DPM Over Existing Tiles

[fro0tylo0py]

Hi,

I want to retile one of my rooms, but first I need to raise the floor with 50mm unbonded screed. The room is a bit damp, hence why I believe there is no DPM present so wish to install my own before raising the floor ready for tiling so my question is:

I'm a bit stuck for time with other jobs on, so can I lay the DPM over existing tiles, then lay my screed and then lay new tiles? Will the DPM over existing tiles just cause further problems such as condensation? I could remove the tiles first but will take me days as they are firmly stuck down and the concrete underneath could possibly pierce the DPM which i why i thought it would be better to lay the DPM over the smooth tiles.

Please advise. Many thanks.

 

[Ajax123]

Yes you can put a pm and screed over existing tiles. Provided the substrate is sound and not moving. There isno reason to anticipate a dew point at the tile - dpm interface. Make sure you use a minimum 1200gauge polythene for our DPM though. Also if you use a traditional 1:4sand cement screed you will need. Put either fibres or d49 mesh into it.

If you are worried about a dew point you could put 15mm of insulation down first and then your DPM followed by a 35mm calcium sulphate screed instead of traditional sand cement.

 

[fro0tylo0py]

Hi many many thanks for your info . I live a country with extreme temperatures, in the summer an average of 32*C sometimes as high as 40*C and in winter sometimes as low as minus 20. A calcium sulphate screed is not possible to buy over here as far as I know. Is this a premixed screed available from specialist supplier as never heard of it?
In view of the extreme temperatures, do you have any idea whether simply laying DMP over existing tiles will create a potential damp problem such as condensation, penetrating damp into surrounding walls? Or to be safe, shall I lay the 1.5mm insulation boards, then DMP and just do traditional screed of 3:1 mix without the calcium sulphate? Will actually be 65mm screed, not 50mm which is better


Thank you!

 

[fro0tylo0py]

Sorry forgot to say why does it have to be 1200gauge polythene and is there a way to check as what i bought today (it was all that was available) I can't seem to find anything about the guage