Hello All...I am working on the original tongue and groove floors in our old farmhouse. They were covered in old fiberboard as a subfloor, and then linoleum. The floor trusses go N to S, then the 1x12 subfloor boards go E to W. When they put the t/g flooring in, they went the same direction (N to S) as the trusses, so over the years..the planks have curved down between the trusses. I have some ideas of what to try..but looking for someone that has had positive results doing this..thanks! Matt

Don't know what your ideas are but you are not going to undo years of sagging planks and make them tight again. It is possible to install solid blocking between joists from underneath but it's a huge job - you'll need blocking every 16". Even with blocking, you may not be able to raise all the floor evenly without it going too far.

What were you planning to do?

Confusing post. Need to work on some trems so we can figure out what it is your talking about.
This is a truss.
http://www.bing.com/images/sea...534246CF9064&first=1
It's used to hold the roof up.

You may mean joist?
What do you mean by fiber board?
Is it OSB, plywood, partical board?
If your not sure tell us what it looks like.
It's very common in old houses to have a sagging floor, could be from undersized joist, to long a span, cracked joist, insect infestation, fungus damage.
All these need to be checked out to figure out how to fix it.

Hello All...thanks for the responses. I did mean joists..long day working on trusses in old shed too.
Blocking was my thinking, just not sure what to use for the block and secure with lags/screws/nails..and if I should use a standard floor jack or a 3T floor jack with a 4x4 post or something. Planks are tight..and I realize it will never by perfectly level. It was a run that should have had another beam, so not sure if I should try raising the joists in the middle first, and then check the dips for blocking..or do the blocks first to check results.

Most often I suggest adding 24 X 24 X 8" deep footings with rebar in the middle every 8' I make a beam out of doubled up 2 X 10's lift it in place with three bottle jacks (more for longer runs).
I have precut 1/4 thick 6" X 6" plates I sit on top of the jacks so the piston does not just crush the wood.
Depending on how bad it is you may have to do a small lift, let it sit over night and go back and lift some more the next day.
Once it's level I use concrete blocks as piers.

You will want to add any beams and do your floor leveling prior to installing any blocking. Since the 'blocking' is really there to raise and support the sagging floor planks you can gt by with 2x4 or 2x6.

I still don't think it will work very well, especially considering the effort involved.

If the whole floor sags in the middle you would never want to just try lifting one joist at a time like that.
Far more likle to mess up the flooring and or snap a joist. Plus you would end up with a wavy floor.