You're on the right track. Even with a series of 1/2" holes in the top edge, the 2x4 will still be strong enough. But were you suggesting locating the pipes under the 2x4's, essentially 3.5" from the plywood? Can radiant pipes be put in like that?
When you knock the blocks into place, you will easily be able to remove the 1/8" of sag in the plywood. That's gonna be a lot of joist hangers.
Casey

My gameplan DOES put the pipes beneath the 2x4's, so as you say, 3.5" from the plywood. As far as I know, the pipes can be installed that way, since (as I understand it) the principle of under-floor radiant heat depends on heating the air pocket(s) between the joist bays, rather than being butted up against the floor itself. The pipes just go into little hanger-clips that get screwed to the floor -- or in this case, the 2x4 reinforcing.

Since posting this, I was struck by the odd thought that there's all this visible sag in T&G plywood. Odd, but... I need to get this dealt with ASAP.

16" OC is indeed the norm for joist spacing, but since you're dealing with 1-1/4" flooring, you could probably get away with 24" OC spacing of your new framing.

I would go with 2X6, however; I just don't think 2X4 will do the trick. As for the tubing holes, I'm assumning they're about 1" Dia or less...? As long as they're placed in the upper half of the member, they won't compromise any significant structural integrity. If they're in the dead-center, there won't be any compromise at all.

you have not mentioned what the main span is. How long are the 4x6's? Supported how?

Does the whole floor "bounce" or is your concern only with the 48" spacing between the 4x6 joists?

I can't say that I've ever heard of this configuration of a floor support system. Sounds like a DIY project.

I'd check the local codes and have someone with structural experience review your plan before proceeding.

I agree with Sombriel Mongrel - 2x4 on edge has more than enough strength and stiffness for what will amount to a 44 inch span.

The main potential weak link in your design is the attachment to the existing 4x6, which would be well handled by the joist hangers. But, as S.M. also said, that's going to be a lot of joist hangers. Have you considered just using a ledger system, i.e., attaching a 2x2 near to the bottom edge of the 4x6 to rest the 2x4's on? This would save you all those joist hangars and should be just as effective in supporting your cross members. A couple of toe-nails into the 4x6 at the ends should be enough to keep them vertical.

I would also suggest just using a jack to lift up the sag in the plywood as necessary, rather that pulling all the nails out of one side. Would be much less work with the same effect.

If a ledger system would work, that would be great! By rough calculation, I'd need about 350 Simpson brackets to finish the job. Obviously, 2x2's are a LOT cheaper. Out of curiosity, would screws be acceptable fasteners, instead of nails? There's not a lot of room in this crawlspace, and I don't have a nail gun that fires anything bigger than finishing brads.

Addressing a few other questions/comments, and correcting an earlier misstatement:

1) The flooring is T&G, so there are no nails to pull out to jack the stuff up.

2) The "bounce" I mentioned has to do with multiple unsupported pieces of flooring butting together in open air. This is what needs to be corrected, because it not only bounces, it's sagging as well.

3) The plywood IS laid down so that the joists are parallel with the 4' dimension of the ply. There is NO continuity over the supports.

4) The span is 30' at the shortest point and about 50' at the largest. The ply is laid down directly over the joists. The joists are supported by concrete piers with short 4x4 uprights. There IS cross-bracing, but there's no rhyme or reason to the way it was installed.

5) While adding new joists to divide the 48" span was my original thought, it would be exceptionally difficult for a number of reasons. Therefore, if any of the other options being discussed are viable, then they are vastly more preferable.

6) We are nominally the first owners of this house (it being the house DW grew up in). It was built to code in '75, by lunatics, apparently, but not by DIY'ers.

"The span is 30' at the shortest point and about 50' at the largest. The ply is laid down directly over the joists. The joists are supported by concrete piers with short 4x4 uprights."

FWIW: the span of the 4x6's is between supports not the full distance (I assume the plan dimensions of the floor are 30' x 50'). No way a 4x6 would span 30 ft much less 50 ft.

Based on your latest description I would plan on lifting the plywood and reframing. (this is essentially what you're doing). I wouldn't do it from the crawlspace if removing the flooring is possible without too much difficulty. You say nothing is nailed.

Sketch out your existing framing along with the support points. Take the sketch to a reputable framing contractor or local engineer and have them recommend a framing plan and fix it once and for all.

good luck

As far as attaching the ledger goes, 1/4" or 5/16" lag bolts should do the job, say about 3-4" long. Brads won't help, of course, except maybe for holding it in place while you put the lag bolts in.

I would pre-drill the ledger board, with a bolt hole every 16" under where each cross-member will rest.

I am in the middle of a similar project. 1 1/8" subfloor with 2x6" joists 48" apart. My floor is also bouncy, and my house was built even cheaper than yours :) They used metal "jacks" to hold up the joists, and the joists are formed sheet metal. Very cheap.

Anyway... I am adding 1 row of new 2x6 for every existing joist, so I will have a 24" space between joists. I think this will be fine for a 1 1/8" subfloor. I am sistering the joists so they can span the length of the house, which is roughly 50 feet as well.
Since I am also replacing the jacks (you have piers already), I am running a 4x6 below all the joists, and pouring new piers for the 4x6 to sit on. For every jack I replace, I am jacking up the house at that spot to keep the house stable while I do the work.

Its an incredible amount of work, and I am crawling under the house to do it. I suggest that if you can, you cut the flooring up for easier access.

I think that you have a good idea, and it will work to just cross brace every 4 feet at the plywood seams. This would effectively give you a supported 4x4 spot for the plywood and will help alot. Though I would use 2x6 and drill for your heating.

Be sure to use liquid nails or some other good subfloor glue, and to also use nails through the subfloor to your new cross brace, and not screws. Screws will not pull the subfloor into the cross brace. Your floor stands a good chance of squeaking if you do not.

good luck. Its alot of work.

cheers-
Dustin

ah, one more thought. You mentioned that there is cross bracing, but that it was in random places. (You might know this already, but...) The cross bracing you have is probably only to prevent to joists from warping / turning in at the bottom- and not for additional floor support.

cheers-
Dustin

Hi
I too have a house built in the 70's with 48" centers but the floor joices are 4x10 and sagging 3/4 inch plywood on top. I am curious as to which method you used: the joice hangers or the 2x2 supports? time wise how long did it take and if there were changes that you would make what were they? after the fact do you think it would be ueasier to work from the top down?

coffeebrake