Considering strength was not an issue once BS680 was written 70 years ago; a strength test was not incorporated and this one has been a downside in the face of poor brought in slates. Gladly; the new standard deals with this issue. Firstly; their stone strength; the ?modulus of rupture?; try assessed on the sample of 40 test pieces and from this specific strength values combined plus over the slates is determined. These kinds of values can then be applied to an equation which provides a minimal plain slate width; ebi.