Squeaky T&G subfloor

We had a hardwood floor installed in a large room last December, at which time several "interesting" structural details became apparent:
1) The subfloor is 2x6 tongue and groove douglas fir planks, laid at a 90 degree angle to the joists
2) The joists are 48" on center, and as I recall are mostly built up out of 2x8 or 2x10 (don't remember which)
The floor squeaked like crazy before the job, and I screwed every board down with one or two deck screws to supplement the nails. This stopped much of the squeaking, but there are still some "deep squeaks" (more like groans) that remain. I was unable to pin the source of the squeaking down precisely, but it seems to be coming from somewhere on top of a post where pieces of a built-up joist butt together (maybe inside the layers of boards - it's a tough thing to pinpoint) What is the recommended way to handle such a thing? Just drive a bunch of screws into the joist members?
Another issue - the floor doesn't feel "solid" on one half of the room - you can definitely tell you're walking over a crawlspace. I don't know if this is because of minor deflection that is slightly perceivable, the echo underneath, or some combination. On the other half, the floor seems very solid, but the constuction is different; there is a finished room underneath, and floorboards run the other way (and presumably the joists, too - but since it's finished, I can't tell). Are there reasonable retrofittable cross-bracing schemes to be recommended for stiffening up a floor from underneath?
Andy
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Yikes!
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This calculator doesn't seem applicable - it only shows 2x members. What we have are sistered 2x8s or 10s. Further, this only deals with spacing up to 24", nothing wider. My joists are 48 o.c.
Andy
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Thanks.
This is what I was wondering - whether retrofitting in more joists is generally possible (obviously whether it is specifically possible in my case is ultimately the question, though as I'm sure some will point out, somebody with experience will need to make that determination - I just wanted to know if such a thing were advisable/doable before calling around about it).
I guess the key question is whether those joists are going to need extra piers or not - the crawlspace is filled with footings on top of which posts are placed, supporting the joists above. Adding more of these, I gather would be the most difficult aspect of all this.
Andy
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How far is the span of the existing joists, Andy? Not the o.c. dimensions but rather how long are the joists? If all you do is add additional joists, they will require the same support as the existing ones. If they're not supported over a long span (and keep in mind I'm not a structural engineer) all you will do is weight down the areas between the existing supporting ones when you screw down the floor to them. It sounds like it might be an untaking for you - but to protect what you've got from any future complications - I believe I'd make the investment. 48" O.C. sure is a long span for floor support below a living area.
Jim

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Ok, here's the revised scoop. I hate going down in that crawlspace, but my hazy memory clearly needed the boost. Maybe I have my terminology all wrong and the "joists" I've been referring to are more properly "beams" (as in pier and beam, which sounds like what I have in the crawlspace).
The joists are not sistered 2x boards as I stated earlier; they are solid 4x8s. They sit atop either 4x4 or 4x6 solid posts (it's a mystery why some are 4x4 and some are 4x6 - there didn't seem to be an obvious pattern to me). These posts are distributed 4 feet o.c. in one direction and 6 feet o.c. in the other. The joists run on top of the posts in the "long" (6 foot spacing) direction, and they are butted together on on top of the posts with a 4 foot long 2x6 nailed to them to keep them together. I now realize after coming back up that I forgot to measure how long the sections of joist were between butts - but I would wager they span at least two of the posts (and would hence be at least 6 feet.
Andy
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Standard post and beam construction uses 2 2x8's or 2 2x10's for beams with posts every 8 feet with the beams on 4 foot centers (there are probably other standards). That's what my house uses along with T&G 2x6 across the beams. Also the beams are composed of 2 pieces 16 feet long but the one piece is offset one post so you get what appears to be a continuous beam (nailed together) but no plates. Your construction appears to be a simple variant and should not be flimsy or need anything additional.
As for the squeaking, most of the squeaks probably result from movement within the tongues and grooves. You need somebody to walk around uptop to help you locate the exact location of the squeak/groan while you are underneath. If it is in the subfloor, your only course now is to screw up from underneath. Use 1 inch material underneath to tie the subfloor boards together and make sure it extends at least two subfloor boards on each side of the squeak so that you can put 2 screws through the 1 inch material near the squeak and 2 screw in each of the adjacent subfloor boards.
On your one room, depending on your house layout, a change in the direction of the beams in the middle of a room doesn't seem right. And, you should get no deflection of the floor unless you jump really hard and are heavy. Most likely you have some pieces of bad/poor quality subflooring (broken or partially rotten). If so, you can always add an "I" of 2x6 or 2x8 material at any weak spots (use three 4 foot pieces to make the "I" and use metal hangers and screws). If all of the 1/2 of the room is really unstable you probably need to grit it out and add some more beams or get a professional to advise you.
Andy Francke wrote:

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Andy Francke spaketh...

That was point Andy, to demonstrate that you have a serious problem; however, I note from your later posts that your information was inaccurate and the problem may not be as serious as first appeared. Still, it might not hurt to have someone take a look.
--
McQualude

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