Unlevel Foundation Solutions

[nikki]

Hi. 
 

We bought a house with a large extension that had already been started which included the foundation and some of the blockwork. It was all signed off by

building control before we bought it (according to the building control officer).

 

Unfortunately, upon starting work to finish it our builder found that the foundation is not level (as was the blockwork 🤦🏼‍♀️) despite building control signing off on it all.
 

My question now is:

when we level the floor, should we do this with the screed layer on top of the insulation (so the screed will be thicker in some rooms) or should we level it out before we lay the insulation down so the screed on top will be an equal thickness across the whole extension. Or another options I haven’t thought of? The extension is approx 90m2 so it’s a fairly big area. 

[Bonner]

Building control don’t care about being level, only meeting regulations. How much difference between high and lowest points? My subfloor was about 40mm out, I filled some low spots which reduced the difference to 20mm and made the rest up in the screed.

[Dave Jones]

is the floor or the walls out of level ? Have you checked its square ?

[nikki]

47 minutes ago, Bonner said:

Building control don’t care about being level, only meeting regulations. How much difference between high and lowest points? My subfloor was about 40mm out, I filled some low spots which reduced the difference to 20mm and made the rest up in the screed.

Thanks! I’ll check and report back. I think it might have been around 90mm though 🙃

[nikki]

44 minutes ago, Dave Jones said:

is the floor or the walls out of level ? Have you checked its square ?

Both. Seems the original builder used a tape measure instead of a level when doing the walls as the walls followed the slope of the foundations. Our builder has fixed the walls now (we have roof on and windows in now) and they checked plum/square and the walls are all fine now. Just the foundation that’s still unlevel.
 

We wanted to do the insulation on the floor ourselves though to save some money so need to figure out what the best approach is to handle the unlevel foundation issue 🙂

[JohnMo]

14 minutes ago, nikki said:

Thanks! I’ll check and report back. I think it might have been around 90mm though 🙃

Could you simply add more insulation in the low areas?

 

Use EPS sheets to make up the difference - they are cheap enough.

[Jenki]

You don't mention your location, so assume not Scotland.

For a detailed reply we need to know the dimensions, current floor make up, height between current build stage and FFL any. Details you have will help with a solution.

What U value do you need to achieve?

 

[saveasteading]

20 hours ago, nikki said:

when we level the floor, should we

I would suggest both. 

First to check...by foundations not level do you mean all the way up to floor level?

Its very common for footings to go in to very poor level control. Lasers can make it worse as the builder who doesn't understand numbers assumes that the machine does.

Then it can be corrected by the bricklayer who may use string-lines, which are sometimes more accurate.* A story for another day.

 

If the perimeter at floor level is out, then that may cause all sorts of issues, so explain please.

 

 

in principle though you can adjust the insulation level to maximise the potential, then the screed will make up the balance.

2 layers of insulation is good anyway as you can overlap the boards

 

 

 

[nikki]

On 22/05/2024 at 08:18, Jenki said:

You don't mention your location, so assume not Scotland.

For a detailed reply we need to know the dimensions, current floor make up, height between current build stage and FFL any. Details you have will help with a solution.

What U value do you need to achieve?

 

No, not Scotland. We’re in Mid West Wales. 

The floor level is off through out the entire extension (approx 90sqm). The difference in floor level is about 8cm in the worst part. It’s a gradual slope from one corner to another so it’s not just pockets of it that are unlevel - it’s the entire slab. FFL will be approx 15cm higher than current FL but obviously will be quite a bit higher than this in the lowest corners.

 

I don’t have the u-value we need to achieve at the moment. The plans were submitted by the previous owner and never passed on to us when we bought the house. Waiting for building control to send them to us so we know the u-values needed for all aspects of the build (not just this). Our builder said 80mm celotex will be ok though.

[nikki]

On 22/05/2024 at 23:43, saveasteading said:

I would suggest both. 

First to check...by foundations not level do you mean all the way up to floor level?

Its very common for footings to go in to very poor level control. Lasers can make it worse as the builder who doesn't understand numbers assumes that the machine does.

Then it can be corrected by the bricklayer who may use string-lines, which are sometimes more accurate.* A story for another day.

 

If the perimeter at floor level is out, then that may cause all sorts of issues, so explain please.

 

 

in principle though you can adjust the insulation level to maximise the potential, then the screed will make up the balance.

2 layers of insulation is good anyway as you can overlap the boards

 

 

 

Yes all the way up to the current floor level. The difference is approx 8cm in the worst part. It’s a gradual slope across the entire slab so unfortunately it’s not just corners or pockets to be corrected. 
 

2 layers of insulation in some areas might work I guess. I’m just trying to get my head around how that would work i.e. cutting a slope into the first layer or something.

[nikki]

On 22/05/2024 at 08:13, JohnMo said:

Could you simply add more insulation in the low areas?

 

Use EPS sheets to make up the difference - they are cheap enough.

 

2 layers of insulation in some areas might work I guess. I’m just trying to get my head around how that would work i.e. cutting a slope into the first layer or something.
 

Unfortunately the current floor level is unlevel across the entire slab (90sqm) and not just corners or pockets so that makes it harder to fix I think. Difference of 8cm in the worst part.

[saveasteading]

6 hours ago, nikki said:

how that would work i.e. cutting a slope into the first layer

No slopes. Cover the whole area with the thickest board that the floor allows. Then add another layer on top, in one or two thicknesses. There will be steps but the screed takes care of that.

[nikki]

13 hours ago, saveasteading said:

No slopes. Cover the whole area with the thickest board that the floor allows. Then add another layer on top, in one or two thicknesses. There will be steps but the screed takes care of that.

Ah ok! That makes much more sense! Thank you!