Concrete floor and foundations - quiz

[Tim S]

Hi,

Very hypothetical questions for which there probably won't be definitive answers, but that's OK, I'm doing an "expectations management" thing here:

"The bungalow" has a brick (both leaves) cavity external wall and all single brick internals walls. The ground floor is poured concrete slab (not beam/block). The building is 1950's in East Sussex, privately built (not council stock). I don't have any reason to believe the house was built on a structural raft.

Which of these statements is likely to be true:

a) The external walls sit on their own foundations, mechanically separate from the floor slab;

b) The floor slab(s) pretty much sit on the dirt.

c) The internal non load bearing walls sit on the slab without foundations.

d) Internal centre load bearing wall has foundations and is separate from the slab.

e) There is a DPC or some sort of water barrier in/under the slab.

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Answers or educated guesses would be most welcome.

I'm going to find out anyway, eventually, but I'd like to get some idea of what to expect so I can plan things like "is it OK to build new brick walls straight onto the slab" and "do I need to maintain an existing DPC" and "if I dig out a section of floor slab right upto a wall, does the house fall down" sort of things... :)

Many thanks,

Tim

[Tanner-'op]

Yes!

No! - They should sit on a hardcore base

No! - They should sit on the concrete base.

No! - As above

Yes! - There should be some form of DPC, if not a polythene barrier, then possibly something like Bitumen or Synthaproof applied to the top of the concrete base and under the 2" floor screed.

Yes!

Yes! - Unless you are going to insert a new one.

Very unlikely.

Tanner-'op

[Tim S]

Hi, thanks for the detailed reply :)

Tanner-'op coughed up some electrons that declared:

Ah good. That's as I was hoping.

OK, silly me, of course hardcore is involved. But the hardcore sits on the dirt? What I really meant was the floor slab is not likely to be hanging on the foundations?

Sorry, I'm lost. Do you mean the concrete layer of the floor slab, not including screed?

OK.

OK. It was the 1950's bit that caused me to question - I don't know what they were doing back then. Sounds like judicious use of synthaproof would be in order when replacing or repairing bits of the floor.

Splendid. Got 4 internal walls to build. Two will be brick because they will form a new shower room and I hate drywall for shower/toilet rooms. The brick walls are quit short and support very little apart from there own weight so there's probably not much to worry about.

BCO will soon complain if I'm doing something daft, but I'd rather not give him the chance.

The other two will be wood stud drywall because there's no reason not to.

Good. I didn't think it was very likely, but neither do I fancy causing expensive damage or quick building a brick tomb for myself :)

Cheers

Tim

[Roger]

The message from Tim S contains these words:

My parents had a house built in 1953 or 1954. I don't recall very much about the construction (which is why I have made no attempt to answer the other questions) but I am sure the foundations for the outside walls were wide (perhaps 2 foot wide) and shallow. I don't think they were much deeper than they were wide. I think the foundations were concrete strip on a layer of brick rubble but I wouldn't like to swear to that.

Downstairs floors were concrete. There was still a wood shortage at the time so suspended floors were at least uncommon and may well have been banned.

[Tim S]

Roger coughed up some electrons that declared:

Heh - probably end up with deeper foundations on the conservatory :)

That's interesting. I did wonder why suspended wood floors seemed rare after the 30's.

Cheers

Tim

[The Natural Philosopher]

sort of. Ther will be a damp stopper somewhere in the floor slab: this may be contigouus with the outside DPC in which case the walls and floor may actually be connected, or it mauy not.

Yes.

One hopes so.

I am trying to remember standard practice..ISTR that in general te external DPC is lower than the internal one..so floors are normally above damp by some margin.

I suspect that the way they would have approached this sort of site build would have been to build strip foundations for the wall - probably not very deep - maybe not even on poured concrete, and build up to damp about 2-3 bricks above soil level.

Then in goes outer danp course, and a course higher, in goes inner damp.

Now at some stage the builder will shovel in his rubble and old cement crap and build up the interior to the inner damp level, possibly finshing with a sand layer; if its was a modern slab he would then lay a plastic sheet over it, and cast his concrete on that or maybe caat concrete up to damp, then plastic sheet and screed over.

However in them days it MIGHT have been cast concrete and paint with bitumen paint..and not even screed over.

My parents' house built in 1953 and the ones built across the road from it should give me a clue, but memory is hazy..I remember walls with DPC in them allright, and the 'floor' being full of broken bricks, and all eventually concreted over, but I cant recall and damp proof membranes going down. Maybe they did use screed over plastic sheet even then.

[The Natural Philosopher]

Poured concrete was cheaper than wood, basically.

And it doesn't rot,

Its cold tho. But then came the 60's and FITTED CARPETS.

[Tanner-'op]

No - the slab will be independant of the wall foundations.

The floor slab - Yes. Discount the screed for *any* structural support as that is laid after the walls have been constructed,

Snipped

Tanner-'op

[<me9]

Comments more like 60s/70s or later construction. I wouldn't like to confirm the same applied in the 50s.

[Tim S]

Tanner-'op coughed up some electrons that declared:

Thanks

Ah. So when I start digging, I might expect the bricks to be an inch or two under the floor surface, depending on how much screed went down.

Again, thanks. This is very helpful.

Cheers

Tim

[Tim S]

The Natural Philosopher coughed up some electrons that declared:

OK, concensus. All good to know.

Ah, this *is* interesting. Hadn't considered there might be a difference between inner and out DPC levels.

Right. Something else to look for.

If I get the lekky meter moved, that will be a good time for some exploratory work as I need to dig a big hole next to the outside wall and core drill through the floor and wall for some ducts (not big ducts, probably a couple of 60-odd mm) for SWA cables to outside.

The "real" work revolves around converting the end of a small and useless (for us, suited my parents just fine) room to a shower and loo. As it happens, there's a natural nook that could house a 1x1m shower in such away that a wet room would work without piddling water upto and over the loo and out the door (one of the usual complaints about wetrooms). It would also be the place for people doing gardening, hairy dogs and naked mud wrestling women to oblute and/or clean up in, so a hoseable floor is a bonus.

So, need to slope the floor and install drainage. Might as well lift it and relay with insulation and UFH it too. The area isn't that large so the amount of work, rubble and concrete mixing isn't huge. But I was wondering if there'd be any gotcha's. Probably not by the sound of it.

Many thanks indeed.

One day, there will be photos.

And my wife will buy you all a drink ;->

She thinks I'm planning to much. I keep pointing out all the near disasters on Grand Designs...

Cheers

Tim

[Tanner-'op]

Did my apprenticeship in the mid 60's and I can remember the old college tutors giving us information on the types of foundations used in 'earlier times', and all of what I have had said would have been around in the 50's - with the construction methods developing rapidly to accomodate the prefabricated building boom of that time - and if I remember correctly, leading to the 'development' of the first set of 'National' Building Regulations around the mid to late 60's (prior that, the old local byelaws covered building control).

As a matter of interest, I have been involved in many repairs on local authority housing of that period and have seldom come across foundations and concrete bases laid directly onto the dirt - a bad practice due to chemical/sulphate reactions between the earth and concrete (a problem that also arose in the 60's and '70's when old colliery waste was used as a hardcore).

Tanner-'op