearing off the surface, it has probably given up (and evaporated into thin air).