Hi there this might seem like a really odd question but i'm going to ask anyway (i'm a woman so that might explain it). We are building a games room in the garden and are at the floor laying stage. The hardcore is down with sand on top of that. The water proof membrane has also gone down and we are going to lay bricks on top of the membrane (the outside wall). Now the question, how on earth does the cement stick to the membrane? It doesn't seem posible. Could anyone enlighten me please?
We are assuming you mean that the first few courses of the outside wall have been laid and the membrane under the slab has been turned up and over the brickwork at DPC height. You now intend to continue up. The answer is that under enough pressure, and the weight of the bricks above is enough pressure, the mortar will stick to the membrane, but you are right in so much as it does not stick well. There is no lateral (sideways) force against the brick, only vertical pressure so the bricks do not move sideways. This is one of the reasons bricks have a "frog" or indent in them to stop any sideways "slip".