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Old 11-08-2009, 02:15 PM  
patch1239
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Default reinforcing a subfloor

We just had a structural report done on a house we are selling and the recommendation was to reinforce the subfloor. Here is my question. When we had the house remodeled we added cement board prior to putting tile down. Is that considered having the floor reinforced? The joist are 20 inches apart and in good condition. The inspector claims that some of the subfloor was "soft" and needed to be reinforced or replaced. To replace the subfloor we would have to tear up the new tile floor. He was not aware of the cement board or tile when he wrote the report...and he has not yet returned a call that I have placed to him. Any ideas out there. I really don't want to have to tear up the whole floor.



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Old 11-09-2009, 08:16 PM  
Adk
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It sounds to be that he had no clue what the floor consisted of to begin with and just that the floor joists were 20" OC.
I can tell you why he's not returning your call.....Nothing like looking like a douche and an ignorant one at that when you write an official report.
Time to have someone else look at your floor.



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Old 11-13-2009, 02:10 AM  
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I am planning on installing carpet on my third floor. I will need to put something down to raise the carpet to the level that the wood floors were at. The thing that I would like to fix in the process is that the floors shake some if you walk a little heavy on it. Little things will shake on the other side of the room. It is more of an annoyance than anything serious. I guess my question is would laying another layer of plywood help stiffen it up, or is it a lot more complicated to fix?

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Old 11-13-2009, 08:44 AM  
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Originally Posted by Superpack View Post
I am planning on installing carpet on my third floor. I will need to put something down to raise the carpet to the level that the wood floors were at. The thing that I would like to fix in the process is that the floors shake some if you walk a little heavy on it. Little things will shake on the other side of the room. It is more of an annoyance than anything serious. I guess my question is would laying another layer of plywood help stiffen it up, or is it a lot more complicated to fix?

Regards,
It is generally best to start a new topic for your own question, but I'll reply here as this pertains to the OP's question too.

Bending strength comes from thickness. That is why a floor joist is laid so the wide part is vertical and supports the weight. The best way to make a strong floor is to use a wider board in the original construction i.e a 2x10 vs a 2 x8. Once the floor is constructed, the best way to reinforce the floor is from underneath by "sistering" a second board to the side of the existing floor joist. The maximum stress is in the center, so the sister does not have to match the joist from end to end, but must be firmly attached (glued and screwed).

A second technique is to attach each floor joist to the one next to it, so the load is shared. This is done with blocking (a perpendicular board of same width as joist attached at each end to joists) or diagonal braces which connect the bottom of one joist to the top of the adjoining joint.

Adding a layer of plywood will make the floor a little stiffer, but this will not be nearly as effective as the above two techniques.

Google "reinforcing floor joists" for more suggestions

What to do about bouncy floors
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Old 11-13-2009, 03:40 PM  
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Actually, you can add as much stiffness to a floor by laying a thickness of plywood across the floor too. You should adequately attach it with screws and glue into the joists. Roughly, a 3/4 plywood panel(Not OSB!!) on the floor is equal to adding 2" in joist depth if you can stand the added height of the floor.

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Old 11-13-2009, 08:12 PM  
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Patch, the cement board is not structural, only dead weight. Read this on sistering: http://www.finehomebuilding.com/PDF/Free/021184090.pdf Recommended 1-1/4" minimum plywood under c.b.u. and tile.

SuperP, here is a good one on ply layering: The design of renovations - Google Books

Craig, may I ask where you got that good information on ply/joist rating?
Be safe, Gary

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Old 11-13-2009, 09:00 PM  
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Default quick question....

How the he@% does he know the sub floor is soft under hardy board and tile? If theres that much flex in the floor, your tiles would be popping up all over! Did you do the floor after he gave his report?

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Old 11-14-2009, 06:56 AM  
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How the he@% does he know the sub floor is soft under hardy board and tile? If theres that much flex in the floor, your tiles would be popping up all over! Did you do the floor after he gave his report?
That was my exact thought and that is why I said what I did.
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Old 11-14-2009, 08:04 PM  
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I don't really understand why people suggest "sistering" joists. I graduated from university with a Bachelor's degree in Mechanical Engineering and the design of beams is part of the course work that every mechanical and civil engineering student takes.

Instead of sistering floor joists, I'd suggest that you talk to any architectural or structural engineering firm about ADDING wood to the bottoms of your existing joists.

If you can use a glue that forms a bond that's as strong or stronger than the wood the joist is made of, then you can ADD wood to the bottom of a joist to make it stronger. That is, you can turn a 2X8 into a 2X10, or a 2X8 into a 2X12 for that matter. All that is necessary is that the glue you use and the wood you add be as strong or stronger than the wood you have now.

If you were to use a construction adhesive like LePage's PL Premium to glue and screw two pieces of fir 1X2 together (side to side), and once the glue sets up you were to take the screws out and bend that 2X2 beam until it breaks (by using a car jack, for example, to apply tremendous force), and you find out that it broke in the wood, not at the glued joint, then you have proved that the glue is stronger than the wood. If the glue is stronger than the wood, and the wood you add is as strong or stronger than the wood you have, you can do nothing but strengthen the joist by adding wood to it ANYWHERE. There rest of this post will be about where you need to add it for maximum effect.

Travelover was correct in saying that the height of the joist is one of the most important considerations to the rigidity of a joist, and he was correct, but the length of the joist is equally important. Apart from changing the cross section of the joist to make it look more like an I beam than a rectangle, or changing the material the joist is made of (magically changing the wood into steel, say), the height of the joist and the span of the beam are equally important in determining how much a beam deflects under any given load.

This web page gives the basic formulas for determining the deflections of beams:
Deflection of beams

For a solid rectangular beam like a floor joist, the ends of the beam are supported at both ends. Neglecting gravity, the joist will deflect the same amount if it's supported from below and a weight is rested on it, or if it's prevented from moving upward, and a vertical upward force is applied to it:



In the above drawing, the gray triangles are the supports and are assumed not to move. A force is applied at the center of the beam and the deflection of the beam is calculated as:

deflection = F*L*L*L / 48*EI

where
F is the force applied
L is the span of the beam, which is the distance between supports
E is the "Modulus of Elasticity" of the material the beam is made of. This parameter takes into account that a fir joist will bend less than a spruce joist, and one made of solid steel will bend even less. This web page gives the values of E to be used for Southern Yellow Pine, Douglas Fir, Hemlock and Spruce for various grades of these woods. For steel, use E = 30 million psi. Steel is about 15 to 20 times as stiff as wood. So, just be aware that the Modulus of Elasticity, "E" is the parameter that accounts for the strength of the material the beam is made of, and the value of E won't change unless you replace the joist or beam with one made from a different material.

http://www.southernpine.com/speciescomparisons2x6.shtml

I is the Moment of Inertia of the beam, which takes into account it's cross sectional shape. This parameter takes into account that a round solid steel bar will be stronger and bend less than a hollow steel tube of the same diameter, or that a square hollow tube will be stronger and bend less than a round hollow tube of the same height. The mathematics in the equations developed to predict beam deflection presume that the beam is a bundle of fibers. When the beam bends, those equations predict that the fibers on the top and bottom of the beam will be in compression and tension, and the fibers in the middle of the beam won't have any stress on them at all. Consequently, it's the material that's furthest away from the middle of the beam that's in the most tension and most compression and helps the most in resisting any bending of the beam. This is exactly why an I beam (with it's top and bottom flanges) is very much stronger than the same beam with those top and bottom flanges removed. And, it's also why the same 2X12 can be used as either a very rigid floor joist or a very bouncy diving board depending entirely on it's orientation relative to the applied force. So, just be aware that the Moment of Inertia, "I" is the parameter that accounts for the shape of the beam relative to the applied force.

For a solid rectangular beam (like a joist), I is calculated from the formula:



For a round beam, like a round pry bar, I is calculated from the formula:



So, just for argument's sake, let's plug the formula for I for a rectangular beam into the formula for a beam's deflection under a concentrated load at mid-span of the beam:

We have:

deflection = F*L*L*L / 48*EI

substituting I = b*h*h*h / 12, we get

deflection = 12*F*L*L*L / 48*E*b*h*h*h

or, deflection = F*L*L*L / 4*E*b*h*h*h

Now, if you sister the joists, all you're doing is effectively doubling the width of the beam, b. The result, predictable from the formula above and from common sense, is that the deflection of the sistered beam will be half of what a single beam is under any given load.

But, if you start playing with the h (height) of the beam, or the L (span of the beam), then you get much greater changes in the rigidity and deflection cuz these terms are CUBED in the formula for deflection.

So, for example, if you were to cut a hole in the basement floor, pour a reinforced concrete footing, and sit a jack post on it to support a beam supporting the floor joists in the middle of their spans, you'd effectively cut the span, L, in half. The result would be that the floor deflection would be 1/2 cubed, or 1/8 of what it was before.

Similarily, if you doubled the height of the joists, h, you'd also get 1/8 of the deflection you had before.

If, for example, you used PL Premium to glue and screw some fir strips 2 inches high by 1 1/2 inches wide to the bottoms of 2X10 floor joists, then the reduction in the deflection in the floor would be:

(9 1/2 * 9 1/2 * 9 1/2) / (11 1/2 * 11 1/2 * 11 1/2) or (9.5 cubed)/(11.5 cubed)

or 0.56, or you'd have only 56 percent of the deflection you had with the 2X10's.

That is, by adding 2 inches of wood to the bottoms of 2X10 floor joists, the deflection of the floor is 56 percent of what it was before, or approximately the same amount as if you'd used vastly more wood to sister each joist. And, strengthening the joists by adding wood to them completely avoids all of the potential additional work involved in running electrical wires and such through the sistered joists.

Aslo, that above estimate of reducing the deflection of the floor by 56 percent is based on turning the 2X10s into 2X12's. You COULD glue 2X4's edge-to-edge to the bottoms of the joists for a much larger increase in rigidity, or even glue them on to make the bottoms of the joists resemble the bottom half of an I beam (which would allow you to use shorter screws. Both of those gameplans would result in an even larger increase in rigidity of the joists, and therefore less deflection of the floor.

But, the above is not really strengthening the subfloor, as the post is entitled. It's strengthening the floor joists that support the subfloor. It could be that the inspector was saying that there's too much flexing of the subfloor between the joists. In that case, strengthening the joists won't address the problem he's complaining about. AND, it's based entirely on the assumption that you can find a glue that'll form a bond that's as strong or stronger than the wood you're gluing to. I really don't know whether PL Premium is, but it'd be an easy matter to test it.

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Old 11-15-2009, 06:13 AM  
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The glue and screw method though ideal from green train of thought is many times not even close to ideal.
It's great when it works.
The ideal thing abpout sistering joists is that you lose no ceiling height from below and since this method is used primarily at basement level where it is an ideal method in many of our old houses here. If you add another 2" you might be smacking your head every 16" or every step you take as you walk to your dryer.
When most people sister joists they don't glue as well. They slap a joist up there and screw it together. Quick and easy. Poor installation technique!!!
The glue and screw method is not quick. The old joists have to be prepped. Yes they have to be sanded without any dips and dives. Most old houses have joists that are rough cut. This rough cut surely won't cut the mustard.
New joists in more modern construstion need to be sanded as well yet not as aggressively.
If we are talking fir joists like in many houses you can almost pick any glue to form the wood to wood bond. Be careful in the northeast here. Many many houses were constructed with spruce yet most contractors think it's fir because they have no clue about wood usage history. Spruce for it's weight dry is extremely strong.

Nestor, great post above!!!



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