Damp in basement floor

gingerman · 27 Jun 2017

Hi all
I have a basement which is below ground level on the 2 external walls. These walls were re-tanked about 15 years ago and no obvious moisture has come in since then. After having a musty, damp smell I lifted the carpets and there are now damp patches on the concrete floor ( please see pics ). I have put a damp meter on the 'dry' spots on the floor well away from the walls and the meter still shows dampness, and where the obvious damp patches are next the the outside walls damp is showing on the meter up to about 30cm above floor level, but not much above that. The property was built in about 1900. Any advice o how to treat this problem, I was wondering whether trying the concrete with a de-humidifier first then applying something like https://www.kingfisheruk.com/epoxy_primer-item-70 would help?
Thanks for any advice.

vinn · 27 Jun 2017

What was the original tanking? How was it done, what materials were used?
What does re-tanking mean? Same as Q's above?
Were all the walls tanked - what about the floor: does it have a membrane (DPM)?

Damp could have been penetrating the floor from the time the carpet was laid?

The de-humidifier will merely take up ambient moisture but do nothing for the cause(s) of the damp.

The Kingfisher stuff is new to me - but, in a sense, its irrelevant, given that moisture is rising up the walls.

If you removed a length of skirting & examined the back of the skirting, you might find that its rotting. You would also be able to see the lower wall condition.

gingerman · 27 Jun 2017

When it was re-tanked, all previous existing waterproofing and backing coats were removed to ceiling height. An initial base render coat of sulphate resisting cement/sand (1:4) incorporating a modified polymer additive. The base render was overcoated with two coats of the Thoroseal waterproofing system at a combined loading of 2.5Kg/m2., again incorporating a modified polymer additive. The base of the waterproofing was sealed above floor line with Waterplug rapid setting hydraulic cement. Following that further render coats of cement/sand (1:4) added incorporating an approved additive to reduce the effected of hydroscopic salts, and then finished with a Thistle skim.

It was re-tanked due to a flooding incident 15 years ago.

Floor - don't know whether it has a DPM, don't have any records of that - but have been living here for 20 years and not had a problem before recently.

vinn · 27 Jun 2017

Removing a length of skirting will reveal if any membrane is in position.

The tanking treatment that you describe seems to have worked - except at the base of the wall. However, so far as I can see on the pic, there's very few signs of damp near the floor.

Ground water pressure can change, and an increase in pressure could blow off the render you re-tanked with. If the render moves then all applications to the render will move too.
But we dont yet know what the cause(s) might be.

Is the house on a hillside or slope in any direction?

What was the "flooding incident" - are you near a stream or river?

gingerman · 27 Jun 2017

I'm not at the property at the moment but will remove some skirting next week.

What do you mean by "there's very few signs of damp near the floor." - do you mean the damp is located next to the tanked walls and not generally across the floor - if so then that's correct, although using a damp meter ( I know they can be inaccurate ) on other apparently dry parts of the floor shows they are damp.

The basement is below ground level on the effected side and ground level reduces from just above ceiling height at the back of the property to basement floor level at the back of the property.

We have a small brook at the end of the garden which is normally about 4 feet below basement floor level.

gingerman · 30 Jun 2017

Hi, pic showing the wall behind the skirting.....

vinn · 30 Jun 2017

There are obvious, but so far very minor, traces of damp on the exposed bit of wall - whats the condition of the back of the skirting? Can you pic it?
There's no sign of a membrane but given the amount of tanking layers it would be difficult to see.
Wrong paint has been used - the best kind of paint to use on render is a masonry wash.

Given that you've had 14 yrs of reasonable use then it might be safest to leave well alone - any hacking off and re-rendering might cause more severe difficulties.

I doubt that there is a membrane, I think the moisture is coming up from below the slab.
Unless it gets much worse then let the slab breathe, and the moisture evaporate into the room.
Dont cover with a carpet or a water proofer.

Tanking basements is always a tricky business, & doing it your way, its best done by at least using a DPM (membrane) tied into any wall tanking.

gingerman · 30 Jun 2017

Thanks, will take some pics of the skirting when next there in a couple of days, it has rotted in a few places.

I stuck a dehumidifier in there a couple of days ago and went to check today - the visible signs of damp on the floor had gone but the dehumidifier was full, have emptied it and will check again shortly. Does this suggest only a minor quantity of water coming through the slab?

If the hydraulic cement is intact, and assuming there is some moisture coming through the concrete slab, then presumably this could travel up the plaster skim but the concrete wall behind that should be dry?

If there is only a minor quantity of water penetrating through the slab would it be worth applying a penetrating damp proofing sealant on the floor once it was dried out for a while to stop that re-appearing when the carpet is re-laid?

Having said that, the visible signs of damp were all around the edges of the tanked walls, not in the middle of the concrete floor, do you have any thoughts on that?

vinn · 30 Jun 2017

The moisture is finding the line of least resistance as it rises from below - ie the join at the slab and the wall.
Its probable that using damp proofing sealants will only disperse the moisture differently and perhaps more intensely.
You will not dry out the slab.

The affected perimeter of the slab must be kept clear of any floor covering.

Skirtings will continue to rot so perhaps remove all skirting (dont replace it) and make respectable the finish with a 4:1 mix of sand & lime.

gingerman · 30 Jun 2017

Is there no way of providing a barrier to the moisture from below at this stage? Don't understand why an epoxy floor sealant applied right up to the hydraulic cement wouldn't provide a barrier? If the line of least resistance is being taken, why wouldn't there be visible signs of damp on the floor adjacent to the non tanked walls?

vinn · 30 Jun 2017

If you attempt to provide a barrier then you could end up with more moisture being pushed up the wall.
There's also moisture penetrating the wall brickwork below the level of the FFL and the tanking.

gingerman · 30 Jun 2017

Isn't the point of the hydraulic cement to form a barrier against water rising up from the slab, and wouldn't the tanking prevent water ingress from the outside? If the hydraulic cement is exposed and a barrier put in place to prevent water ingress from the slab, attached to the hydraulic cement, with skirting above that, can't quite see where water could rise to? Could you produce a sketch of what you mean?

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