hey all, just a bit of advice needed, basically i need to know how long i need to wait before i can tile onto new concrete, is it a matter of waiting a few days til its gone totally hard or is there a rule of thumb like there is for plaster, ie have to wait weeks? obviously i know the thickness of the concrete is a factor, so without a spefic job in mind its really guidelines i'm after

Originally Posted by quackfly

hey all, just a bit of advice needed, basically i need to know how long i need to wait before i can tile onto new concrete, is it a matter of waiting a few days til its gone totally hard or is there a rule of thumb like there is for plaster, ie have to wait weeks? obviously i know the thickness of the concrete is a factor, so without a spefic job in mind its really guidelines i'm after

Normal drying time for concrete is between 0.5mm and 1.5mm per day depending on the grade, finish, depth and the cement content. Typically a 150mm house floor concrete e.g. Gen 1 with a hand float finish will take in excess of 6 months to dry properly.

However the moisture in the screed is not the important thing as such. It is the relative humidity at the surface of the screed i.e. the rate at which the moisture is escaping. This is dependent on the ambient RH and the pore structure within the concrete but enough of that techy stuff for now.

If you need to "rush" the job on a bit you can suppress the RH by using a surface applied Epoxy DPM. There are lots of these available from most major flooring suppliers. a couple are Universal One COat from Tremco (I have used this myself), One Coat DPM from Laybond Products and Merlin Barrier Coat from Merlin Coatings (Others are available). No idea of costs I am afraid. The concrete, being made with Portland Cement does not suffer in any way from having moisture trapped in it and these membranes are used pretty routinely throughout the soft flooring industry. I know most of them will be suitable for concrete surfaces as soon as you can walk on them. If you want to look at these as an option PM me and I will let you have some contact details.

De-coupling mats are another alternative.

Alternatively you could just wait. The concrete needs to acheive 75% RH when measured using a calibrated Hair Hygrometer (approx 4% moisture) to be considered dry enough for floor coverings.