Re-inforcing joists with CLS timber

[David Lindon]

I have joists 5cm x 20cm spaced roughly 40cms apart spanning a 3.5m square room and the floor has a slight bounce allowing the flooboards to flex and squeak. I'm not sure what type of wood the joists are made of but it is quite soft. I'm going to lift and replace the floorboards and at the same time I plan to re-inforce the joists by running some CLS timber along each and bolting with some coach bolts. Are there any potential issues with this approach? For example expansion and contracting in different temperatures on different types of wood?

 

[martygturner]

Worked for me...found the easiest way was to omit the coach bolts and glue and screw. Getting a drill in to go through the cls and the joist can be a right faf given that you will be shrinking the joist space down by 100mm , the first one goes in then when you turn round to to the next joist the drill does not fit in the hole....
I would also suggest getting a couple of acrow props in below on a plank to push up a bit say 1inch on the floor, that way when is done and you take the props out the floor will try to return to its slightly lower level and then you will place the new cls under tension, effectively locking out the bounce.

 

[RandomGrinch]

Worked for me...found the easiest way was to omit the coach bolts and glue and screw.

Yep, same for me in the loft - I used some coach screws, basically following this advice...

Post in thread 'Strengthening Loft floor' https://www.diynot.com/diy/threads/strengthening-loft-floor.326918/post-2424203

I also propped from below, to take out some of the bow.

...but have you the space to raise the floor level?
Adding some noggins, or sistering some joists, may be enough to stiffen the floor and take out some bounce?

 

[cdbe]

Your joists would appear to be within spec for the span, are you sure they're not flexing (twisting sideways) which can give similar symptoms? I would check for noggins or strutting first. If the joist need strengthening I would used something a bit more substantial than CLS (stud wall timber), which I can't see having much effect at a 3.5m span. I would have thought 6x2 C16 as a minimum.

 

[Dereekoo]

Your joists would appear to be within spec for the span, are you sure they're not flexing (twisting sideways) which can give similar symptoms? I would check for noggins or strutting first. If the joist need strengthening I would used something a bit more substantial than CLS (stud wall timber), which I can't see having much effect at a 3.5m span. I would have thought 6x2 C16 as a minimum.

No appear about it, they are well within spec (assuming minimum of C16 grade wood) and calcs indicate max deflection of 5mm against allowable of 10mm. Why do you think it is bouncy, is it something you feel or is it just because the floorboards squeak? If its just a perceived thing because floorboards squeak punch down all nails fixing floorboards. Noggins do not stop joists flexing as such because they add no structural strength in the vertical direction, at best they stop the joists from leaning over and loosing some of their rigidity but if there are floorboards in place this is already happening.