Concrete floor and dampness

[TheScullster]

"BartC" wrote

As others have said, try to identify the source of the damp before laying final floor coverings. Do you have any hot water pipes or heating pipes in this area? If you have the usual hot water circuit for the time (with hot water from an indirect cylinder heated by your boiler), then a small leak may not show immediately as water flowing through your meter. The vagueness of float valves in storage tanks means that small quantities of water can be drawn without make up water being immediately needed to replace it.

I have a 1973 property of similar construction to yours. When we moved in about 10 years ago it was discovered that a damp patch was not due to a dodgy patio door, but down to a fractured elbow on a hot water pipe that had clearly been leaking for some time. This had caused rotting of some skirtings and door frames. The damp was just at a level where an inspection didn't show anything too alarming, but once in, the effects were found to be quite wide spread. Water had tracked under most of the original tiled floor covering, so this has all been removed.

One issue with pipes laid in solid floors is that a leak can develop, but the resulting damp may creap along the pipe channels and affect other areas.

If you don't have a good handle on pipe routes, I would try to establish these as it may help pinpoint the source of the problem.

Phil

[Peter Parry]

If you have a combination of damp and swelling floor in a 1960's house there is a possibility that the floor was constructed using blast furnace slag or other industrial waste. This was common from 1945 to

1970 especially in areas near steelworks or collieries. The sulphate bearing hardcore reacts with the concrete and causes irreversible damage. See

metaliferous slags caused similar problems

It would be worth asking if this is a known problem in housing in your area as if that is the case resurfacing the floor will be ineffective.

[John Williamson]

broken link.

[BartC]

OK, thanks.

The actual swelling is slight, and might be just salts pushing up the tiles. I'm not getting of those major symptoms described, although the house *is* in a colliery area.

But, the floor sounded hollow around there (more like a wooden floor!). I dug up few inches, and underneath a harder top later of a few mm, the concrete was dry and powdery (I could dig it out with my fingers).

So, now what? I admit I don't even have any idea what kind of tradesman to call for an opinion (and suspect they will recommend works whether they are needed or not).

However the main problem was the dampness (the floor was still doing it's job of being walked on!) (And if it was subject to sulphur attack, the document suggests nothing need be done in early stages. I think it also says somewhere that allowing evaporation of moisture will make things worse by speeding up the reaction.)

[NT]

If its dry under the surface, you've got no water coming from below, that's good news, and you can simply leave it dry out. End of problem.

NT

[Tim Watts]

Sounds like something you'd find lurking in Chatham...

[Peter Parry]

If you are in an area where this is common you will find most local Independent Chartered Surveyors will be very familiar with it and probably able to advise you without extensive testing.

[BartC]

I'm trying to understand in more detail how the concrete flooring works in this house built around 1967.

Presumably, the walls are built first, including interior walls, with their own DPC in the brickwork.

Then, concrete floors are added, up to each wall, in the form of a proper concrete layer, then a thinner, smoother layer called a 'screed'?

Where would the damp-proof membrane have been in this construction; would it have just have been the black bitumen coating on top of the screed layer? (This corresponds to the level of the DPC in the walls.)

What I don't understand is how the wooden structures (stairs, understair housing and door frames) fit on top of the concrete: they seem to go below floor level (and below that bitumen layer), yet do not suffer from dampness (apart from one side of one door frame; the other two original doors were removed as the downstairs became open-plan so don't know what they were like).

(And is it likely the CH pipes - in an unusual single-pipe circuit layout - would have been fitted at the time of the construction? As many go below the concrete, but I can't see any signs of major disruption where they run (although I can only see where one pipe runs, in the hallway; elsewhere there is stone tile or laminate and I haven't looked.)