Plastering before screed/ Plastering over of DPM membrane

2 Dec 2021
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United Kingdom
Hi. We are building an extension and laying new floors inside the existing house as well. We are managing the job ourselves, but seems like we are making lots of mistakes, relying on "specialists" trades instead of doing our own research to get the right solutions.
I know we got it wrong, so please just help if you can.

Problem no 1
We have hardwall on most of the existing and new walls which was installed all the way down to the concrete slab level. We used Thistle hardwall. A few of the are also done with multifinish, ready to paint them.

Insulation and screed with UFH (I believe it will be liquid screed- 50mm)will be installed in a few weeks time.

Doing things in this order made sense as I have seen how messy some of the plastering can be and we wanted to make sure that the screed will be nice an clean at the end.

Where we have plasterboard, we will cut it to make sure it sits above the screed level to avoid it getting wet. Will we need to do somethign with the hardwall where it will sit within the depth of the screed?

Problem no 2

The dpm will go over the concrete slab, then insulation and UFH with screed.
We have a 300mm step down from existing house into the new extension generrally, except for one room which is level with existing floors.
The dpc was installed on external skin to suit external levels and then it comes up in the cavity and laps over the block and then down the inside face of the blockwork and will lap with the dpm.
The dpc was installed to suit the higher level of the floor as that was where they started building the walls, meaning that in the bulk of the extension the dpm/dpc membrane is close to 300mm above FFL. Again, we are using hardwall rather than dot and dab plasterboard.
Is there a way to plaster or fix something over the membrane?
We will have tiles in this area.

Thank you in advance for your help
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