A metal tie used to tie the outer leaf to the inner leaf of an external cavity wall and so provide the required stability. Available in a number of different patterns, to meet the requirements of BS 1243, and made of galvanized metal or stainless steel. All types are formed with a centre twist to prevent the passage of water across the tie and on to the inner leaf. Building Regulations require ties to be placed no further apart than 900mm horizontally and 450mm vertically (6 courses). It is normal practice to stagger the alternate layers of ties.