I'm attempting to lay some laminate flooring over a level floor made of screed.
At the edges of the floor where it meets the wall I can see that under the screed it looks like the builder has already put down a damp proof membrane (DPM).
So if theres a DPM under the screed do I still need to put another layer of DPM over the screed before laying underlay and a new laminate floor?
Concrete wicks water, so even if the membrane under your screed layer is undamaged, if any moisture gets into that layer it's going to be redistributed as evenly as possible throughout that layer.
What you don't want to do, with a DPM under the screed, is create a moisture trap - which a second DPM would do.
Instead of a membrane, use a foam underlayment material which will keep the wood from touching the screed directly (avoiding any moisture transfer) but which will also allow the screed to breathe and thus dry over time.
