Sandstone appears to be a roofing material used mainly for castles and churches.
Lots of floor tiles are available second hand. They tend to be much thicker than the commercial roof tiles. We wondered what thickness traditional roof slates are:
I found these two articles on english restoration of slate roofs.
the second suggests that english slates are traditionally 5mm or so. OTOH, the thicker the slate, the stronger the tiles are.
This article is about the excavation of a medieval hospital in sussex – http://archaeologydataservice.ac.uk/catalogue/adsdata/arch-285-1/dissemination/pdf/vol_148/05Barber&Sibun_ADS.pdf. They found stone fragments that appear to be roof tiles and have “thickness varying between 6 and 11mm”
This article is about the slate industry and in 1923 2nd quality slates were “3/8 inch or more thick”. (9mm)
Great video of slate roofing – it looks like we can avoid drilling into slates if we use the hook fixing method – a european method.
http://www.spanishslateuk.com/hook_fixing.asp Of course, then we we need to work out how to organise hooks suitable for tiles “10-15mm” thick.