Dropped floor - concrete slab

[ChrisL69]

Hello. 

First post here so be gentle please 🙂

 

We are near completion of a house purchase built on clay soil in Norfolk that we are taking on as a restoration project. It is a lovely architect designed 1969 house in a quiet village. The survey (RICS 2) said everything we expected, but also said the floor in the corner of the living room has dropped and is hollow. He marked it as a 2 in severity and gave no further comments. When I spoke to him he said it has dropped by 10mm over an area covering about 2m x 2m and needs investigating before exchange. He said a drain survey is the next step - there are no drains anywhere near that part of the house so we are assuming drain damage is unlikely to be the cause. There is evidence that a tree located about 4m from that corner of the house was cut down in throat few years (the stump is about 30cm diameter and is weathered), and 14 years ago (Google Street View) was about 12 feet tall with a canopy of 3 m or so. There is no cracking or evident movement in external walls. 
 

The property owner died a couple

of years ago so we cannot ask her the history, and the executors aren’t replying to questions at the moment (presumably they don’t know). So my questions for experienced folk (and any SE reading) are:

 

1. Is this subsidence? The surveyor wouldn’t say the word directly. 
 

2. Is it likely that the tree has dedicated the soil and no further movement will occur?

 

3. Do we need a drain survey if the nearest drain is at the other side of the house? It only has foul drainage, with surface water draining into gravel soak aways. 
 

4. Should we ask for a price reduction, and if so how much?

 

5. Can we just assume everything is stable now the tree has gone and level the floor?

 

6. Is there anything we might be missing?

 

7. Are we worrying unnecessarily?

 

The floor drop can be seen in the attached pictures. 
 

 The surveyor did summarise that the house is a good purchase provided we are prepared to spend some

money on it. Vague advice…

 

Many thanks in advance for reading and replying. 

[ProDave]

I would certainly want to pull that carpet up for a look. I bet the surveyor did not do that.  Go armed with a long spirit level and your camera again.

[saveasteading]

Subsidence seems the likeliest thing. Ground shrinkage would be more general than just  that corner.

It might only be the slab, not foundations.

If it is clay shrinkage then it shoild tecover over the winter

Are there any cracks in walls, inside or out?

[ChrisL69]

Thanks for the thoughts. No cracks anywhere inside or out, and there hasn’t been any redecorating for many years so they haven’t been hidden with filler. There has been an extra radiator put in at some point where the drop is, and it looks like the pipes are within the concrete floor - could that be relevant?

[ChrisL69]

51 minutes ago, ProDave said:

I would certainly want to pull that carpet up for a look. I bet the surveyor did not do that.  Go armed with a long spirit level and your camera again.

No he just looked at the gap under the skirting and stamped on the floor. Unfortunately we’re moving from 150 miles away so are reliant on the surveyor and vendor. 

[Mike]

1 hour ago, ChrisL69 said:

1. Is this subsidence? The surveyor wouldn’t say the word directly.

Subsidence is a loaded word more commonly taken to mean the dropping of the ground, taking foundations wall and floor with it. If only a localised part the floor has moved that may be why the surveyor didn't want to use the term, even if it has, in linguistic terms, subsided.

 

1 hour ago, ChrisL69 said:

2. Is it likely that the tree has dedicated the soil and no further movement will occur?

It could have been the tree. Or maybe poor compaction of the soil or hardcore beneath the floor slab. Maybe there was an old well / bomb shelter / tree root / something else just there.

 

BTW, as I recall, there isn't much clay in Norfork, is there?

 

1 hour ago, ChrisL69 said:

3. Do we need a drain survey if the nearest drain is at the other side of the house? It only has foul drainage, with surface water draining into gravel soak aways.

It wouldn't hurt, and they're not unduly expensive. A structural engineer's survey of the building may be better though, if you want additional reassurance.

 

1 hour ago, ChrisL69 said:

4. Should we ask for a price reduction, and if so how much?

That depends how keen you are to buy and how competitive the market is.

 

1 hour ago, ChrisL69 said:

5. Can we just assume everything is stable now the tree has gone and level the floor?

It wasn't necessarily the tree...

 

1 hour ago, ChrisL69 said:

6. Is there anything we might be missing?

It's more a case of whether your surveyor has missed anything.

 

1 hour ago, ChrisL69 said:

7. Are we worrying unnecessarily?

If that's really the only fault, and building's been there since 1969, then I'd not worry that much, but my risk aversion may be different to yours. And, budget permitting, I'd be tempted to take up all the ground floors anyway, then insulate them and install underfloor heating.

 

[Oz07]

Id not be as concerned if it was just the slab and not foundations. Look for cracks in walls to see if foundations have moved. Internal corners often don't get the hardcore compacted as well so sometimes sound a bit hollow anyway. Maybe the moisture levels changing in the soil haveade the ground move a bit in that corner?

[saveasteading]

7 hours ago, Mike said:

Subsidence is a loaded word more commonly taken to mean the dropping of the ground

Once you have had the word used, you need to advise insurers for ever, so it's a good thing the term is not used here.

As the structure is OK, it probably isn't.

 

You have a small chance of a big problem.   Ultra local moisture effects are unlikely......the slab is protected by walls and footings.

 

 

A large chance of a small problem.

 

Tell us more about the hollow sound. Is it when walking, or tapping?

Could there be a top screed that is loose after pipes were cut in?

 

[ProDave]

9 hours ago, ChrisL69 said:

No he just looked at the gap under the skirting and stamped on the floor. Unfortunately we’re moving from 150 miles away so are reliant on the surveyor and vendor. 

This is a major purchase, usually your house is the most expensive thing you ever buy.  It IS worth a good long day trip to thoroughly look yourself.  Depending where you are anything from 3 to 6 hours driving.  If you don't think you can do it in a day, book a B&B.  Get that carpet up and look at the slab, in particular look for crack.  and as I say buy or borrow a good long spirit level.

[saveasteading]

I might take a chance.

I cannot advise you to.

You must get an SE look at it, even informally  walking round with you.

 

The tree is long past being relevant I think.

 

Any recent holes in the ground nearby to see what the ground is? But a local SE will know anyway.

[Galileo]

Off topic, but was this a common enough style back in the 60s? I have a strong feeling that I've seen that living room before somewhere! There was a discussion about a window or door to a first floor balcony that was awkward due to the roofline cutting through it. Should probably just drink less coffee..

[Mr Punter]

16 hours ago, ChrisL69 said:

it looks like the pipes are within the concrete floor - could that be relevant?

 

I could be if they have been leaking and made the soil weaker in that area.

 

Get the vendors consent to roll back the carpet.  Check the concrete / screed for damage.  Tap it all over to see where there are voids.

 

Double check gutters, downpipes and drains as well as water main.

[ChrisL69]

On 12/09/2024 at 00:25, Mike said:

If that's really the only fault, and building's been there since 1969, then I'd not worry that much, but my risk aversion may be different to yours. And, budget permitting, I'd be tempted to take up all the ground floors anyway, then insulate them and install underfloor heating.

That would be my preferred plan, but the budget doesn't stretch nearly that far. I'm thinking right now that if the walls aren't affected then the problem must be fairly minor....

[ChrisL69]

Thank you all for all the advice. As far as I can tell there are no drains or water mains anywhere near the affected area. The walls aren't affected so the drop is isolated to the slab. There is the evidence of a tree that was once close by, and the surveyor has highlighted dangers from other large shrubs a few metres away, and the soil is clay so I am thinking possible desiccation and/or a void caused by the tree roots rotting away. I am also suspicious of the additional radiator (making three very close together) and any pipework with joints that have been chased into the concrete floor.

[ChrisL69]

So I'm having real difficulty engaging a structural engineer to assess the dropped slab. Most have refused to consider the job, but a couple have chatted on the phone. Two have said it will just be poor compaction under the slab and not to worry unless there is a crack. One has said a full site inspection is essential at great cost!

 

My father in law has lifted the carpet today and taken photos of the floor. The drop is 10-15mm in the corner and there are surface cracks at right angles to the drop. The cracks are level and very shallow. The floor sounds noticeably hollow in the corner and the walls reverberate when the floor is stamped on. I have attached photos for any shared expertise please...

 

Thank you all.

plan.pdf

[Mr Punter]

If that is an uninsulated concrete floor you may want to consider a new insulated floor.  Otherwise, some levelling screed.

[Iceverge]

Have a really really good look for bad cracks in the wall, ceiling, around windows etc. 

 

If they're ok I wouldn't worry in the least about floor. 

 

The house is 65 years old and if it's lasted this long it'll last another 650 years if minded. 

 

 

 

 

[saveasteading]

The cracks are normal. The gap under the skirting is common, just through some settlement of the slab or a rough job with the skirting.

 

If the slab was settling there would be other signs.

 

SE would be taking on a risk for a small fee. Good business sense to decline I'd say.

[Richard Anthony Johnson]

Saw this thread and had to sign up. I recall seeing that the site is in Norfolk. It would be use fuil knowing where in Norfolk it is.  The County has a mixed bag of geology, with Clays Silts and PEAT to the west, local to the fens, Chalks and Gravels to the North west of Downham Market, then diamicton heading East, which in effect is a mixed bag of everything, laid down by glacial runoff and glaciers. 

The note about the tree, felled 14 years ago an set 4 meters from the property is a potential indicator of prior movement due to shrinkage of and clays and silts (hence why its good to know the location). The ground takes up to 10 years to recover after a tree has been felled but water demand differs by species. 4 meters is very close. Was it done on advice and is there evidence of repointing on the brickwork outside? do any of the bedjoints/ perps seem wider than others or is the mortar a different colour in places.

Guttering and downpipes can be an issue, as indicated by others and I usually find more often than not that localised subsidence has been caused by a failure to maintain the rainwater goods. However, this has more of an impact on fine grained silts and sands, where the water can flow through the ground, dragging fine particles with it, causing consolidation of the supporting soils. Also,  failed gutters would be expected to affect the foundation of the building, not the slab.

I've certainly had one site where the contractor didnt compact to the head of strip footings inside the building, before laying the slab.  The result was a 10mm drop in the finished floor to the perimeter of the building. 

Your building is 1960's. It'll have relatively shallow foundations by today's standards so I'd rule out subsidence, more because of the insurance, resale and mortgagability issues but also because settlement or consolidation are more accurate words.  It's also worthwhile noting that the building is 50 over years old and the skirting looks just as old.