25–27mm dished beam-and-EPS structural ground floor — likely cause and remediation?

[sisturix]

Hi all,
 

I’m looking for some advice on a badly uneven ground floor in a 2020 new-build.

 

The floor construction has been confirmed by the developer as a proprietary suspended system consisting of prestressed concrete beams, EPS/Jablite infill and approximately 75mm of reinforced structural concrete topping. There is no conventional floating screed. Glue-down LVT was installed over roughly 3mm of smoothing compound.

 

After lifting the failing LVT, I surveyed the floor on a 500 × 500mm laser grid. Both main rooms are distinctly bowl-shaped: high around the perimeter and low in the centre.

• Living room: 25mm total variation over approximately 4.5 × 4.5m.
• Kitchen/dining room: 27mm variation over approximately 5.5 × 4m.
• Calculated departures under a 2m straightedge reach approximately 6.9mm.
• There are particularly steep ramps near the patio doors.
•  

The same pattern appears to continue into the rest of the ground floor.

 

I’m trying to understand the likely cause. Possibilities I have considered are:

• curling of the reinforced topping from differential drying shrinkage;
• the topping having been struck out incorrectly;
• deflection of the beams;
• incorrect topping thickness or reinforcement position.

The biaxial bowl shape seems less consistent with simple beam deflection, but I am not a structural engineer.

 

The remediation options I have considered are:

1. Mechanically remove the old adhesive and smoothing compound, then install a bonded deep levelling compound (unsure about extra weight)
2. Use Fermacell dry levelling compound and dry-screed boards, possibly with Therm25 UFH.
3. Install an adjustable low-profile raised floor such as SubFloor steel rails with a structural deck.
4. Limited grinding of the highest areas combined with levelling material, subject to reinforcement cover.
5. In the worst case, structural removal and recasting of the topping, although this would be highly disruptive.
    

The intention is eventually to install wet UFH and a floating engineered-wood floor. Added weight and finished-floor height are important because of the entrance door and bottom stair riser.

Has anyone encountered this kind of deformation in a beam-and-EPS floor with a reinforced structural topping? What investigations would you carry out before choosing a repair? And which remediation approach would you regard as the most technically sound and proportionate?

PS: I guess I could attempt to raise a claim with NHBC, particularly seems it was mention in the first 2 years as well to the developer, but I have a feeling it won't go anywhere

[Oz07]

This reads like AI. Usually the beams are bowed upwards and you use SCC for the floating concrete top with insulation in-between. The concrete levels the surface out so its thinner in the middle, thicker at the edges. Has the floor moved since the concrete was poured? Do you mean you surveyed a 5x5m square, seems a big room but 500mm square seems to small to judge anything. 

[sisturix]

Haha, I'd love if it was AI, but it's very real and disturbing issue  I surveyed a 5x5m room, by splitting it into 50x50cm squares, and taking a reading in each of that point (laser in the middle of the room). Basically 100 measurement point equally spread throughout the room. I'm not sure if it moved since it was poured, I don't necessarily think so since I remember installing bookshelves and needing to shim a lot. Problem became more apparent 2 years in with the LVT separating a bit. 

The problem i have now is that due to this bowl shaped floor I can't have new flooring that requires SR1, and can't retrofit UFH that also needs a level surface. 

Edited 10 hours ago by sisturix

[sisturix]

Oh, one more detail regarding the floor structure, initially I thought it's beam and block + PIR + screed. My thought was to remove old screed, replace PIR, pour new screed with UFH pipes clipped into the PIR. However, the floor is something similar to https://bewi.com/products/structural-board-thermal-floor-system?lang=en-gb Because there are no blocks, the concrete pour on top is structural, so I can't mess with it.

[-rick-]

You are well within the warranty period and this has the potential to be an expensive repair (could also be relatively cheap). Get a structural engineer who is used to dealing with warranty issues to have a look and give an opinion. If they think it's a big/expensive issue then they can write a detailed claim to NHBC and get the ball rolling. If it's not a major issue then can propose the fix.

 

@Oz07 is right that these floor usually start with a crest rather than a hollow. Though they are expecting to move a bit when weight is applied. But going from a crest to a hollow seems a bit much to me (an amateur). 

[-rick-]

To add, if you do just decide to DIY something and there is an underlying structural issue that then reappears later, you may be invalidating the NHBC insurance (or at least giving them a lot of ammunition for an argument) and exposing yourself to much higher costs down the line.

[sisturix]

Yes, indeed, I did notify the builder now with a view of going to NHBC after. I will probably have to wait for the process to take it's course, and maybe source independent structural advice on the side. Sounds like 6 months without a floor 😑

[-rick-]

Just now, sisturix said:

Yes, indeed, I did notify the builder now with a view of going to NHBC after. I will probably have to wait for the process to take it's course, and maybe source independent structural advice on the side. Sounds like 6 months without a floor 😑

 

If I was in your situation, I'd want to get that independent SE opinion now both because it will provide some protection against getting fobbed off but also if there is a real problem, a professionally written claim with specific demands will almost certainly speed things up.

[Nickfromwales]

This needs a call to the warranty people. 
 

Mention to the developer that you are about to begin the claims process to see if that fires a rocket up his arse, if not, continue independently via the warranty advisor; taking their initial response as questionable would be wise, and post it up here for comment (anonymise it completely with zero specifics on it please).

 

It just sounds like an incompetent groundworker, on a desperately bean-counted (typical) developer site; get in, get out, max profit, minimum effort.

 

If you have radiators then why do you want to add a full wet UFH system? This will be a HUGE pita, and some local comfort ‘underfoot’ heating can be achieved with an electric solution (for more sporadic use). 

[Nickfromwales]

2 minutes ago, sisturix said:

Yes, indeed, I did notify the builder now with a view of going to NHBC after. I will probably have to wait for the process to take it's course, and maybe source independent structural advice on the side. Sounds like 6 months without a floor 😑

The good people at rugsRus will help you through the temp flooring annoyance    
 

 

 

 

 

 

 

 

 

Note: I just made that name up, please don’t post any cheques to them, you will not receive any rugs in the mail. Lol.  

[Nickfromwales]

@sisturix  

 

I don’t think the slab is curling, as it’s mesh reinforced so would be quite a trick.

 

This is literally a groundworker on his mother’s side.

 

If you want to draw a line, just self-level this and accept the £2k cost to correct it. Or, if you want to go to war, pursue it as above. 
 

A possible solution is to approach the builder / developer and ask them to pony up £1k towards the repairs as a one-off payment and that you will accept that as full and final resolution aka “case closed”. He should agree, I would! 

[ProDave]

13 minutes ago, Nickfromwales said:

A possible solution is to approach the builder / developer and ask them to pony up £1k towards the repairs as a one-off payment and that you will accept that as full and final resolution aka “case closed”. He should agree, I would! 

And if it then deflects further over time?  I am not sure I would do that.

[-rick-]

I think the most likely explanation is as Nick suggests but personally I think I would want someone with professional insurance to put that in writing before accepting 'full and final' settlement. If something has shifted underneath that slab then it'll cost a lot to fix. (I assume a 75mm reinforced slab should really resist bending, but if it's sloppy work how do we know it's actually 75mm?)

Edited 9 hours ago by -rick-

[ProDave]

You need to know if this is an insulated ground supported slab that may have settled (or been laid badly)  or a suspended beam system that may have deflected too much.

 

Surely the builder can tell you how they built the house?

 

Do you know if any others on the development have a similar problem?

[Nickfromwales]

22 minutes ago, ProDave said:

And if it then deflects further over time?  I am not sure I would do that.

It’s reinforced concrete….where’s it going to go?

[Nickfromwales]

5 minutes ago, ProDave said:

You need to know if this is an insulated ground supported slab that may have settled (or been laid badly)  or a suspended beam system that may have deflected too much.

 

Surely the builder can tell you how they built the house?

 

Do you know if any others on the development have a similar problem?

If the site is still ongoing, you can take a sneaky look of an evening. 

[sisturix]

43 minutes ago, Nickfromwales said:

If you have radiators then why do you want to add a full wet UFH system? This will be a HUGE pita, and some local comfort ‘underfoot’ heating can be achieved with an electric solution (for more sporadic use). 

I switched to a heat pump recently, and thought it's going to be more efficient, more pleasant underfoot, and I was anyway going to do some changes. However, after I understood how bad the levels are, even if I give up the idea of UFH all together, I still need to have some remediation in order to install hard floors. Whatever I do I'll need to raise levels I suppose, so that will impact stairs, entrance door etc.

[Nickfromwales]

Just now, sisturix said:

I switched to a heat pump recently, and thought it's going to be more efficient, more pleasant underfoot, and I was anyway going to do some changes. However, after I understood how bad the levels are, even if I give up the idea of UFH all together, I still need to have some remediation in order to install hard floors. Whatever I do I'll need to raise levels I suppose, so that will impact stairs, entrance door etc.

You’d only be infilling the depressions, so don’t make the mistake a lot of floor layers do, and go and add to the high points! All around the periphery, stairs, front / sliding / rear doors will get zero leveller; you may need to just knock these back with a bit of grinding, which can massively reduced the amount of infilling that’s needed.

 

 

Apologies on costs I stated above, you need to add the new flooring / labour / disposal costs on top of that. 

[sisturix]

40 minutes ago, Nickfromwales said:

If you want to draw a line, just self-level this and accept the £2k cost to correct it. Or, if you want to go to war, pursue it as above. 
 

A possible solution is to approach the builder / developer and ask them to pony up £1k towards the repairs as a one-off payment and that you will accept that as full and final resolution aka “case closed”. He should agree, I would! 

If it would be 2k cost to remedy this I wouldn't bother with developer / NHBC. However, my understanding is that seems you need to level from almost nothing (high spots on the edges) to 27mm (in the middle of the bowl), you need a compound that does well on that span. I got quoted 3500 + VAT for such a pour on an average on 12mm, with no guarantee for SR1. I Suggestions on compounds / who to pour would very much welcome!

The other complication is that in the deeper spots, ±30mm of say Mapei Renovation Screed 3240 + flooring would probably marginally go over the 0.5 kN / sqm that the beams are rated for in terms of floor finished (assuming my understanding of the floor deck is correct).

[sisturix]

13 minutes ago, ProDave said:

You need to know if this is an insulated ground supported slab that may have settled (or been laid badly)  or a suspended beam system that may have deflected too much.

 

Surely the builder can tell you how they built the house?

 

Do you know if any others on the development have a similar problem?

I've attached photos of the likely floor structure. It's like beam and block, but with EPS instead of blocks, and then structural concrete instead of screed 🤷‍♂️

Builder initially refused to confirm due to copyright / intellectual property nonsense. Then they mentioned the beam + EPS infill structure, 155mm x 120mm beams with a 75mm reinforced screed. I guess a structural engineer will need more details around beam spacing, orientation, etc - that so far they refused to produce. This is obviously not my area of expertise, but as an outsider I find it crazy that there's no legal framework to source those calculation from developer / building control (NHBC) but instead guess or do invasive surveys.

I can probably check with a few neighbours. I wouldn't be surprised is the same, but worth confirming.

 

[ProDave]

So it is suspended?  But you only say "likely"

 

Just an amateur suggestion, to determine if the beams are sagging, or it is the screed that was laid with a depression, drill and measure the screed thickness at the centre and the edges.

[saveasteading]

27 minutes ago, sisturix said:

photos of the likely floor structure

Likely? 

You need to know.

There's lots of good info above as if this was prestressed planks rather than beam and block. B and B seems more likely and the prognosis is less bad.

They can span 5m so this sounds simply to be an incompetent slab laying, but could be something else.

But 27mm is extreme. As is the straightedge.

 

What failed, exactly,  please?

 

If there is ongoing development you have enormous clout. First get more help on here, then visit the sales office.

Local publicity could kill all sales, so they will want to keep you quiet.

But you must also send a formal letter to the company that you have this issue.

 

Tell us more.  Is that definitely the construction?

What failed?

 

Sorry to question this  but I'd question myself too..... are you sure the laser level is accurate?  They do go wrong.

 

 

 

[Nickfromwales]

75mm is scraping the barrel for a constructional, reinforced slab. 
 

Takes us back to my above comment, on what arguably is, the worst possible place to be bean-counting…..

 

Developer built sites are always cash cow, sausage factories. They don’t bother with quality control and building control are useless wazzocks; probably 2 out of 10 of these new builds actually get walked into, and the other 8 will just be a paper trail.

 

Currently dealing with a BCO that had to admit to the client that he’d been on site, met the builder, checked all the major structural steels / pads / foundations, and signed them all of as kosher.

 

Turns out that he simply glanced across the site, saw there were a load of steel in situ, assumed the builder knew how to follow printed, written installation instructions, yet did no such thing.

 

We called the SE to site and he noted the vertical steels were the wrong sizes, in the wrong place, rotated 90° out of the required orientation, and were simply poking into the virgin ground with zero foundation / pads underneath them; the builder had consciously tried to deceive, by simply pouring a “pineapple ring” of concrete onto the ground around the base, squared off with some off cuts of OSB, to look like a full pad had been poured as per.

 

All professionals on site, myself included, agreed that this was the worst job / builder we’d seen in over 30 years.

 

BCO signed it all off as “correct”.

[sisturix]

15 minutes ago, ProDave said:

So it is suspended?  But you only say "likely"

 

Just an amateur suggestion, to determine if the beams are sagging, or it is the screed that was laid with a depression, drill and measure the screed thickness at the centre and the edges.

The floor is definitely suspended, I see air bricks, seen similar properties build next door, and the builder confirmed beam + EPS infill. Likely is just the BEWI Thermal Flooring, there are other competing insulated beam infills solutions out there. 

The problem with drilling is that I would compromise the DPM.

Edited 7 hours ago by sisturix

[Nickfromwales]

1 minute ago, sisturix said:

The floor is definitely suspended, I see air bricks, seen similar properties build next door, and the builder confirmed beam + EPS infill. Likely is just the BEWI Thermal Flooring, there are other competing insulated beam infills solutions out there. 

The problem with drilling is that I would compromise the DPM.

You’d just inject a silicone or mastic deep into the holes afterwards.