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Old 09-28-2009, 02:59 AM   #1
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Stiffen joists

I am looking to stiffen up my floor joists under a new kitchen and bath. My old house has true 2x8 joists (not 1.5x 7.5) over a 12 ft span sitting on a new glulam beam spanning 14 ft. I thought I might glue and screw 3/4 plywood to each joist to stiffen them up to accept porcelain or perhaps stone tile. I would also use 2x8s to make blocking between the studs afterwards to add stability. I was going to cut the plywood in 8 ft and 4 ft pieces to cover the 12 ft span, alternating the 4ft section on each joist.

Will this give me the stiffness and deflection factor I need to tile?

I hope this made sense.

Last edited by jjneedshvac; 09-28-2009 at 03:00 AM. Reason: spelling
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Old 09-28-2009, 05:49 AM   #2
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Ayuh,.. It might,.. Or you could add uprights at the center of the span...
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Old 09-28-2009, 07:43 AM   #3
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You didn't say what centers your existing joists are set at. If they are on 12" centers you may want to sister new joists to old. If they are on 16", you might be better off to add joists in between.
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Old 09-28-2009, 08:07 AM   #4
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I'm doing the same thing for my kitchen to stiffen up the floor getting ready for tile. I'm ripping 2x10s down to get the width of the old stock and sistering with glue and nails.
I tear things down and build them up.
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Old 09-28-2009, 08:55 AM   #5
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By my computations, true 2x8 joists centered 16 inches OC spanning 12 feet with a live floor load of 40 psf (typical for a bathroom) and a dead load of 10 psf would have a maximum deflection of approximately 0.2 inches at the center, for a d/L ratio of about 1:600. This is OK for most tile, maybe not for large format porcelain, but remember you need to add two layers of subfloor, which will stiffen the joists somewhat. Unless you are planning on putting a very heavy tub (i.e. cast iron) full of water on the bathroom, you should be OK without doing anything. However, you may want to hire a structural engineer to check these calcs, as your load may be different than I assumed, the joists may be made of a species of wood that is not as stiff as normal, or there could be an unusual geometry situation that affects the loading on your joists. Or my calcs could be wrong.
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Old 09-28-2009, 09:52 AM   #6
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Hmmmmm...something I had not considered
I thought my 2nd floor (new) bath (shower) 2x6 floor joist spanned 8'
They don't - they span closer to 11'
The bathroom wall ends at 8'...but the joists span to the next wall at 11'

When you say new kitchen & bath - just remodeling the area?
If totally remodeling why not add some joists?

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Old 09-28-2009, 02:45 PM   #7
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How many layers of plywood would you need? Have you tried Googling "strengthening floors"? As others have said:
Be safe, Gary
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