All cast stone cills should be installed in accordance with BS 5628-Part 3.
DPCs and Cavity Trays
One piece and multi-stone cills should be provided with a DPC for the full length and width of the cill bed. The DPC should be overlapped by the vertical DPC at the jambs and should be turned up at the back and ends for the full depth of the cill. Allow the leading edge of the DPC to project 5 mm from the wall face below the cill.
Bedding of Stooled Cills
To prevent cracking of the cills due to the effects of differential settlement and thermal movement, the mortar bed below the cills
should be trowelled smooth, allowed to set, and then cleaned off before the DPC is laid. One piece and multi-stone cills should be laid on top of the DPC with mortar only below the stoolings and ends. The open section below the cills should be sealed with a flexible material only on completion of structure.
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