DIY Home Improvement, Remodeling & Repair Forum > Flooring - 24 OC joists

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02-19-2010, 12:52 PM
aaronled
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Join Date: Jan 2010
Posts: 6

Sorry for the delayed reply - busy month

Thanks, Wuzzat, for all the info!

As I said, I do have construction experience; however, I have only installed floor joist on new builds / additions.

I said in a previous post that I would LIKE to add the joist without ripping up the subfloor (and damaging the existing flanges). I would like to position each joist laying on its side where it's going to go (in the centers - not sister joist), add glue on the flange, then beat the joist upright into place, then someone on top will screw down via chalk lines.

This may be a long shot, but is there anyone out there that has done this? CAN it be done?

Furthermore, Would this help to make the floor feel more solid? Would it be worth the money / effort? The floor is not 'bouncy' per se - but it would be nice to stiffen the floor (as well as close the 24" gap before installing the new tile / hardwoods)

Thanks again.

02-19-2010, 01:49 PM
Wuzzat?
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Quote:
 Originally Posted by aaronled I would like to position each joist laying on its side where it's going to go (in the centers - not sister joist)
If your current deflection is L/360, what deflection would you like to have? L/480?

From the deflection design goal, the joist size [this gives I, the moment] and material [this gives E, the modulus of elasticity] can be figured.

But since I = b(h^3)/12, increasing h [height] by 10% gives a 33% increase in moment [(1.1)^3 = 1.33], but increasing b [width] by 10% gives only a 10% increase in moment.
That's why joists are on edge. It's the best use of the material for the money spent.

In your case you might do better running small steel I beams down the middle between the joists. I have a list of moments and E values for various types of steel I beams.

Or, if you can find a way to fasten a 2x under each existing joist to increase its height and therefore its I, you can dial up however much stiffness you want, at a loss of some headroom.

Last edited by Wuzzat?; 02-20-2010 at 01:52 PM.

02-22-2010, 10:43 AM
aaronled
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Join Date: Jan 2010
Posts: 6

I think what I wrote may have been taken out of context. Yes, I would like to lay the joist down on their sides in between the existing joist - BUT THEN, they would be stood on edge like the existing ones (the laying down on the side bit was just to convey the gluing and installing method.) The end result would be just like the existing joist - only now they would be 12oc instead of 24oc. I was trying to paint a picture of how I would go about it - sorry if it is confusing or not coming out right!

Having said that - your suggestion about the small steel beams sounds interesting. This makes me very curious - as it SEEMS it would be an easier way to install (could be visualizing it wrong though!) This might be worth looking into seeing if it's a viable option...codes, price for materials, etc...

The BIGGEST goal here is to provide a nice flat (without dipping) sub for new floors - however, if I can increase the L value, then that would be great too! As another poster pointed out, i could just beef up the sub-ply by adding more sheets...however, I'm not so keen on this idea being that it would seem to add a lot of extra weight on an already questionable deflection rating... Again, the basement is not finished yet (but soon will be) - and I think now would be the time to move on this (if I do)!