Counter flashings are an intricate part of sealing a flat roof. There is a lot of work in installing a counter flashing correctly. Cutting the wall with a grinder to create a slot for the lead flashing is very important. You need to make sure that any water running down the wall does not leak in behind the upstand of the waterproof membrane. The slot needs to go beyond the plaster work and into the block wall to give adequate protection. A lot of time and effort goes into this part of the job because it is usually the weakest point on any flat roof and it needs to be done right.