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Question about a concrete floor

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Removed User 31 Aug 2019

I've got a concrete floor in a cellar. It seems to be permeable to water. Cardboard boxes and suchlike get soggy bottoms after sitting on it for a year or two. Is this normal and is there anything I can do to stop it?

 BnB 31 Aug 2019
In reply to Removed User:

> I've got a concrete floor in a cellar. It seems to be permeable to water. Cardboard boxes and suchlike get soggy bottoms after sitting on it for a year or two. Is this normal and is there anything I can do to stop it?

Yep. Put the boxes on a table

In reply to Removed User:

Yes, it can be sealed ... at some considerable expense. Known in the trade as 'tanking'. The ground below your cellar is probably naturally damp, and it'll be below the damp course, so the bottom of your cellar acts as a piece of blotting paper, sucking up the moisture. If anything, a warm centrally-heated house above makes it worse.

Post edited at 22:43
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 sbc23 31 Aug 2019
In reply to Removed User:

There may be no effective damp proof membrane below the concrete slab. Even if there is, there will also probably be poor ventilation to the surface of the slab wherever you have something sat on the surface (eg the box) or in corners/alcoves etc.

There are two solutions.

You can improve the ventilation to the surface. Put the boxes on some kind of spacers/pallets/racks to allow the surface of the slab to evaporate.

You can apply a surface damp proof membrane (epoxy paint). Something like F Ball F77. This will bond to the surface and seal the moisture in. This isn’t always perfect. Other floor paints (acrylic etc) are generally pretty rubbish. Any moisture below the paint, even if it appears surface dry, will just blister the paint and it falls off. There are premium products available like RIW Toughseal that form a plastic-like tanking seal but it’s about £500 a tin.

 summo 01 Sep 2019
In reply to Removed User:

I'd agree with the above. Are the walls very damp too? 

Are you able to improve the drainage around the house? 

If the ceiling is quite high and you can afford to lose a little height. You could put in a damp proof membrane (posh word for thick plastic sheet), then add a thinner layer of concrete on top. 

The problem is if the walls are single skin and are very damp, what you call damp is just them breathing the moisture from outside. Better to stop from the outside, not in. 

If you can't do any of the above, get some thin bits of granite (non porous) and sit boards on them, to put you stuff on top off. Don't cover the whole floor, leave space for the air. 

 Billhook 01 Sep 2019
In reply to Removed User:

And even if you can seal the floor from moisture coming in, you''ll still have the problem of warm air from the inside of the house condensing on the cold floor.

 pec 01 Sep 2019
In reply to Removed User:

If you want a cheap and easy solution and the concrete is in sound condition (not broken or cracked) you can paint 2 or 3 coats of bitumen damp proof membrane onto the floor such as this

https://www.wickes.co.uk/Wickes-Bitumen-Damp-Proof-Membrane-Liquid---5L/p/1...

and then cover it with self levelling floor compound like this

https://www.wickes.co.uk/search?text=levelling+compound

You don't need to clear the whole cellar to do it, you can just do 1/2, 1/3, 1/4 or whatever at a time and once it's dried move stuff onto the bit you've done to do the next bit.

I did mine about 7 years ago and have had no damp rising through the floor since but cellars have inherantly high moisture levels in the air so cardboard can still go a bit damp from that.

 wintertree 01 Sep 2019
In reply to Removed User:

Have you considered condensation?  If the floor is uninsulated and the air temperature cycles daily then the floor will hold items in contact with it at below-ambient when the room is at its warmest, causing condensation to form on to the objects.

You might wander round with an IR thermometer or thermal camera (£230 for a “Seek Compact” to go on a mobile phone and the novelty and utility never wears off) before you commit to expensive tanking.

Using some low wooden tables or a wooden raised floor is probably enough to stop such condensation.  A dehumidifier is an option but an expensive and annoying one if you have to carry the water up and out daily to drain it.

Sovereign Chemicals’ Hey’di K11 is the nuclear option for sealing things - but it’s not cheap and needs a lot of careful prep to work properly.  It also won’t make a jot of difference if your problem is condensation.  

 NaCl 01 Sep 2019
In reply to Removed User:

I have a partially below ground garage that used to have water coming through the wall from the garden on the other side. I used this stuff as it was recommended to me. I've been genuinely amazed at the difference it has made. 

https://www.toolstation.com/cementone-tanking-slurry/p79194

Its quite thick when painted on according to the instructions and might make some difference if its condensation but I wouldn't want to say for sure. 

Removed User 02 Sep 2019
In reply to wintertree:

I'm pretty sure it's not condensation, it doesn't get much colder in the cellar than in the house.

Permeability of the concrete also explains why I sometimes get swelling of laminate flooring in another part of the flat which is on the same level as the "cellar". Again the laminate flooring lies above the same concrete base although separated from it by something like chipboard sitting on batons.

 gribble 02 Sep 2019
In reply to Removed User:

I have a Sheffield cellar that is prone to dampness.  Some of them get quite extreme with damp.  I've got round it by a layer of thick plastic sheet on the concrete floor, then some cheapy old carpeting, and a decent dehumidifier running 24/7.  Seems to work well enough apart from stuff (timber, cardboard) having direct contact with the floor, and I can get round that by raising it up a little.  Cheap and effective enough.

 Tricky Dicky 02 Sep 2019
In reply to Removed User:

You could try taping a square of polythene (400mmx 400mm say) to the floor and seeing if moisture forms on the inside (rising through the concrete) or outside (condensation).

Removed User 02 Sep 2019
In reply to Tricky Dicky:

That's clever, I'd not thought of that.

 Toerag 04 Sep 2019
In reply to Removed User:

> I'm pretty sure it's not condensation, it doesn't get much colder in the cellar than in the house.

The critical thing is 'does the concrete get below dew point?'  If it's the coldest part of the house and gets below dew point then it will get condensation.

Post-construction tanking isn't likely to be super-effective. If you trap moisture in the floor it'll just start working its way out through the walls instead. As it's only the base of things stored in the cellar that are a problem I'd say putting them on some sort of supports with a layer of DPC under the supports will be the cheapest and easiest thing to do.

 Toerag 04 Sep 2019
In reply to summo:

> If you can't do any of the above, get some thin bits of granite (non porous) and sit boards on them, to put you stuff on top off. Don't cover the whole floor, leave space for the air. 

Granite is porous.  The builders of my house used slivers of granite to put the downstairs floor joists on and I've had to replace all of them due to rot where they sat.  A much better solution would be plastic blocks - not porous, not wicking, and insulating.


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