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Old 01-13-2014, 03:44 PM   #1
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best way to stiffen this floor


I am about to install some new flooring in my living room and while I'm at it I want to take some of the bounce out of the floor. The floor is solid at the perimeters, but the whole room will rattle if I stand on my toes and drop my heels in the middle of the room (I'm not a huge person).

The basement below is unfinished and I can easily get in there and make some repairs. I'm just not sure what the best option will be.

The subfloor is 1.5" x 5" t&g planks. Those are laid horizontally across some solid lumber 13.75" x 3.75" joists that are spaced 48" on center. the joists span 21.5 feet from an exterior block wall to a 2x6 interior supporting wall.

Currently there is a layer of 1/2" plywood nailed on top of the planks. but it is only nailed around the perimeter of the sheets and I will be replacing all of it due to damage in some areas. I plan to make sure the plank subfloor is securely fastened to the joists when I remove the plywood.
I'd actually like to replace the plywood with 3/8 or thinner if possible. A layer of 1/2" strand-woven bamboo floor will be nailed on top of it all when it's done.

The plumbing and HVAC stuff all runs on the bottom of the joists at one end. There are a few NM cables running through the holes in the joists, but they can be easily relocated if necessary.

I think I could sister the joists easily enough, or even add a few joists in between. I think blocking would be easier and less expensive, but I don't know how well it will work with the 48" spacing between joists. I'm kind of hoping that some expert on here will tell me that screwing down the planks and adding the new hardwood floor will do the trick all on it's own.

I really don't want to add a wall or posts.
The rest of the house is actually pretty solid. this room is the biggest and the only one that really bounces.

The joists look a lot like cedar, but I'm no wood expert. They are fairly soft, dark red, almost no knots. I'm in the Western Washington area and I'm sure locally available lumber was used.
I'm pretty sure the planks are pine. They are in great shape. The house was built in 1968. It was a custom build by a guy who was in the construction trades (owned a bricklaying company).

There is almost no tile or stone in the house, although I'm certain my wife will want to be adding some in the future so I might as well get the floors in shape for that.


*** Editing to add new info ***
I just pulled up one of the sheets of plywood and I can see that the planks are nailed down to the joists at each end with a single nail. Is that normal? The nails are popping a little so I plan to add some screws.
I added a couple pics of the top and bottom sides of the floor.
Attached Thumbnails
best way to stiffen this floor-photo-2.jpg   best way to stiffen this floor-photo-1.jpg  

Last edited by dftc; 01-13-2014 at 04:37 PM. Reason: New information and pictures
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Old 01-13-2014, 06:34 PM   #2
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48 Spacing is over double the span in most homes. (16" O/C) And way to long a span.
Adding a center beam would be the easiest, least amount of money.
You would have cut the slab and pore footings for the post to support the beam.
There must have been a shortage of nails that year. No it's not normal. There should have been two.
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Old 01-13-2014, 06:49 PM   #3
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My last home, which was built around the same time had the same spacing between joists and I've seen it in others. I think it's kind of normal in this area for homes of that age. It is pretty thick t&g and pretty big joists.
I'm not sure I understand why I need to break the slab and pour a footing. I can easily place the ends if any new joists on top of the block wall and the center supporting wall. Are you saying I also need to add posts to reduce the joists span?
I will definitely add some screws or nails to the planks. It's odd because the nails are all at one corner of the plank, like they meant to do two, but forgot to add the second one on every board.
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Old 01-13-2014, 06:54 PM   #4
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There logic may been a house that old may have been built with green lumber. With one nail the wood could shrink and not split the wood down the middle. We have seen many floors built like that many times here before.
Every one of them had bounce and the tiles where cracking.
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Old 01-13-2014, 07:32 PM   #5
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I actually found this link from a response by GaryInWa to an old thread that seems to show my 1.5" t&g floor can span up to 60", so I should be fine with 48", in theory.
http://publicecodes.cyberregs.com/ic...9_5_sec003.htm
I have no idea if the joists are big enough for the 21 foot span. All the info I could find was for 2x joists up to 2x10 at the biggest. These joists are much bigger than that.

What would be the best way to secure those planks to the joists better? The nails in there now are popping up in a few spots so I am inclined to go with screws unless someone has a good reason not to.
Would the self drilling decking screws work, or should I drill pilot holes? Would one or two be best for each joist connection. There is already one nail at each one and I don't want to split any planks. The planks are about 5" wide.
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Old 01-13-2014, 07:54 PM   #6
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I cannot tell from the photo if that is western red cedar, but it could well be. If it is, then the western red cedar association lists the structural properties of select #1 western red cedar as an allowable stress of 950 psi, and a modulus of elasticity of 1.1 million psi. Using these values, and a span of 21.5 feet (your input), and a C-C spacing of 48 inches (your input), and a dead load on your floor of 5 pounds per square foot, and a live load of 40 pounds per square foot, a beam depth of 13.75 inches, and a beam width of 3.75 inches, I get the following values:

Factor of safety against bending failure: 0.9
L/D (stiffness factor): 266 (dead plus live load)

This suggests that your joists are overstressed, however there is always a factor of safety built into the allowable bearing stress, and it is likely your floor is not loaded with 40 psf, so that is why your floor is still standing.

As for stiffness, 266 is below recommended for 12 inch or larger porcelain tile or natural stone, and is below recommended for a wooden floor. Installation of plywood and subfloor stiffens the floor up a little, and blocking helps a little bit, so if you are going to use hardwood the floor is probably OK, but not suitable for tile.

You can stiffen the floor by installing a cross beam as noted by joecaption. Alternatively, you can stiffen the floor by sistering the joists. Using the same size joist as the sister would double the stiffness of the floor, which would reach about 530. You would immediately notice the difference.
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Old 01-13-2014, 08:18 PM   #7
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Thanks.

I guess I will look at sistering the joists. I really don't want to add a beam and posts in the basement since I'd loose headroom.

I'm not sure where I'm going to find joists that big. I'm guessing these days I'd be looking at a glue-lam beam. That's going to get very pricey.

Is this something I'd have to do to every joist, or would every-other be ok?
Would using a smaller piece of lumber than the original joist be ok? For example, can I sister a 2x10 to my 4x14 joist, assuming I support the ends properly. I know it won't be as strong, but would it do anything positive?
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Old 01-14-2014, 06:52 PM   #8
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The stiffness of a joist is proportional to the moment of inertia of the beam. I am not going to bore you with a lot of math, but any lumber you sister to the existing joist is going to stiffen it up. If you use a piece of lumber of the same dimensions, and made of the same wood, you double the stiffness. If you use smaller size lumber, you get less stiffening.

The moment of inertia of a rectangular board is bd^3/12, where b is the width in inches, d is the depth in inches, ^3 means cubed. So your original joist is 3.75 inches wide, 13.75 inches deep, for a moment of inertia of 812 in^4. If you sistered on a standard 2x10 made of western red cedar, you would be adding a beam 1.5 inches wide, and 9.25 inches deep, with a moment of inertia of about 99 in^4. So the net gain would be about 12 percent (99/812). You can figure out the improvement in stiffness for any size sistered board this way.

If you sister on a piece of wood that has a higher modulus of elasticity, you get a stiffer end product, but most construction lumber has about the same modulus, so it is hard to add stiffness by using higher modulus lumber.
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Old 01-14-2014, 06:54 PM   #9
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L/360 for ceramic and L/720 for stone. A center beam would be efficient but you want/need the headroom. You could add a joist between existing joists or double the existing joists but you need to hire an engineer for the correct solution.
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