Hi All, I feel a bit of an idiot asking this question as I feel I should know it however, using the formula given in BS 5306: Part 8: 2000; Gross Floor Area (m2) x 0.065 = Fire Rating What does "0.065" actually relate to please? I have various notes and read numberous book, but they only tell you how to calcuate A & B ratings ... they don't actually explain the formular in any detail whatsoever. Chat again soon. Ian :-)