Help slab built too high

Ligia · Jan 01, 2025 12:34 am

My slab has been built too high (about 10-15cm) above the plan floor levels. I’m worried that this means the slab is structurally compromised. The whole slab is 410mm deep, waffle. With bored piers of about 90cm - 100cm length. Soil is H2/P class. Theres sections where it looks like the whole slab (all 40cm of it) sits on top of the ground.

I expect fill will be added around the perimeter of the slab. Although I haven’t got clarity on this from the builder yet. I learnt too late about the waffle pod sagas on reactive clay … now I’m quite worried that the slab is not set deeper into the soil and what issues can arise.

We will be constructing a perimeter concrete skirting around the slab if that makes a difference… except for the sections on boundary which we obviously cannot control 😢

Any advice welcome.

Ashington Homes · Jan 01, 2025 4:00 am

Ligia

My slab has been built too high (about 10-15cm) above the plan floor levels. I’m worried that this means the slab is structurally compromised. The whole slab is 410mm deep, waffle. With bored piers of about 90cm - 100cm length. Soil is H2/P class. Theres sections where it looks like the whole slab (all 40cm of it) sits on top of the ground.

I expect fill will be added around the perimeter of the slab. Although I haven’t got clarity on this from the builder yet. I learnt too late about the waffle pod sagas on reactive clay … now I’m quite worried that the slab is not set deeper into the soil and what issues can arise.

We will be constructing a perimeter concrete skirting around the slab if that makes a difference… except for the sections on boundary which we obviously cannot control 😢

Any advice welcome.

personally I wouldnt stress however reach out to the structural engineer who designed the slab and ask their opinion.

Structural engineers are the best and I have always found them easy to talk to so just pick up the phone and put your mind at ease

cheers

Simeon

Ligia · Jan 01, 2025 8:30 am

Ashington Homes

Ligia

My slab has been built too high (about 10-15cm) above the plan floor levels. I’m worried that this means the slab is structurally compromised. The whole slab is 410mm deep, waffle. With bored piers of about 90cm - 100cm length. Soil is H2/P class. Theres sections where it looks like the whole slab (all 40cm of it) sits on top of the ground.

I expect fill will be added around the perimeter of the slab. Although I haven’t got clarity on this from the builder yet. I learnt too late about the waffle pod sagas on reactive clay … now I’m quite worried that the slab is not set deeper into the soil and what issues can arise.

We will be constructing a perimeter concrete skirting around the slab if that makes a difference… except for the sections on boundary which we obviously cannot control 😢

Any advice welcome.

personally I wouldnt stress however reach out to the structural engineer who designed the slab and ask their opinion.

Structural engineers are the best and I have always found them easy to talk to so just pick up the phone and put your mind at ease

cheers

Simeon

We would/will, unfortunately he’s notoriously hard to get hold of. Not very professional in that respect (even outside of festive season), doesnt respond to emails or answer phone.

Eng. B A · Jan 01, 2025 2:58 pm

Can you post the soil logs and what type of building is it?

Ligia · Jan 01, 2025 10:13 pm

Eng. B A

Can you post the soil logs and what type of building is it?

Here’s screen shot of soil log. It’s a modest double Storey house. Brick veneer at ground and cladding on second floor.

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Eng. B A · Jan 02, 2025 6:25 am

If you are only have 200mm of uncontrolled fill, the slab could be sitting in firm ground according to soil log. However, if the engineer specified 410mm deep slab, then he may meant for the slab to be found in the next layer of very stiff clay, this may be due to better soil bearing capacity at that depth. Without the DCP information and design loads, we can’t confirm whether the slab is still adequate or not. You need to contact the design engineer for confirmation.

building-expert · Jan 02, 2025 11:36 am

There is no excuse for the builder not building to design floor levels. It has now left you wondering not only about adequacy of the slab but also about builder's competence. If there was an issue on site this should have been communicated early and a variation raised together with engineering assessment and architectural assessment of the need and consequences of raised levels.
Perhaps there was a genuine need to raise the floor level, if so you should have been told beforehand.
As the owner I would not expect less than independent assessments listed above (paid by the builder)
I think you need to watch your builder and if you have already not done so get your own independent building consultant to look after you during the build.

Ligia · Jan 02, 2025 1:41 pm

building-expert

There is no excuse for the builder not building to design floor levels. It has now left you wondering not only about adequacy of the slab but also about builder's competence. If there was an issue on site this should have been communicated early and a variation raised together with engineering assessment and architectural assessment of the need and consequences of raised levels.
Perhaps there was a genuine need to raise the floor level, if so you should have been told beforehand.
As the owner I would not expect less than independent assessments listed above (paid by the builder)
I think you need to watch your builder and if you have already not done so get your own independent building consultant to look after you during the build.

Yes this has been a huge stress for us. There’s a fair bit of background. We are with a small custom builder. We provided all documentation .. surveys and architectural plans/ engineering. Unfortunately it appears to date that the land survey was incorrect. This was found just before Christmas and no one is around to consult with. The builder advises he has built to spec. Which we believe, as honestly I feel in my gut he’s not out to screw us.. and upon closer review of the land survey we can see the error. We believe the survey mistake should have been picked up earlier though as there were red flags that a seasoned professional (not just builder, but surveyor and our private building inspector) should have picked up. The garage slab (detached garage) is so high that it’s actually inaccessible and will now be demolished.

At this point we want to move forward. We aren’t wealthy people and cannot afford to pay mortgage and rent for more time than necessary. We will be asking the land surveyor what went wrong in their survey and getting them to assess although the land has now been cut so not sure how that works. We just hoping to get on with the main house and have that completed. Then tackle the garage issue separately.

building-expert · Jan 02, 2025 3:14 pm

Ligia

building-expert

There is no excuse for the builder not building to design floor levels. It has now left you wondering not only about adequacy of the slab but also about builder's competence. If there was an issue on site this should have been communicated early and a variation raised together with engineering assessment and architectural assessment of the need and consequences of raised levels.
Perhaps there was a genuine need to raise the floor level, if so you should have been told beforehand.
As the owner I would not expect less than independent assessments listed above (paid by the builder)
I think you need to watch your builder and if you have already not done so get your own independent building consultant to look after you during the build.

Yes this has been a huge stress for us. There’s a fair bit of background. We are with a small custom builder. We provided all documentation .. surveys and architectural plans/ engineering. Unfortunately it appears to date that the land survey was incorrect. This was found just before Christmas and no one is around to consult with. The builder advises he has built to spec. Which we believe, as honestly I feel in my gut he’s not out to screw us.. and upon closer review of the land survey we can see the error. We believe the survey mistake should have been picked up earlier though as there were red flags that a seasoned professional (not just builder, but surveyor and our private building inspector) should have picked up. The garage slab (detached garage) is so high that it’s actually inaccessible and will now be demolished.

At this point we want to move forward. We aren’t wealthy people and cannot afford to pay mortgage and rent for more time than necessary. We will be asking the land surveyor what went wrong in their survey and getting them to assess although the land has now been cut so not sure how that works. We just hoping to get on with the main house and have that completed. Then tackle the garage issue separately.

On further information you provided its clear builder is not at fault but you, through your own contractor(land surveyor) supplying erroneous data to the builder.

Ligia · Jan 02, 2025 5:19 pm

building-expert

Ligia

building-expert

There is no excuse for the builder not building to design floor levels. It has now left you wondering not only about adequacy of the slab but also about builder's competence. If there was an issue on site this should have been communicated early and a variation raised together with engineering assessment and architectural assessment of the need and consequences of raised levels.
Perhaps there was a genuine need to raise the floor level, if so you should have been told beforehand.
As the owner I would not expect less than independent assessments listed above (paid by the builder)
I think you need to watch your builder and if you have already not done so get your own independent building consultant to look after you during the build.

Yes this has been a huge stress for us. There’s a fair bit of background. We are with a small custom builder. We provided all documentation .. surveys and architectural plans/ engineering. Unfortunately it appears to date that the land survey was incorrect. This was found just before Christmas and no one is around to consult with. The builder advises he has built to spec. Which we believe, as honestly I feel in my gut he’s not out to screw us.. and upon closer review of the land survey we can see the error. We believe the survey mistake should have been picked up earlier though as there were red flags that a seasoned professional (not just builder, but surveyor and our private building inspector) should have picked up. The garage slab (detached garage) is so high that it’s actually inaccessible and will now be demolished.

At this point we want to move forward. We aren’t wealthy people and cannot afford to pay mortgage and rent for more time than necessary. We will be asking the land surveyor what went wrong in their survey and getting them to assess although the land has now been cut so not sure how that works. We just hoping to get on with the main house and have that completed. Then tackle the garage issue separately.

On further information you provided its clear builder is not at fault but you, through your own contractor(land surveyor) supplying erroneous data to the builder.

At the risk of sounding arrogant. We are actually fairly smart people and poured over every piece of documentation. Unfortunately we aren’t in construction, first house. Trusted that professionals would provide the advice and skills necessary. In any case, who is at fault is less of a priority. Just want a house for my three kids to have space to play and live.

I’m assuming when I raise it with the land surveyor he’s going to point to their various indemnities and the fact the builder should have raised discrepancy as to why the land levels that were meant to be flush with the adjacent driveway was suddenly 40cm higher. Looking forward to how that all works out. But I expect as you say, the owner is at fault…

building-expert · Jan 02, 2025 7:37 pm

I was referring to your relationship with the builder.
In your relationship with land surveyor he is at fault if there is an error with levels. I think you should get past the indemnities however as you say your priority is to house your family.
This is why essential information should be peer reviewed. It does not mater how smart you are if you don’t know what to look for. It simply means you are going outside your expertise.
Building Surveyors are not architects and engineers and cannot be expected to review designs other than for statutory compliance.
In engineering matters BS will ask that design is certified by design engineer then independently peer reviewed and certified. Relying on certifications from two insured engineers BS will accept the proposed design with confidence.
I won’t ask about your independent inspector but you already know you need to rethink.