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Old 10-04-2009, 07:11 PM   #1
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To sister, or not to sister?


Basic problem: house is 60 years old, and one corner has settled a little (in the diagram, that would be the front left corner). The settling happened years ago, and was evaluated as being stable, etc. Given that the settling is in one corner, the flooring is 'twisted'. There also appears to be some bowing in the joists.

I want to put in new hardwood flooring throughout the first floor of the house. There is way too much 'wave' in the subfloor for this to be immediately possible. I've ripped out the old flooring (and part of the old subfloor, a layer of 1/2" plywood) to expose 7" x 3/4" planking (with 1/4" - 3/4" gaps between the planks), running at a roughly 20 degree angle to the joists.

The joists are 1 3/4" x 9" (actual dimensions) and in that part of the house they have a roughly 10'6" span. In removing the plank subfloor I'd be able to expose the entire length of the joists, minus a few inches on one side. The joists are butted against the central beam and sit on a 2x3 ledger, but I can't (without removing the subfloor) see how they are attached to the outside of the house. Sliding in joists from outside is not possible as the house is terraced, with shared exterior walls on both sides.

Question 1: what is the best way to level the subfloor? Does sistering the joists make sense here, followed by a new 3/4" tongue and groove plywood subfloor?

Question 2: On top of the planking and along the length of the entire center beam of the house (see figure), there is a load bearing wall (supports the second story). This wall sits directly on top of the planking which sits directly on top of the central beam; will cutting the planking along the wall won't impact its integrity? (Should I be bringing in a structural engineer?)

Question 3: the joists are not regularly spaced, but are in fact 24 inches in some places, 20 in others, and yet 16 in others (no idea why this is so). What should the minum spacing be if I want to put a 3/4" plywood subfloor on top, followed by 4" wide 3/4" hardwood. Should I add extra joists to make this so?

Question 4: In the entrance and kitchen, I'd like to put in tile. What should the minimum subfloor be here? Preferably everything will have the same finished height, and adding new joists is not a huge problem as the subfloor is likely going to be removed anyways.

Thanks in advance,

Chris
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Old 10-05-2009, 03:28 PM   #2
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Bump. Lots of reads, but no suggestions?

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Old 10-05-2009, 03:43 PM   #3
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Best bet is contacting a local engineer. Lot's of stuff going on in your structure, and the safe bet is to have someone come in and take a look. Particularly with the tile area. Depending on what product you choose, you might need to beef it up to an L/720 floor. That doesn't even sound like an L/240 floor in its current state.
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Old 10-05-2009, 04:55 PM   #4
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Based on John Bridge's deflection calculator, it appears to be somewhere around an L/300 floor. This is assuming a 20 inch spacing, but I've actually got some places where it's 24 inches.

My plan was to sister old joists to level and drop in extra joists between any 24" gaps. Sticking to 1 3/4 x 9" lumber, the doubled joists (and reduced spacing) should get me to L/750 or so on the 14.5' span, and even with the 24" spacing, I should be good to L/1100 on the 10.5' span side.

This is only calculating for a simple joist supported rigidly on both sides. In my case, however, one side is supported by a beam, the inverted 'T' described above. This beam in itself has a span of 12 feet, so likely adds quite a bit of flex to the whole setup. I'm not sure how much this adds to the problem, and strengthening this beam is effectively impossible.

Thanks for the response.

Chris
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Old 10-05-2009, 08:13 PM   #5
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If you don't want to bring in a local contractor, look for a professional supplier. They have the ability to size joists appropriately, calculate all your loads and supply you with the lumber and hardware you'll need. I don't know where you're located so I can't make any suggestions, but you can go to http://www.ilevel.com/buynow.aspx and put in your zip code to find iLevel dealers in your area. Call around until you find one with an inside sales dept. that helps home owner's and walk-in customers. Many times, they'll do the engineering required as long as you buy materials form them.

Sistering your joists sounds like a good idea. For the tile areas, you'll want to be at 16" o.c. max and L/600 min. Check the tile installation guide for the specific tile you choose and it will tell you the min. L/ ratio for that tile.
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Old 10-05-2009, 08:27 PM   #6
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I checked the strength and deflection of the 10'6" long, 1-3/4 x 9 inch beams, and they appear to be OK, assuming 40 psf live load, 20 inch spacing, and average dead load. You may have other issues here, specifically if you are getting settling, it may be the foundation, not the joists themselves. If it is a foundation problem, sistering the joists, while it won't hurt anything, won't solve the problem either.

I would get a fluid level (if you don't have one, you may be able to rent a good one at a rental store) and check the elevations on your joists and your floor. If you can't get a fluid level, use a laser or a long level. I hate to speculate, but your issues may involve failure of the subfloor, not the joists, or could involve differential movement of the foundation. I agree with previous poster, you have a relatively complex structure, with variably spaced structural elements, a non-standard main support beam/joist connection, and you probably need some professional advice from an engineer who actually visits your house and makes the appropriate measurements. Sistering may be expensive, unnecessary, and ineffective.
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Old 10-06-2009, 09:00 AM   #7
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Quote:
Originally Posted by Daniel Holzman View Post
I checked the strength and deflection of the 10'6" long, 1-3/4 x 9 inch beams, and they appear to be OK, assuming 40 psf live load, 20 inch spacing, and average dead load. You may have other issues here, specifically if you are getting settling, it may be the foundation, not the joists themselves. If it is a foundation problem, sistering the joists, while it won't hurt anything, won't solve the problem either.
The settling is apparently from the first 20 years after the construction of the house, and hasn't worsened since. It's isolated to one corner of the house, and really only puts on bedroom out of true. I'm only interested in putting hardwood in there, so sistering is more about relevelling the floor than strengthening it for tile.

The kitchen and entrance are both still level, but insufficiently stiff for a tile installation. Sistering (or adding new joists) in these sections is with the goal of putting tile down later. We haven't quite settled on what yet, so we'll probably aim for L/720+ and that way we'll be able to put it effectively whatever we want.

Quote:
I agree with previous poster, you have a relatively complex structure, with variably spaced structural elements, a non-standard main support beam/joist connection, and you probably need some professional advice from an engineer who actually visits your house and makes the appropriate measurements. Sistering may be expensive, unnecessary, and ineffective.
I tore down a piece of drywall (it's only screwed in, not taped and mudded, so not a big deal) from the ceiling in the basement workshop. From anywhere I can poke my head in, it appears the joists are on a regular 17" spacing. The strangeness is only in the front third of the house and I have no idea why. I'm curious to find out what's under there when I rip up the subfloor.

And I'll be taking everyone's advice and talking to a structural engineer. I really don't want to muck around when it's come to the structural integrity of the place!

Thanks for the responses.

Chris

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