Blackjacking damp concrete floor?

Saladin Ane

I've a damp concrete floor in a two storey extension built around 1956. AFAIK there was no dpc - just the blocks layed on the slab. Everything else is OK, just the damp floor. If I gave two coats of blackjack - floor and up the walls 12" - and then stuck 2 x 1s onto the floor with blackjack (to take floorboards/ply), would this solve the problem?

The floor is level and measures 12' x 8'.

Any advice appreciated.

Theres a good possibility its damp due to condensation rather than rising damp. If it varies with external humidity its a good clue to condensation.

The Continental Op

Tape a piece of clear plastic over the floor with the tape covering all the edges. Leave for 24 hours and look for condensation on the floor side of the plastic. If there is none the floor is dry.

Saladin Ane

THe reason I think it's no dpc is; the floor around by the three outside walls is damp after lots of rain (like last Tuesday). Will try the taped plastic trick in the centre of the room (which incidentally) looks dry. There may be no membrane under the slab as well. Will post result in a few days.

Also, would condensation appear if the temperature of the slab was the same as the air?

Lumen

Also, would condensation appear if the temperature of the slab was the same as the air?

No, but that's unlikely.

The reason it would be more damp around the edges is because this will be the coldest part of the slab. If the soil is damp around the slab it will also be much cooler than dry soil.

The floor will nearly always be the coldest part of the room unless it gets lots of solar exposure.

monseiur

https://www.boards.ie/discussion/comment/119716854#Comment_119716854

Is it possible to lower the level of the ground ouside or install a good drainage system ?

Saladin Ane

Everything seems to be good. Blackjacked the floor and 6" up the walls. Stuck the 2"x2" to the blackjack (with blackjack) 😁, then plywood and flooring. The old walls are fitted with 25mm polyeurathane-backed sheetrock and at the periphary of the ply I drilled a dozen 1/2" holes for some ventilation. These are hidden behind the sheetrock which finishes 1" above the floor.

GreeBo

Did you do the polythene test?

The concrete could be soaked under the blackjack and growing all sorts of nasties if you have sealed it in...

Saladin Ane

https://www.boards.ie/discussion/comment/120967225#Comment_120967225

Yes I did. And it was dry but the periphary looked damp. Put in a couple of heaters for 24 hrs, the floor was bone dry and I blackjacked it immediately, giving it a second coat the following afternoon.

As for the nasties? That should keep them at bay for my lifetime. 😋