Because strength was no problem when BS680 was written 70 years ago; a strength test was not included and this has become a problem in the face of weak imported slates. Gladly; the new standard deals with this issue. Firstly; their heavy metal strength; the ?modulus of rupture?; looks assessed on the sample of 40 test pieces and from this specific strength score combined as well as across the slates tend to be determined. Such values can then be applied to excellent equation which gives a minimal standard slate depth; ebi.