Considering strength was not an concern once BS680 was written seventy years ago; a strength test was not included and this particular has become a downside in the face of poor imported slates. Enjoyably; the latest standard deals with this issue. Firstly; that the rock strength; the ?modulus of rupture?; try calculated on a trial of 40 test pieces and from this specific strength values along and over the slates is determined. All values can then be applied to one equation which gives a minimum basic slate thickness; ebi.