I am building some decking on top of an existing flagged patio. I am going to put a thick weed membrane down on top of the flags before I lay the joists. I want to then raise the joists up slightly so that they are not sitting in the water when it rains. I though of getting some 40mm waste pipe and cutting it into 10mm discs to put underneath, but this will be very time consuming to cut. Does anyone know where I can get something like this pre-cut - or any other ideas to raise up the decking?