Sloping floors upstairs

[Newhomeowner23]

First time poster as recently completed on our new home but have so far found this forum very helpful reading, so a thank you to those who contribute in advance 🙂

We have a question about flooring and NHBC tolerance. We’ve just had our snagging survey back which has highlighted 1 bathroom and 2 bedrooms have creaking and uneven floors. We hadn’t noticed these initially so glad to have had the Snagger find these.

The Snagging Inspector has raised them as they are outside the  NHBC tolerance which I understand is 3mm deviation across 1 metre for floors up to 6 metres, max 20mm out of level for floors over 6 metres across, and flat within a +\- 5mm deviation measured using a 2 metre straight edge with equal offsets. What we have is the following:
- bathroom (16mm deviation in level measured with snagger's 2m spirit level)
- bedroom (10mm deviation in level with a 2m spirit)
- bedroom (17 mm deviation in level with a 2m spirit)

All sloping seems to occur towards centre of house/stairwell

We’ve raised this with builder who so far have agreed to investigate, although we have had some initial suggestions such as settlement, which I’m not sure about since downstairs is ok. According to the carpenter who visited today (who had fixed the creaking but did not investigate the floor slope) he has suggested it could be deflection of joists and because the joists span the entire top floor across rooms they are allowed to be deviating by 20mm according to NHBC but I’m not sure if that is right, given the other NHBC guidance allowing max 3mm deviation within 1 metre for rooms up to 6 metres seems to apply, since all impacted rooms are within this size?  Obviously going to ask for further investigation on this and to take up boards to understand the issue, but just looking for a bit of guidance really on how to apply the NHBC standards in these case and what possible causes/solutions could be.

Many thanks in advance for your help & suggestions

[New Home Expert]

Your post demonstrates the value of a snagging inspection.
And thank you for recognising the value of the forum.

I suspect what you have are composite joists. These do deflect and in smaller span homes they will span across the whole house. This means there is a possibility that there is an out of level floor within the rooms upstairs.

Please pay no attention to the word settlement. It is builders speak for "it is normal" or "nothing can be done"

If your upper floors are outside the NHBC tolerances then this is a clear defect and needs addressing.  It may indicate that the I joist manufacturer's installation instructions and/or warranty standards have not been followed, such as a large hole cut in the web of the joist mid span or damage or cuts to the flanges and a whole host of other things.

If this is the case, you will probably have to move out for a month or two whilst any defective joist are removed or replaced.
Expect push back and further delaying and "explanations" from the builder.

Who is the house builder?