need to achieve thin screed with dpm

First off hello everyone and thankyou for this informative forum which has been a great help as I renovate my house.

I needed to lower the floor of landing and bathroom so i removed the existing screed which was about 50mm thick from a concrete subbase - came away rather easily. There was approx 5mm of something on top of the screed which i guess was acting as the dpm.

I now need to make up about 30mm and also install a dpm. Enginered toungue and groove flooring with underlay will be laid on top and tiles in the bathroom. I was thinking of laying a plastic dpm and using a grano 2 part, sharp sand 2 part, cement 1 part screed as a floating / unbonded screed but dont know if 30mm is thick enough for this.

Another idea I have is to sbr bond the grano screed to the concrete and finish with an epoxy coating such as trimol 40 which says it can be used as a dpm. I really dont know what the best product to use is though and would need to know what kind of thickness is required so that I can lay the grano accordingly so it finishes at 30mm.

Or.. could i paint on a bitumen dmp to the concrete, followed by an adhesive and grano screed on top of that making it bonded - what adhesive is best?

My preference is for a the plastic dpm if 30mm is not too thin - I already have a big roll of the heavy duty stuff.

Many thanks for your advice guys.

The stuff on top of the previous screed was probs self-levelling compound, used to level the floor.

You won't get a workable screed on plastic at 30mm, best answer would be to lay the dpm, 30mm insulation board and float the floor on the insulation, easier, cheaper and warmer.

For the tiled areas, you should use a liquid dpm, see the stickies at the top of the flooring section, you will need the expensive f76/76 stuff as opposed to cheapo bitumen dpm for a proper job.

On top of your liquid dpm fit some marmox or similar insulated tilebacker boards, warmup/aquapanel available in 10mm marmox do a 6,10, 20,30mm board to suit.

many thanks deluks,

I finished the landing today. i took more concrete out to give me about 80mm of screed over plastic with a bedding of soft sand beneath the plastic. Luckily it was another layer that came away quite easily. underneath it was a truly old mix of charcoal and broken bits of pottery etc - must be as old as the house (over 100 years). for the bathroom I will probably take your advice or could again take the concrete away and use plastic.

I will take a look at the sticky and tile backing boards. Thankyou