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Need Help Stiffening Joists

I am remodeling a 1928-era farmhouse. My current project is remodeling a basement room. The ceiling joists run 15 ft, and are 2X8 (actual size 7 1/2 X 2) and are 24" OC. In other projects I have simply sistered up a new joist beside each existing one and cross blocked as appropriate. I have never been satisfied with the stiffness of the strengthened ceiling. There is always still some "bounce" on the floor of the room above. Replacing the 2X8's with 2X10 or-12 is not an option as the unfinished cement slab to unfinished ceiling height is only 80 in, and I don't want to lose any headroom. The existing joists rest at each end on a poured cement wall capped by a 2X8 plate. And, whatever method I use to stiffen the joists, I have to work around numerous protruding nails from the sub-floor above.

I have thought of installing OSB "I-Beams" between the existing joists so that I-Beam to existing joist is 12" oc, but that is an expensive option, and I'm not sure I can force them into the support plates at each end.

I also thought of simply ripping 7/16 OSB to 7 1/2 in and sistering (glue and screw) to existing joists (after pre-stressing the joist upwards a bit) Problem there is I can't find OSB in 15 ft lengths. A possible solution may be to laminate 2 layers of OSB so that the ends overlap ( a 4 ft, 8 ft and 4 ft layer laminated to 2 8 ft layers and both sistered to existing joist).

Or I could just add additional 2X8 joists as I have done before, cross brace and live with the slight "spring". If this is the best answer, should the new joists be sistered directly to the old, or should they be placed mid-point between existing to give 12" OC?

Any suggestions/comment appreciated. Sorry for the length of this post

Re: Need Help Stiffening Joists

If those joists are spanning 15 ft without any intermediate support midspan they are grossly over extended even if they are true 2 inch width.

Combined with the 24 inch spacing you definately have bounce .... Even sistering dimensional 2x8 with cross blocking will still give bounce as you know. The reason for the bounce is the span of 15 feet unsupported. Using OSB to try and stiffen the joists will be a wasted effort.

Even using manufactured I joists would likely result in a 9 - 10 inches depth at 15 foot span with the possibility of 12 inch spacing .... definately 16 in spacing minimum.

If you don't want a support midspan then you would need to have an engineer recommend an alternate method.
Perhaps something like a 1/4 in thick steel the 7 1/2 depth bolted to each 15 foot joist ..... as an example.

Re: Need Help Stiffening Joists

I agree with Canuk. ¼" steel plate drilled every 16" top and bottom and bolted to joists would be your best bet. However it's not going to be cheap.

Re: Need Help Stiffening Joists

Thank You, All. Not the answers I wanted, but I expect the right ones.

Re: Need Help Stiffening Joists

I also agree with the other responses. However, for future reference, you wouldn't need a 15' long piece of OSB because the center 8' of the span is really the important area for stiffness. If you look at the moment diagram which is directly related to the bounce in your floor, you could reduce the bounce by only stiffening the center half of the beam. Again, I don't think this will do much good for you because the joists are already far too small for that span.

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