Putting down floating floor on horrible subfloor

I have a house that was built in 1980 and utilizes a subfloor of a compund I have never seen in a house, before or after this house was built. The material is a gray, almost pressed paper looking material that is a full 1-1/2" thick. The problem is that over the last 32 years, the subfloor has drooped a bit between the floor joists, and the joists themselves may not be the straightest, either. Nevertheless, we are determined to put an engineered flooring in and I am at An impass. I purchased and laid 3/8" plywood over about 250 feet of flooring (1 bedroom and 2 hallways and a bathroom) and began laying my flooring. The plywood did not take out enough of the differences from the peaks to the valleys to make the flooring install correctly. I realized this after installing it in the bedroom, and I pulled it back up to address the subfloor.
The subfloor has up to a 3/8" difference across different parts of the floor. I am considering using a self leveling compound, but posts I've read state that it's not the best on big floors and the bedroom itself is about 150' and it needs it all over the floor. Additionally, There is still some flex where the plywood did not completely adhere to the original subfloor and I fear the self leveling compound will break up over time.
I am at a standstill and completely clueless as to what to do. I have already committed several sheets of plywood down with glue and nails, so ripping out the original subfloor and replacing it is a bigger task.
Any thoughts or directions would certainly be appreciated.
Thanks, Mark
--
Mark Bolton


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wrote:

You will have to put up cross-bracing between the joists at an absolute minimum. Probably the best thing to do is to sister new joists along side the original joists, but doing this only after jacking up the oriiginal joists to somewhat highr than perfectly horizointal and then putting in the sistering. You may have to put in both the sistering and the cross-joists if you want a really stiff and level floor. The only other possibility is to put in a new heavy-duty joist/beam cross-wise under the lowest point of the sagging joists and then jacking up the new beam somehow until the floors are level. You may have to do this separately for each room, depending on the floor joist pattern. Glad this is not my house.
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Mark Bolton wrote:

I'm afraid your best option is to get out your circular saw, cut through the funky subfloor around the perimeter and rip the stuff out. Replace with two layers of 3/4" ply glued to the joists with construction adhesive and fastened with screws or screw shank nails and more adhesive between layers with the joints staggered. Then install your new flooring.
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Yep. Anything fastened to the failed subfloor is only "polishing faeces".
Harry K
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The above sounds like a straightforward plan that will work to me. The only concern would be that the OP mentioned that besides the subfloor sagging between the joists, there may be a problem with the joists themselves. That can be determined and corrected if necessary after the old subfloor is removed. If just a couple joists are involved and there is no issue from below, eg visibly sagging ceiling, then I'd probably sister the bad joists without jacking the old ones up to level. You have the subfloor attach to the top of the new sister joist. I would also make sure that the joist sizes are adequate for the span.....
This is some very weird construction technique for a house.
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Mark Bolton wrote:

Sounds like Homasote, a rather pricey sound deadening board.
I've used it in combination with existing 3/4" subfloor, but it apparently is also available as an all-in-one floor decking, which sounds like what you have:
http://www.homasote.com/products/4-Way-Floor-Deck.aspx
I'd contact the mfr. for advice before doing anything extreme like removing it.
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*I have seen floors like you describe. They developed cavities over time for whatever reason. I think that stuff is for sound and/or fireproofing.
What I have seen done is the cavities get filled in with a Dash Patch type product such as this: http://www.pacoa.com/dash-patch-25lb-floor-leveler-12125.html
Home Depot sells a similar product under the "Henry's" brand name (I think).
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wrote:
...snip...

Maybe I'm missing something, but if the problem is that the subfloor material is substandard and "has drooped a bit between the floor joists" what is sistering joists going to accomplish with respect to the "droop"? Nothing as far as I can tell.
Once the substandard floor is removed (which you have to do to sister anything onto the joists) the "droop between the joists" will obviously be gone and you can assess any other issues at that point.
Sistering may help if the joists themselves are also a problem ("...and the joists themselves may not be the straightest, either") but until you determine whether or not the "may be" is actually true, making the decision to sister might a bit premature.

Well, I wouldn't say there's "no way" but I'll give you there's "no easy way".

We can't give the correct way until we know what is really wrong. Is it just a subfloor issue or a structural issue or both? I don't know if you can really determine that until the drooping subfloor is removed and you know what going on underneath it.
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As usual, I agree with all of the above. The first step is taking out the failed subfloor. It would be a good idea to also take a look at the ceiling from below and see if there is any visible sagging, cracking there.
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wrote:

Sorry about jacking it up, your OP didn't say it was a second floor. But, you could still jack it up if you spread the top of the jack out using several increasingly large pieces of wood on the top of the jack so you don't apply the jacking force all in one place but rather spread it out over a few square feet.
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On Thursday, July 19, 2012 5:15:25 PM UTC-4, Mark Bolton wrote:

first pull up your junk sub floor, inspect and repair if needed your joists, shim level joists, then install new sub floor, and your probably better off to go with a real hardwood finish floor, you can buy it pre finished.
self level material on a bad floor is just a waste of time.
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On 7/19/2012 5:15 PM, Mark Bolton wrote:

Does it look like this?
http://www.homasote.com/products/4-Way-Floor-Deck.aspx
http://www.homasote.com/products/440-Soundbarrier.aspx
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