Tiling On Damp (ish!) Concrete Floor

[BenW]

My bathroom is on the ground floor of my circa 1900 end of terrace house in Sussex. It had lino on the floor and when I took it up I found the underside of the lino to be mouldy looking - though the concrete floor underneath seemed relatively dry to the touch. Thinking it was just condensation (the floor is always freezing and it is especially cold at the moment - minus 7 degrees overnight!) I left it to dry out for a few days then levelled the floor using a cement based levelling compound which dried quickly and evenly. I'm now preparing to tile the floor but have noticed that if I leave something sitting on the floor for a few hours when I pick it up there is a dark patch underneath that makes the floor look damp but it's still dry to the touch. The item I picked up however often has moisture on it.

Does it sound like I have rising damp in the floor or is it condensation caused by the cold floor? Either way, whatever sits on the floor gets wet so I'm worried about tiling over it. Someone suggested sticking insulated tile backer board to the floor which sounded like a great idea at first but then I thought won't that just trap the moisture underneath? Is liquid DPM the answer? And if so will tile adhesive stick to it?

Sorry for all the questions. I have looked around for advice but got a bit overwhelmed with different suggestions. I'm an experienced DIYer but never had anything like this before.

 

[BenW]

Here is a photo of the floor. It didn't come out that well but you can just about make out a dark oblong shape going from the bottom right corner towards the middle of the photo. I left the lid for the toilet cistern sitting on the floor all day and that dark patch was what was left when I picked it up. You can also see mottled patches here and there like at the bottom of the photo.

 

[Spud]

Sound like you don't have a damp membrane under the concrete or a leaky pipe under the floor perhaps

 

[Qwerty]

If the floor has survived 115 years I'm sure if anything horrendous was going to happen it would have done so by now. Likely there is no dpc. The old lino will have trapped a lot of moisture in the screed over the years. Couple that with high humidity levels and cold screed with moisture in the air being drawn to cold surfaces will nearly always result in the issue you describe.

You could use insulation boards or a decoupling membrane if you wanted. what height have you got to play with? What tiles are you laying?

 

[BenW]

Sound like you don't have a damp membrane under the concrete or a leaky pipe under the floor perhaps

Quite likely there is no DPM underneath. I'm fairly certain the bathroom was actually an outhouse or a brick shed and the main house was extended to join up with it. The walls of the bathroom are a single course of bricks! There is no sign of any damp in the room and in fact if I'd never taken the lino up you'd think there was nothing wrong at all.

 

[BenW]

If the floor has survived 115 years I'm sure if anything horrendous was going to happen it would have done so by now. Likely there is no dpc. The old lino will have trapped a lot of moisture in the screed over the years. Couple that with high humidity levels and cold screed with moisture in the air being drawn to cold surfaces will nearly always result in the issue you describe.

You could use insulation boards or a decoupling membrane if you wanted. what height have you got to play with? What tiles are you laying?

There's plenty of height. I'm 6 foot and there's at least another foot above my head to the ceiling. 6mm board plus 10mm tiles on the floor won't make much difference. Tiles are ceramic.

 

[J Sid]

A bit of ufh would make it feel alot better under foot

 

[White Room]

Quite likely there is no DPM underneath. I'm fairly certain the bathroom was actually an outhouse or a brick shed and the main house was extended to join up with it. The walls of the bathroom are a single course of bricks! There is no sign of any damp in the room and in fact if I'd never taken the lino up you'd think there was nothing wrong at all.

What on the single brick walls...

 

[BenW]

What on the single brick walls...

Walls are painted with a thick black shiny substance which I assume is liquid membrane (but could just be gloss black paint I suppose!). Then there's inch thick polystyrene insulation sheets sandwiched between the wall and plasterboard.

 

[BenW]

A bit of ufh would make it feel alot better under foot

I did think about it. I suppose I convinced myself it would be inefficient pouring heat in to the ground! There is a heated towel rail with the pipework for it running along the bottom of the wall so if there were an easy way of taking a spur off that to run underfloor heating I'd go for it. I seem to remember looking in to it and finding it wasn't that easy though? Again, not something I've ever done before. Electric underfloor heating I have done and wouldn't recommend it to anyone.

 

[Old Mod]

If u put down insulation boards first as @Plan Tec Tiling suggested, then u won't be pouring money into the slab.
Insulation boards, electric UFH, and a coat of smoothing compound and your done.

 

[J Sid]

Should be an easy install electric ufh if you have a power supply.
If height is not an issue then put a 10 or even 20mm insulation board down. It won't cost alot in running costs

 

[J Sid]

To quick for me marc.

 

[White Room]

Stick some Zypex modified and concentrate that'll stop moisture coming through.

 

[Qwerty]

Stick some Zypex modified and concentrate that'll stop moisture coming through.

Good stuff that. Beats black jack any day! Still see builders using black jack now even when stuff like xypex is about