Mattress Forum

You  might be better putting a foundation on it.   You can see how many slats a Flobeds foundation has.    Plus the slats are attached to the sides of the foundation which will help keep them from dipping down.  Adding more slats might help, but I think with enough weight they will still dip down (since they are not attached they can pull away from the sides an bend), and that seems to be an issue for you already.  Of course the more slats you have and the thicker they are, the less dipping you will get. 
 

I was thinking of using my current wooden foundation (the one that came with my innerspring) but putting a sheet of plywood over it to add stability and help prevent dipping.

Don't know if that would be enough, or if I'd still need to rig up some sort of center rail that could still be unscrewed when the bed frame needs to be taken apart for moving.

Since I've got 5" of medium to medium-firm latex here (1" 24 ILD Talatech, 1" N3/27 ILD, 1" FBM 20 [more like 28-30 ILD], and 2" med.-firm Dunlop), I might be able to add a 3" firm Dunlop base and have a good start there. 

The 2" Dunlop that I have is the Overstock topper that I had stopped using because I thought it was cratering (assumed it was cheap synthetic stuff). I don't see evidence of cratering in it now, but I've been using it for only a few nights, so at this point, I don't know whether it will crater eventually or whether it was actually the mattress under it that was cratering before. Either way, it's useful right now. 

If the curent slats are dipping, then you'll need to replace them and add a center brace. If the current slats are straight, I'd add as many additional slats as you can keeping a maximum distance of 2.5" between each one. I know there's plenty of debate as to whether or not latex needs to breath. I could not find anything other than some people's opinions as to whether or not this is true, but decided myself to err on the side of caution and stick with slats rather than plywood. My Home Depot sells 1x3s (actual size - 3/4" x 2.5") in untreated yellow pine for less than $2 per 8' board. If you add 12 sticks, you looking at less than $25 worth of wood. If you don't have a saw, you can even get them to cut it for you for something like $0.50 a cut.

I don't think they're dipping, but I'd have to get under the bed with a flashlight to check, I guess.

Thanks for the suggestion. Home Depot is where I got the extra slats a couple years ago; I have a hand saw, but had HD cut the lumber for me (faster & easier). Still have a couple extra slats in the basement.

If I go the foam route, I'd prefer a solid base under it, but need to have a sense of whether this old frame could handle the weight. (I know no one can tell me without actually seeing the thing.) It's held up this long, so it could well hold up another 50 years. The dressers have certainly lasted well.
 

Same here.  

Either way I'd bet an old solid wood frame is sturdy enough for whatever you stack on it.  

Thanks, Jason (and thanks, megalops).
 

I've been saying my foundation is wooden, but I guess it does have some steel in it, too. It's the Leggett & Platt "Semi-Flex limited deflection" unit.

Relevant links if anyone's interested (these things come with a lot of mattresses now):

L&P Semi-Flex Web page (a little info)

L&P Semi-Flex PDF (details about the semi-flex foundation)

I am spending entirely too much time pondering mattress components and options....

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Edited the next morning to add:
Whatever this foundation is made of, I'm feeling the slats on it through 5" of medium latex + my old thin fiberbed. [Correction: I'm feeling the metal grid. Youch. The wooden slats are just supporting the metal grid. I should have looked at the L&P PDF more carefully.]   Yesterday morning wasn't bad, but today I woke up feeling like I've been pummeled. I might be back on the couch tonight. And/or I might have to get a few inches of inexpensive high-density PU foam, just to buy myself some time.

I am also obsessed with finding the right mattress, but have a few comments;

Do not put a solid piece of plywood under your mattress - it won't breathe and can mold. 

I would try the old foundation, but make certain the slats are spaced and secure.

I bought a metal bed with a really cheesy set of supports, that I think was sagging and causing back pain.   

So I replaced it with plywood, supported by 2x4s - seriously sturdy.   Of course, as you point out, I had to address moisture issues.    So I got a spade bit and drilled at least a hundred 1+" holes in it.   I *think* it's enough, haven't noticed any moisture, or seen any mold, on the bottom of my foam.   I keep thinking I should drill more sometime ...

I've thought about doing that -- or just getting some pegboard, if that's sturdy enough. (I think it would be, since it'd be resting on top of my foundation, not directly on the bed frame). I'd probably put it under a sheet or inside my allergen casing or something, so it's not directly under the mattress.

Gonna check the bed-frame slats next and then get back to work....Well, what do you know... got me some dips there.* (I flipped the dipping slats over.)

I'll have to add a few more slats to the bed frame, probably, and see about rigging up some sort of center support** from the bottom. The center support would have to rest on the floor, because there's nothing it can attach to on the bed frame.

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* Even when my innerspring mattress is directly on the floor, the coils do not feel supportive enough, so I don't know how much of a factor those little dips were. The foundation seems quite sturdy.

** Or get something like the "adjustable center leg bed frame support" gizmo from Woodcraft (but I'd need a few of them), or some sort of steel support gizmo from The Sleep Shop (they have several options), or some sort of steel center support from Planet Bed (their "V-Rail" thing looks neat but might be overkill for me).

Those "BedBeam" things look very sturdy, but I don't think they'd work with my bed frame (ditto for some of these other metal support systems). My cross-slats aren't attached to the rails; they lie *on* the rails (or on the "cleats" if that's the proper term for that small ledge on each rail), so they're about a half-inch higher. The BedBeams and Metal Support Slats look like they're meant to be flush with the rails/cleats/ledges/whatever they're called, so I'd have to add half-inch-high blocks of wood under them where they'd attach to the ledges if I want to use them with my wooden slats (rather than instead of them).

Ditto with the Strong Arm Center Supports and these Mighty Lift supports; I'd have to shim them if I want to use them in addition to my slats, rather than instead of my slats. I guess these things & the BedBeams are intended to replace the wooden slats. (Um... duh?)

The Lazarbeam Center Support would work with the slats I have, since it's just meant to help keep them up.

So... replace the wooden slats with steel slats (that could get expensive); get something like the Lazarbeam; use those standalone things from Woodcraft; or find a very low-tech way to add center support to my wooden slats?

I could just get a long sturdy piece of lumber and give it a few sturdy "legs." Since any mattress I get is going to rest on the self-contained foundation, rather than on the bed-frame slats, I don't think I have to worry about the frame getting bowed outward to the sides. (Do I?)

(Pardon all my thinking-out-loud here. Those steel support systems are not returnable, so I don't want to get stuck with something that turns out not to work because I forgot some important detail, like, you know, how my bed frame is actually constructed.)

Of course, when I add the center support, I'll have to find another place to put the mattress, which is now stashed under my bed. It'll have to lean up against a wall somewhere or just go out to the curb.

Yesterday I covered the foundation with a sheet of luan so I wouldn't feel the metal grid from the foundation poking me. (Luan was probably not the best choice, but it was cheap, I didn't see any decent-looking pegboard, and I was delirious from lack of sleep. If I need something different later, I'll get what's needed.)   So now I have the Novabond mat under the luan, and the 2 sheets of Dacron from the mattress over that, and then my latex & my old thin fiberbed. (So now I feel a solid hard surface instead of metal wires; a different sort of discomfort but preferable to the wires... I think.)

I might order a few inches of 50 ILD or 35 ILD (or maybe an inch or two of both) from FBM to use as a temporary mattress core under my latex. If that's comfortable enough, it'll give me time to recover from the months of sleep deprivation & mattress-caused pain, and I'll be in better shape to decide if I want to stick with a PU foam base, go for a Dunlop core, or try another innerspring.  That's the hope, anyway. We'll see how things go.

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Added Monday AM: Last night I added one more inch of latex, using the stuff I'd tried zoning with (14 ILD, 32 ILD, 14 ILD; put below the 24 ILD and N3/27 ILD layers). Not wonderful, but it helped enough with pressure points that I might not need the temporary poly-foam core. Not quite sure yet. (Still some lower back pain, but I don't know if it's from the latex, from the dacron being worn out, from my slightly dippy slats, or from a combination of those. Shoulders were much better this morning, though.)