Joist Deflection Bowing Floor

[nitro23456]

Hi all

My upstairs floor is bowing.

Joist are 2x9, span is 4.5M, spacing 450mm centres. Chipboard P5 floor on top which Ive not lifted yet. House 20 years old.

There is a studwall built halfway along the span. This stud wall separates the bathroom from the small bedroom and the centre of the bowing seems under this studwall. There doesnt apear to be any load transferred from the studwall to the floor though as i can lie on the floor and see between the two rooms under the stud base if i try to. So the bowing just appears to be at the centre of the span with the wall on top perhaps a red herring as a potential cause.

Using a laser, i calculate the bowing to be 15-20mm with maybe 4 joists affected.

I plan on ripping the bathroom out, lifting the floor and sistering the affected joists using 2x6 and m12 coach bolts.

Any thoughts on this plan or why this has even happened?

 

[jks7492]

9x2 joists at 450 centres are reasonable for a span that size and under normal loading wouldn't deflect that much. Are there any noggins installed between joists?

 

[nitro23456]

Not sure as ive not lifted any of the floor yet.

 

[jks7492]

Not sure as ive not lifted any of the floor yet.

How do you know the size and spacing of the joists then?

 

[nitro23456]

View media item 107771
Joist size measured from landing at stairs and spacing assumed from nails in chipboard.

Photo showing deflection in centre of room with laser, that is the studwall between the rooms perpendicular to the joists which run underneath it. Stud wall built on top of it.

So the floor is level except where it dips like a shallow "U" for say a 800mm either side of the wall on each side

 

[nitro23456]

View media item 107772
An exaggerated drawing, but u get what i mean hopefully.

 

[tony1851]

Did the plumbers notch out the joists excessively in the middle, where the rad. is?

 

[nitro23456]

Im going to lift a section of board at the weekend and see whats going on.

I suppose anything else is just guesswork.

Bathroom is original.

 

[nitro23456]

Ok, so Ive lifted a section of floor and everything looks ok to me. No cracked joists or anything, just that bracing under the studwall that looks inadequate to me. No other noggins for the whole span.

First picture shows the bracing under the stud wall with the radiator pipe going up.

View media item 107791
Second picture is looking back the other way under the bath to where the block wall is. Again I cant see any issues.

View media item 107792
So my plan is to sister some 2x6 joists with m12 coach bolts in the bathroom to level the floor and add some noggins. I'll tackle the other side of the studwall (bedroom) another time and do the same in there.

Any thoughts? Would 2x4 sisters be ok instead? (Cheaper/easy to manoeuvre).

 

[designinsteel]

The joists might actually be fine in terms of strength. The structural codes say you can expect 14mm of deflection at midspan, or 0.03 x span, which is 13.5mm. But it would be unusual for them to deflect fully like this without a serious amount of loading. Perhaps the wall has a support post over it for the roof or loft storage?

 

[nitro23456]

But the base/sole plate of the studwall is hovering off the chipboard floor underneath i.e. it is practically level whereas the subfloor underneath it is not.

 

[John D v2.0]

It's always possible there was a quality issue with the original timber and it is just bowed, or more likely they are all bowed a bit but some were placed crown upwards and those are crown downwards

 

[nitro23456]

Im beginning to think this must've been like this for a very long time.

Is 2x4 ok to sister with or would you go with 2x6 and is my plan to sister the bathroom only to basically level the floor in there for a new bathroom suite ok? (i.e. from the block wall to the radiator side of the studwall). Il then do the bedroom side in say 6 months time.

I cant think of another way of doing it without removing the studwall.

 

[John D v2.0]

Yeah if it's just to get a level surface for the floorboards then I've used less than 4 by 2 in the past although mainly for bridging wide notches, you wouldn't need continuous pieces so having gaps for the strutting would be ok.

 

[nitro23456]

Thanks for your help.