I tried searching for an answer, but nothing so here we go.

I plan to lay 3/4" BC, but one of the rooms has 2x8 joists 16" OC, the rest of the home is 2x10 & 2x12 from what I can see. Now this 2x8 section of the home spans 24'x 12' wide with a steel beam spanning along the 12' width right in the middle of the room. The subfloor is 5/8" plywood. The floor does seem to have some spring to it, when the dog runs by you while sitting at the dining room table or someone walks by I can feel the flooring bouncing a bit.
The manufacture of the flooring states min 2x10 16"oc.

Will the 2x8 joists be sufficient and just need to beef up the subfloor?

The basement is finished under this room.

From what you said in your posts, that would mean the room with the 2 x 8 floor joists has a joist span of 12'. The room is 24' long and 12' wide with a steel beam running down the middle of the width which halves the 24' span to 12'. 2 x 8's are maximum spanned at 12' long spaced 16" oc. Why the builder ran the joists the long ways instead of the short run is beyond me. Adding more plywood will not take the bounce out of an under-spanned joist system. But it will not fall down. If you want it stiffer, you will need to add additional support from below or remove the subfloor and sister on some more 2 x 8's to the existing ones.

Come to think of it, they may be 12" oc rather than 16". I will double check tonight.
If they are 12"oc would that be considered acceptable?

It isn't a matter of "acceptable". Even @ 16" OC the joists meet span standards. It is more a matter of preference to you. If the subfloor bounces and shakes when someone walks across it, it probably should be beefed up.

Here's a lumber span chart. Notice how the grade and species factor into the spans.

http://www.awc.org/Publications/update/ ... llPage.pdf

Personally the bounce is OK with me...I can live with it since its rather minor in my books, my main concern is that the subfloor will be sufficient as far as strength goes to support the hardwood and not have any issues with creaking or popping floors.

Would adding 2x6 sections perpendicular to these joists help any? Problem with sistering 2x8's is that it would requiring disassembling the piping for the forced hotwater heating, all the wiring and plumbing to make that work.

It may but not a lot. A 5/8" plywood subfloor over joists 16" oc is the minimum requirement for a nailed down 3/4" T&G hardwood floor. Adding 1/2" plywood would have about the same effect but would stiffen the deflection between the joists. These kind of decisions are difficult to make without an onsite inspection. Perhaps consider enlisting the advice of a hardwood flooring professional near you.

thanks Gary, a friend of mine does some hardwood installs, I'll have him take a look at it.

I ripped up the carpets today and found the bounce, some of the nails had lifted a new millimeters also causing a squeak. Nailed them down and it feels more solid, but still going to lay down 3/8 plywood to get it up to 1".


Also the subfloor is for sure 5/8" plywood, but not T&G. Not being T&G is that a bad thing? Is there any better alternative to laying 3/8" ply over it?

The T&G would only be on the two long sides (the 8' side on a 4 x 8' sheet) as the short side is to be nailed directly over a joist and does not need the support of a T&G. IMO, a subfloor cannot perform properly without being T&G UNLESS it is an older 1 x 6 solid plank installed at a diagonal. Those will work ok but are rarely used anymore. All manufactured panel subfloors made today are made with T&G's along the long sides. This ensures no squeaks, a solid feel between the joists/trusses and a subfloor that remains flat.

Now if the existing panel subfloor is truly NOT T&G, then the options are:

1) remove and replace with a T&G subfloor
2) Overlay with more plywood, making sure to NOT marry the seams. All the joints/seams of the new plywood underlayment should not fall on ANY of the existing subfloor seams. Gluing and screwing in this situation would be the most secure.
3) Remove the existing subfloor and install 2 x 6 blocking everywhere a seam lands between the joists. Then re-install the older subfloor with glue and screws.

#1-I want to do that, but the current floors run under the walls, & kitchen ceramic etc so I can't see that working.
#2 - Probably the most realistic option for me, any benefit of laying 1/2" T&G over the existing subfloor, perhaps Advantech or is regular plywood just as good in my case?

1/2" T&G plywood is going to be hard to locate. Lumber yard T&G plywoods start at 5/8" thick and up. Square edge plywood is OK for underlayment use imo.

sorry 5/8" T&G is what I meant. I can also get the Advantech, not sure exact size in T&G for half the price as plywood which makes it attractive, but that shit it HEAVY!

ANd when I lay new underlayment is it best to use wood glue or PL Premium to glue them togther?....btw a 10x13 section has some carpet adhesive on the floor.

Yellow wood glue for adhering wood to wood.

Back to underlayment, I was at the building center last night and took a look at plywood etc. I can get 5/8" T&G ply for $24/sheet or the same size T&G Advantech flooring sheets for $9! I plan to go for the Advantech mainly due to the price, is there anything wrong with doing this?

Personally, I'm more in favor of plywood but I understand the need to keep cost down. Try to look at the big picture and not just the price difference between OSB and plywood. Overall, what is the difference in price between going with plywood instead of OSB? $100.00? $200.00? Then decide if the price difference is worth it. OSB is approved for hardwood flooring and Advantech 5/8" is a steal at 9 bucks. Just make absolutely sure it is installed properly and you should have a stout subfloor.