The purpose of an external sill is to take any water away from the door, so it doesn’t make its way into your property. If your door is sat directly on the brickwork, you will need a sill because the brickwork is a flat surface. Water won’t go away on its own, so we recommend a sill. If your sliding patio door is installed onto an existing sill made of stone or concrete, this should drain water away on its own without the need for an external sill. We recommend that you discuss this with your installer to ensure that no errors are made during the process.