All cast stone cills should be installed in accordance with BS 5628-Part 3.
DPC & Cavity Tray
A DPC the full length of the cill unit and the width of the cill bed should be installed as shown. The DPC should be overlapped by a vertical DPC at the jambs and a slip plane should be formed over the stooling of the cill to prevent cracking.
Bedding of Cill
To prevent cracking we recommend that cills are only laid on top of the DPC with mortar below the stooled ends of the cill only. This is to allow differential settlement and thermal movement.
The then open section of the joint can be sealed with a flexible material on completion of construction (on scaffolding remove and clean).