"Rising damp" from concrete floor

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Bowlzi

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Hi all,

I have lived in my a 1950's semi for 20 years and finally decided to investigate the cause of the small patches on walls and raised carpet.

No mould, just shading of a few places on walls that haven't really changed in the years but as I decorated at the beginning of the year with a lighter, Matt finish paint I could see the patches were still there.

Carpet would raise in long bumps sometimes and others it would be flat. This year they seemed to be worse , especially when it was humid I think.

I lifted carpet and thick underlay and found more water than I expected.

Marley tiles in thin bitumen with water coming through cracks.

Bit the bullet and dug a hole to find out what damp proof I had. Discovered nothing except 4" concrete then earthy hardcore. Very damp underneath.

Lifted all tiles and , because it was quite wet in places, removed the sand and cement top screed.

Let it dry for a few weeks to see if it showed areas of wet so I could rule out pipes etc.

Discovered that the last area to stay damp was very sandy mix of concrete slab only 2" before soil/hardcore.

I have now, after some pondering, dug these soft sandy patches out as I was thinking that they will always be a problem and more moisture will travel up... I assumes that these patches were a bad mix of concrete and have broken down.

I'm now at the stage where I would like some advise other than my own thought process.

Should I be doing anything else before filling these holes and should a liquid dpm be below or level with the wall dpc. ( currently the dpc is level with the top of concrete and not above as I would expect...)

I will put pictures if I can

Thanks for advise in advance,
 
Floor under carpet marked with chalk for wet areas
image.jpg

Water under tiles
image.jpg

All tiles and sand cement screed removed and last remaining suspect area
image.jpg


Small hole dug through 2" soft sandy concrete to reveal hardcore and soil
image.jpg

The same larger hole after I cut back some of the sandy concrete. It mostly comes out as sand rather than come out in in hard lumps.
image.jpg

The mess I am in now and wanting friendly advice to stop me going mad :-)
image.jpg
 
Them type of floors don't have a damp proof membrane.
So I'd put down a liquid dpm like Ardex DPM1c
 
Thank you dazlight, do you think I have done the right thing by removing the poor mix/crumbly concrete. I am wondering if I should do it to the other room too (or at least get the Marley tiles up)

You would recommend the ardex more than any other product like something sika make ( as I could only see black bitumen on the shelf at a builders merchant recently)

Is there any milage in doing anything else while I am here like insulation? I was , prior to doing all this destruction, thinking of putting a wood floor down.

Thanks
 
Hard to say with out seeing it but I have put a dpm of weak screeds a few times with no problems. So might of been best to leave it after taking tiles up. Then giving the concrete a grind.
But I'm it's crumbling then It has to come up.
If it's bad all Over and your staying in the house for many years to come then is it worth getting a new concrete sub floor
 
Thanks. I've been here 20 years so far and probably another 2 to 5 more.
I will grind some of it flat this weekend and next week.
Would you recommend an epoxy concrete to fill the holes back in or would regular sand and cement be ok?
 
Regular. Then liquid dpm over the top

Go on Fball website and have a look at some videos.

F77 dpm , 1200 pro screed
 
Hi,I'm still doing this as not had much time on it recently and was waiting for the delivery of the f77

I've got the f ball f77 now, arrived today.

Two questions
Should I prime the concrete first with something ?

As I have to get back to the old height of 15mm in screed can I use sand and cement because it will be quite expensive to bring this up in the 1200 pro screed. It is about 7m x3m floor

Incidentally, I have put a laser level on the floor there is a difference of 25mm over the 3m width. One side of the room, the wall dpc is the same depth as the concrete and the other it is higher by about 10mm.

Many thanks

Paul
 
It's hard to get a good bond with sand and cement putting it on thin.
Above the dpm you could use a screed called Level flex which goes up to 50mm.
You don't prime under the dpm mate no.
Sometimes I do a weak prime like 10-1 then screed prior to the dpm to get a smooth finish.
 
Thanks for reply and appreciate your patience knowing how hard it is without seeing.

I dont really understand, if there was only 15mm/20mm of sand and cement screed before, would i not just be doing the same or was it originally too thin too?

The concrete slab is not very smooth and has some 10mm holes in it where the sds-chistle went in and some other undulations.
Q - Should i level these out first with the fball1200? (because i am finding it hard to get any of the fball locally from builders merchants etc. would bostik cempoloy Ultra be ok for this too?)

also, because the original concrete level lines up with the wall dpc, I am thinking that i should try to be below or level with the liquid dpm (as oposed to screeding first) i think that was partly why i had rising damp in the walls in the first place because the old marley and bitumen was above the wall dmc and bridged to the plaster...

Thanks
 
I'd say to low before and been lucky.
I'd try and get the dpm on the concrete right up to the base of the walls
You can use stopgap 460 to repair any holes.
I'm not a fan of mixing screeds and dpms but have done before.

What you have to remember is dpm is concrete is always the best way.
If it pin holes though you could have to do a 2nd coat
Prime on top of dpm.


What area do you live Paul ?
 
dazlight said:
I'd say to low before and been lucky.

What you have to remember is dpm is concrete is always the best way.
If it pin holes though you could have to do a 2nd coat
Prime on top of dpm.

What area do you live Paul ?
Click to expand...

I'm not sure i understand what you had written about the dpm is concrete. is there a typo?

I live in Kent just below j5 and j6 of m25

and in reply to your other question - Yes i filled it with cement and ballast a few weeks ago. I also mixed mortar up and filled around the edges where i had dug out at the wall/floor section so that i could expose to the dpm level.
I ground down, with a 5" disk, most of the remaining high bits of concrete/screed.
However when clearing the whole room i could see so many smaller holes/gouges and general uneveness that i decided not to fill in every one as it may be better to put a self leveling screed down... ( i then couldnt find one that would go down on floors above 75% RH)

Wish i hadnt started... :-/

paul.
 
Must be a typo but not sure what I was saying lol

When a sub floor needs sorting it is a lot of work. Costs a lot too
This Is what I do- 2 versions.

Grind floor of old adhesive
Fill deep holes with fball stopgap 460
Prime floor at 10-1 with P131 primer
Screed with 1200 pro
Dpm F77
Primer neat p131
Screed 1200

Or similar with Ardex NA , laybond , Uzin.
 
Thanks again for your advice. Ill see what i can get hold of.
Yes the cost and amount of work is going up.
Paul
 
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