Underfloor water leak and now damp subfloor...

[Arctic Jack]

Our bungalow (built 1985) had an underfloor water leak that first presented itself in the main bedroom. All the water pipes were buried in a few inches of concrete. Insurance was called and a leak detection team moved in. They found multiple leaks throughout the house, along with black mould. Two members of the family, possibly three had health conditions that have cleared up since leaving. We suspected the mould had caused them. Most rooms were found to be ruined. The water was knocked off and industrial drying units moved in and my family moved out.

After many weeks the walls were tested and found to have dried. The surface concrete was dry but the subfloor still contained moisture. The drying team said that the dehumidifiers just weren’t drying it fast enough. Builders recommended just taking the concrete and subfloor up and renewing it. Insurance dragged their feet and didn’t want to, but eventually they gave the go ahead.

We decide to relocate all water pipes into the attic and then chase them down into the walls, at our own cost.

Builders then removed the concrete screed in several rooms but stopped at the subfloor and said something along the lines of - the whole bungalow was built on top a single slab, digging up the subfloor could cause structural damage to the internal partitioned walls. These are only 100mm thick thermalite blocks. Costs could then start to spiral.

At this point builder and insurance guy are put in contact with each other. They’ve together come up with a plan just to lay visqueen and a liquid waterproof membrane over the damp subfloor and then relay the concrete screed.

I understand the builder’s priority is to make money and the insurance guy’s is to save money. With this in mind I’m wondering if this the correct and sensible option, trapping the damp in the subfloor of probably a quarter of the house and just moving on?

Any advice would be great.

[Ken010]

I am a bit confused, sorry about that your explanation is good.

The pipes that leaked in various locations were buried under a layer of Concrete.

What was below this layer of Concrete, or?? is this layer of concrete the "structural" slab from which the walls arise? or was there some fill material between the two layers of concrete?

Ken.

[Arctic Jack]

Sorry, I don't really know building terms.

Top layer of floor = 2 inch concrete layer - water pipes were within this
Below concrete layer = Subfloor slab, where walls are built up from, I'm not sure what this is, it is yellow.

There is nothing separating, between the top layer of concrete and subfloor slab. They're directly on top of each other.

[Ken010]

OK, but I am still confused, really, really sorry about that, I am not being pedantic, or disbelieving you, BUT??

Is this "yellow" you mention a plastic membrane??

Any chance of a couple of images please??

What is confusing is that your description is of a relatively thin concrete layer on top of a Damp proof membrane, this form of construction is very, very common and is intended to stop ground water getting into the underfloor area.

Ken.

[Arctic Jack]

No problem at all...

The yellow is some form of concrete slab that the bungalow was built on. Not a plastic membrane. The floor is structure is currently this...

<2 inch grey concrete>
<Subfloor (yellow)>
<I’m guessing below the subfloor is a plastic membrane>
<Earth>

That two inches of grey concrete has been removed in 4 rooms. The yellow subfloor is now exposed. The yellow subfloor seems damp. Initially the builder was going to remove damp yellow subfloor and replace it. The builder has changed his mind, now he just wants to lay a sheet of visqueen down on top of the yellow subfloor and replace the grey concrete he has removed.

I’m wondering if that is the best idea, since this way the damp is just trapped, going nowhere. I’m worried it’ll eventually find its way to the surface to cause havoc again.

I’ll get photos next time I’m there.

[Ken010]

I am still not fully understanding the yellow colour??

Images if possible would be of assistance

On a different tack, when next back at the property, can you have a look at how many Air Bricks there are around the base of the walls?

I have seen a few cases where there was a leak under the floor, in the by far vast majority of cases the sub floor is simply earth, not a layer of Concrete never mind two layers ?

What you have described so far, is a perfectly normal, modern construction, where as your property is 1985?? they generally tended to have earth floors. So where is this going?? underfloor areas tend to be slightly damp, the Concrete cuts that down a lot, the air bricks in the external walls assist, I am having some difficulty in following the builders rational in ripping up the concrete, why, because the concrete generally has a Damp Proof Membrane below??

Now could be a good time to get an Upgrade in your Floor Insulation??? how ?? When the builder re-places the floor, it should be replaced to today's standards, meaning it should have insulation of some form fitted, insurers call it Inadvertent betterment, suggest you start pressing for this aspect.

Sticking my neck out here??? this entire claim and advice from the Builder has, in my opinion not been undertaken correctly, based on what information you have tendered.

Ken.

[Arctic Jack]

I’m at the house today I’ll take some photographs.

Are air bricks on the internal or external walls? I’ll check both.

My concrete screed has no damp proof directly below it. The membrane is supposedly under the ‘slab’. The builders idea was to dig back to the membrane clearing the damp. Relay the membrane, relay the ‘slab’, relay the concrete screed.

According to the locals, my bungalow and nearby houses were built on swampland. I think to counter this, the houses were built on top of a thick 30cm+ single ‘slab’. The builders that have seen it have always referred to this subfloor as a ‘slab’.

When I describe the slab as yellow, I should say ‘sandy’.

[Ken010]

OK the concrete slab sounds like a Concrete, so called Raft Foundation.

Raft foundations are specifically designed by a Structural Engineer, and are utilised in such areas of poor ground bearing capacity, such as a swampy area of ground, or in areas of mining where the mine workings are near the surface, other similar area where Rafts are designed to be used exist, where ground bearing will not be possible for a simple foundation below the walls.

A raft will disperse the load, the weight of the property over a large area, indeed the area of the overall footprint dimensions of the property rather than normal foundations which will impose a force on the ground which is a much more concentrated load, in ground which is poor at load bearing a raft spreads the load over a very large area.

Generally a structurally designed raft foundation will have an overall depth, you mention 300.mm sounds correct, around the perimeter of this raft the concrete will be deeper to prevent the raft flexing under the load imposed by the external walls, likewise there should be local deepening of the raft under the line of the internal walls.

All the above will have steel reinforcing bars embedded in the concrete, as well as --- a mesh steel layer over the entire area of the raft ---

I think it is possible that your builder has Mis-Diagnosed things??? I for one would not advocate any interference to the concrete raft / slab??? he has altered his repair concept, but??

I can follow the logic of a Damp Proof membrane on top of the Raft / Slab just in case there is no Damp Proof membrane below the Raft / Slab.

Ken.