suspended timber floor or solid concrete... confused

10 Sep 2012
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United Kingdom

I recently bought a victorian mid terrace house built around 1890. It needs some serious tlc.

First job is to sort out the sub floor on ground floor level. joists have rotted away in various places as the ground level is above the dpc's atop the sleeper walls in places, and many of the timber plates supporting the joists are sitting half under ground level.

My original plan was to dig the ground level down, build new honeycomb sleeper walls, install all new timber plates on dpc's and larger (probably 150mm joists) to accomodate some insulation.

The problem is, looking at approved document C on the planning portal website, it says that I need to lay at least 100 mm of concrete oversite on a bed of hardcore under this suspended timber floor.

The level of the oversite should be above the outside ground level to avoid flooding i assume. This would block existing air bricks, and also mean that I would have to raise the finished floor height by at least 150mm if I'm to use 150mm joists.

Also, the reason for a suspended timber floor is to allow moisture to do its thing down there, and be vented away. Surely poring a concrete oversite will force any moisture up through the walls?

So, my questions are:

a. can I ignore the regs regarding the concrete oversite, and just throw down a load of aggregate or similar to stop plant growth etc?

b. is it ok for my subfloor to be lower than ground level outside, so long as it is well ventilated and there are dpc's on the sleeper walls?

c. am I wise to steer clear of solid concrete floors because of potential heave due to the clay in my area, and the damp issue mentioned before?

I am ready to tackle the suspended floor myself, and don't like the idea of tons of concrete poured into my house, but if regs force me to pour 100mm, why not add insulation and make the whole floor solid?

I have an independent damp surveyor coming in a few days to have a look, but I'm not sure he'll be able to advise on these issues, and I'd like to know what other peoples experiences are.

Apologies for the essay...

Any input would be greatly appreciated.

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I'm in a similar situation here, uneasy about concrete too but it might be un-avoidable.

Do you have much moisture under the floor at the moment - is the soil saturated or just damp?

If there isn't much moisture then there won't be anything to go up the walls. What are the walls like at the moment?

As you point out the regs simply don't cater for situations that arise in older buildings that need an individual approach. Both concrete floor or concrete oversite options assume you have a DPC in the walls.

If you can get the depth to do a suspended floor with upgraded ventillation without undermining the walls i would probably go that route. I agree about not sealing the solum, though it really does depend on site conditions. You might never get a problem sealing it, though if you do it's easy to reverse. Sealed or un-sealed as long as your ventillation can keep on top of any moisture generated in the sub-floor you will never get a problem.

Can you reduce the external ground level at all? That should be number one on the list regardless of which option you go for.
is the whole floor fubared?or is it just a case of investigating exactly what can be salvaged?
then start again.i think your over complicating the problem here.
do the floors have intermediate sleeper walls already?
if so then you might be able to remove a brick level to get the depth you require?
as for digging out then yes dig down enough to get the required underfloor ventilation,dont forget to upgrade the air vents where needed.
thanks for the replies mark and gregers.

I probably am overcomplicating things! Just want to do the job right.

Mark, the soil is damp in places. Some of the timber plates and joists have been sitting in this damp soil, and are no more. there is relatively new concrete render on the walls around the front bay window, so I guess they've had damp problems in the past. It was the smell and the bouncy floor that lead me to start pulling up the boards to investigate... I guess I could dig down outside below ground level, like a french drain type afffair?

gregers, it looks like a few people have tried to patch the problem in the past, there are sleeper walls, loads of them seemingly chucked in at random, and mostly just a couple of bricks stacked on top of each other. If I was to remove a brick a course of brickwork, I'd be below ground level.

To me the whole thing seems fubared! If I was renting I wouldn't pay it a second thought, but you know, do it right, do it once, or something... For me its better to start fresh than try to polish the turd.

I'll let you know what the damp man says.

Thanks again.

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well the damp man may say you have rising damp,check to see if your walls are cavitiy?as they may well be filled up with silt.

as for the sleeper walls,dig down and remove excess earth and then dig approx a 6" trench the lay some concrete blocks into the trench then redo the sleeper wall this way you can do with laying a concrete screed over the whole of the oversite.there are various ways of skinning a cat m8.
thanks Gregers.

Pre- cavity walls I think.

Re-the sleeper walls, do you mean bed some concrete blocks into the ground instead of strip foundations?

thanks Gregers.

Pre- cavity walls I think.

Re-the sleeper walls, do you mean bed some concrete blocks into the ground instead of strip foundations?


i have seen it done this way,but do check to see if this is acceptable.
so the damp man came the other day, and told me not to worry too much. No dry rot, no rising damp, just a lack of ventilation and a load of crap shovelled into the void by previous builders.

The solution is to get the boards up, dig out the excess material, and patch repair joists and dwarf walls where needed.

Like you say Gregers, I think I probably was getting a bit carried away. There are better places in this building to be spending my time and money.

Thanks for all your advice.


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