Deflection design for posi joist floor

[SBMS]

Getting our posi joist floor design done. The initial design targeted around 13.5mm deflection in the joists. I asked the engineer to quote for a 10mm and 8mm deflection. 
 

10mm can be done with a wider top chord and at 600mm centers. 8mm requires 480mm centres. 8mm deflection across two floors is about £1600 more plus probably some additional labour time as there are more joists (they are being hung). 
 

Our current build I never thought about this and probably ended up with 13.5mm deflection. It’s a bit bouncy in some rooms. I feel 10mm is significantly better and my gut says 8mm will be overboard and I won’t detect the difference. I think it’ll probably be over £2k more for the 8mm deflection. 
 

Can anyone comment on if they have 10mm deflection - is it enough?

[Russell griffiths]

Go for the best, don’t skimp on the main structure of the house. 
better of forgetting the expensive toilets and build the structure better. 

[SBMS]

17 minutes ago, Russell griffiths said:

Go for the best, don’t skimp on the main structure of the house. 
better of forgetting the expensive toilets and build the structure better. 

Thanks Russell. Well the best is 0mm deflection - where do you stop?

[nod]

Counter batonning the underside with 100x25 saw cut makes a massive difference Closing the centres also Which adds a third to your quote 

None of this is expensive and the grand scheme of things and somthing you can’t change later 

[Thorfun]

We went for 8mm deflection and there still a bit of bounce on our 6m spans. Would’ve been horrendous if we’d gone any higher. 

[crispy_wafer]

1 minute ago, nod said:

Counter batonning the underside with 100x25 saw cut

if anything like mine, this, or res bar will need doing, just to have lovely flat surface to plasterboard.

 

just looking back through my emails

304*75 at 400's worked out at 11.2mm deflection cost was 1585+vat for 25 joists to span 5.2m

 

asked them to get me to 8mm and they came up with

304*122 @ 400's cost was 1980 + vat for same qty's. for 400, along with switching to attic trusses it was a no brainer, makes for piece of mind.  400 centres is a pain for soil pipe, so design in mind

 

 

[joe90]

12 hours ago, Russell griffiths said:

Go for the best, don’t skimp on the main structure of the house. 
better of forgetting the expensive toilets and build the structure better. 

I have no idea what bounce figure I have but cannot detect any bounce at all simply by calculating for 600mm centres but installing at 400mm.

 

Edited November 1 by joe90

[joe90]

31 minutes ago, nod said:

Counter batonning the underside with 100x25 saw cut makes a massive difference

Brilliant (cheap) idea

[SBMS]

53 minutes ago, nod said:

Counter batonning the underside with 100x25 saw cut makes a massive difference Closing the centres also Which adds a third to your quote 

None of this is expensive and the grand scheme of things and somthing you can’t change later 

This is actually a great point. All our floors will be counter battened in this manner by the builder for a bit of a service gap and for plaster boarding. This isn’t accounted for in deflection though, so I wonder how much of an impact regarding deflection this would have? Ie if I keep the 600mm at 10mm deflection I wonder if the counter battening brings this down to 8mm anyway?

[Thorfun]

1 hour ago, crispy_wafer said:

400 centres is a pain for soil pipe, so design in mind

and shower tray wastes. i ended up using trays that had variable waste positioning to accommodate joist locations

[ProDave]

Posi joists over about 3m span I think are usually specified with strongbacks.  Would adding a second set reduce the deflection figure?

 

I have mentioned before, look for little "gotchas"   Our joists were specified for the longest span, but since they span the entire width of the house as one loing joist, that meant in most cases the 2 shorter spans in effect had over specified joists.  Nothing wrong with that.  Except our bedroom, where the long span was broken by the stair well, so half our bedroom got done with the "correct" joists for the span, and the other half the over specified continuous joists.  There is noticable bounce on the "correct" span side of the room and my regret is not spotting this on the plan, and insisting the whole room got done with the same over specified joists.

[saveasteading]

20 hours ago, SBMS said:

10mm deflection - is it enough?

I can't recall ever having an issue with joists selected from the span tables. Remember that in a domestic situation a lot of the load is furniture so part of the deflection is permanent... and any bounce in walking is quickly dampened by the sofa.

But in a dance studio or gym it could be very unpleasant.