Sulphate Attack - Replace with Concrete or Suspended timber?

...it appears many thousands of british home owners have wearily trodden the path I am about to take, due to poor construction methods in the 50's and 60's...

I'm planning to dig out and replace all floors (they are awful, some sunken, some bulging, some cracked) - but I can't establish whether direct concrete replacement or fixing suspended timber floors is the better option.

I have experience of fitting joist timbers off masonary hangers, but I wouldn't attempt to screed a floor.

Conversely airbricks will need to be installed throughout, which sounds like a real pain (cavity walls) and I read the surface under the floor void needs to be sloped to handle any water ingress.

Seems there are pro's and con's for both methods!

Is one method significantly cheaper than the other?

This job is approx 60 square meters (bungalow).

You will have to price it up. One method is no better than another, but each has its pro's and cons which you need to consider, and costs may be similar
A suspended floor will still require oversite concrete below the void, and possibly sleeper walls

It also depends on that type of sulphate attack you are experiencing, as you may just be getting heave of the concrete slab, or the fill may be expanding too - requiring more remedial work

^woody^ said:

You will have to price it up. One method is no better than another, but each has its pro's and cons which you need to consider, and costs may be similar
A suspended floor will still require oversite concrete below the void, and possibly sleeper walls

It also depends on that type of sulphate attack you are experiencing, as you may just be getting heave of the concrete slab, or the fill may be expanding too - requiring more remedial work

Click to expand...

Many thanks for the advice - sulphate has not been proven, a structural engineer reported it is the 'likely' cause. TBH, the floors are in a bad way, as i described previously, and I want to make them good, both for my own use and so as to be able to sell the property on without issue in the future.

With the heave/bulging floors, I don't see any alternative but to dig out and re-lay, and the regs state like for like replacement must be 600mm depth excavation, hardcore, dpc, insulation, concrete, dpc, screed etc etc, so no quick fix possible (to my knowledge). Is there another way to deal with heave in a concrete floor?

I guess the floor with a dip in the middle can be levelled with a good compound, but it won't resolve the root cause.

All of the floors sent the surveyors damp meter crazy, although all of the walls tested were good (which he was surprised about).

For a normal floor replacement on good compact fill, then you only need to remove enough depth to get the new floor and insulation in. The existing hardcore can stay in as a DPM protects the new concrete floor

Many engineers want 1m deep excavation to remove all the fill and some soil below it as standard, but this is only required where expansion of the fill is occurring or likely

Defective drains are commonly associated with sulphate attack as its the leaking water which sets off the reaction. So you may also want to get your drains checked, or if the incoming water main is under the building, that too

Wouldn't it be possible to just dig out, insulate, dpm & concrete with mesh in?
Screed isn't absolutely necessary, you could just fine trowel the crete.
Of course check your drains as previously advised.

Hi I know this was posted a long time ago but just wondered what happen re your floor and what you went for in the end. I'm in the same position at the moment with a floor that needs replacing we are thinking along replacing with solid concrete floors as before.

Any info would be great

tdawson said:

Hi I know this was posted a long time ago but just wondered what happen re your floor and what you went for in the end. I'm in the same position at the moment with a floor that needs replacing we are thinking along replacing with solid concrete floors as before.

Any info would be great

Click to expand...

...as an alternative 'fix' I had the floors damp proofed with mastic asphalt.

This option leveled the floors and prevents rising damp from penetrating the floor surfaces. A certificate was issued with a 25 year guarantee.

Mastic asphalt covering won't stop further bulging in the future, mind you.

We moved out about a year ago. By coincidence I viewed a 160 year old thatch cottage the other week, the ground floors were undulating (to put it mildly). Immediately considered how the floor could be dug up (flags direct to earth), dpm, insulated, screed etc. However, when researching further it's worth noting that water will still want to rise, due to salts or capilary action and if it can't escape via the floor surface it will go somewhere else - namely into the footing of the walls instead. The cottage has cob walls, the thought of damp creeping up those cob walls and the damage it could do over the years really put me off buying the property!

If you do dig out and replace you should really get LA building control in the loop as it's notifiable work, plus they'll give you good advice too.