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I have been doing polished concrete throughout my house. My wife loves it more than I thought she would and now wants me to do it upstairs. I am afraid that the subfloor flexes too much, which wasn't a problem on the slab.

I don't want to put down anything too thick over it or the profile will be too much against the adjacent carpet. What is the thinnest way to stiffen the floor? I am going to use lathe so I don't really care about the cement board. I just want to get the flex out.

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related : diy.stackexchange.com/q/449/86 –  Joe Mar 8 '13 at 13:37
    
Is it just the "floor sheathing" that is flexing too much, or is it the whole floor system? –  decker Mar 9 '13 at 18:38
    
Just the sheathing –  Tatton Chantry Mar 13 '13 at 23:14
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1 Answer

up vote 3 down vote accepted

There are two choices to stiffen a floor system

  1. Halve joist span with a beam.
    The beam-in-joist method has the advantage of minimal final impact on the room below. It does mean spanning the beam to a load bearing walls.
  2. Double up joists in area. This requires removing floor or ceiling to sister existing joists. The joists also have to span as far as the existing joists.

The beam-in-joist solution should be confirmed with a building plan review for point load considerations under the transferred supports.

Update: I did come across a very thorough article from Fine Homebuilding "6 Ways to Stiffen a Bouncy Floor". (its a .pdf file)

Could I suggest a roll or 2 of Ditra (Schluter) under some nice tile?

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No on the tile. Doubling up joists may be an idea, however I think it's a little more than I wanted to do. How much strength do you lose but cutting out the existing subfloor and relaying it in one room without it being tied under the walls anymore? –  Tatton Chantry Mar 8 '13 at 10:32
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if you are looking for a way to stiffen the floor without sisters or closer joist spacing, it is going to be difficult without substantially increasing the thickness of the floor. HerrBag's answer is correct and really the best way to do what you want. –  shirlock homes Mar 8 '13 at 12:44
    
That's what I figured. Thanks both of you. I was hoping for some ingenious thing that I hadn't thought of. –  Tatton Chantry Mar 9 '13 at 2:48
    
bridging/blocking will help if you don't already have it. Although I don't think it "officially" increases deflection numbers it will often be noticeably stiffer when added. Also when "sistering" the greatest benefit comes from the center 1/3 of the span. While it is best practice to run the whole length, you should still see significant improvement just from doing the center 1/3. Just some thoughts, a little extra knowledge can go a long way depending on the situation -cheers –  decker Mar 9 '13 at 18:33
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@Decker agree on full height, solid blocking. Disagree on partial sistering. This may be a concept having its root from where the greatest stress on a joist is: the center 1/3. Sistering only there does not provide significant benefit, because no load is transferred off original joist to the load bearing sides. –  HerrBag Mar 9 '13 at 21:57
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