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Sagging Floors:
Bring In The Sisters?

I just stumbled on your site and I find it very, very good and useful. Here's are me question. Apparently the joists in my house are over-spanned because they are Douglas fir and the house was framed to pine specs. I called him in when I noticed cracks in the drywall over doorways and a few vertical compression lines (bulges) in the drywall. He recommends I sister a few split joists he found add joist hangers on others, and run a central beam down two runs of the house to cut the span. I'm pretty sure I'll hire someone run the beams. (This is in a crawlspace area, so the process might take a while for one guy, and I'm planning to sell the house.) The existing joists are 2x10 (utility I think) notched for a 2x4 ledger on the central beam. 

Should I therefore use 2x4 joist hangers? 

As for doubling, I've never done that after the fact like this. It seems there would be a problem getting a double cut to the exact length of the original to fit from underneath. I'm not sure angling in horizontally with 14.5 inches between the joists is possible. Do I have to finger cut the joist and glue it back? What's the procedure for fitting in a double? What's the practice for nailing and/or gluing the doubles. I don't know but I'm guessing glue and 16 penny nails three to a row and rows every foot or so. I assume I'd then use double 2.4 joist hangers here.

On installing the beam: my report recommends against lifting the floor to level it since it might create more problems. But it seems that when you install a beam you will have to lift the lowest joists to the point of the highest joists or you don't stabilize anything.

The house was built in 1968 in Georgia. I have a moderate level of experience. I worked on a framing crew for a good while, but I'd probably scratch my head for thirty minutes over laying out a stair stringer.


Fred L.

Scratch your head for ONLY 30 minutes before doing stair stringers? You're probably ready for this work.

If the existing joists are notched and resting on a 2x4 ledger, then I'd say that joist hangers will do nothing helpful. The idea of joist hangers is to get rid of the ledger and all that notching.

If you studied mechanical engineering, as I sort of have, you'd realize that floor joists deflect excessively because either they do not have enough "section modulus" or because the material used has too low of a Modulus of Elasticity. Joists fail because the stress on the bottom fiber (the highest stress location) exceeds the strength of those fibers. 

Section Modulus is a property of the geometry (the width and height) and not the material. Section Modulus of a beam or joist is equal to the width times the height CUBED. If you double the height of a joist, the section modulus is amplified by a factor of EIGHT. If you double the width of a joist, such as by sistering, the section modulus is only doubled. This is why we use lumber ON EDGE for joists... it is way stronger and stiffer than laying flat.

Also, if you cut the span by half, the stress on the worst location (the bottom fiber, in the middle of the span) is cut to ONE-FOURTH. Simply building a sturdy beam under the middle of the span should solve most of your problems. But the beam needs to be sturdy enough, and rigid enough, to avoid deflecting. Personally, I prefer to raise the floor a little before installing such beams. I've never seen any problems created, but on a house as old as yours it's likely that the joists have "taken a set", or become permanently curved. This can be undone over a long period of time, but a little quick lifting can take out some of the set. I go by ear... when the snap-crackle-pop sounds get too much for my comfort zone (a difficult thing to communicate in an email) I stop lifting. If I see wood cracking, I've certainly gone too far.

If I was faced with your situation, I would not sister the joists, but I would just build a built-up beam of double or triple 2x8's, with support posts every 12 feet or less. The posts need to rest on something sturdy to spread their load to a large area of soil. A 16" square by 8" deep poured concrete footing sounds good, but a small stack of those big round paving stones might work. Hey, y'know what? I've seen people use these 24" round "paving stones" that are actually pads for supporting mobile homes. I'll bet local mobile home dealers would sell some to you. Then you'd have a quick and easy base for the posts. Make sure the soil is damp and very well packed. The posts do not necessarily have to be cut off and placed UNDER the beam... you could set the posts in place and make the first layer of beam lumber screwed (think Simpson Strong-Drive screws) to the side of the posts. Even better, you could just sandwich the posts with a pair of 2x8's or 2x10's. I've seen this method used for deck construction. Use lots of 16d nails for the joist-to-post connections.

I'm afraid that if you don't raise the middle of the floor at least about 1/4" you'll still experience some flex to the floor because the new beam and post system will settle some. Sure, you can shim between the beam and EVERY joist, but that could take FOREVER. And you'll need to do it several times as the posts pack the soil down a bit. That's one reason to raise the floor a little... the new beam/post system can be placed snugly below the joists and when the jacks are removed the weight will pack down the dirt a small amount, and maybe the floor will be a tad higher than before. But I guarantee that the new posts/footings will settle, unless they are "wickedly tight" when installed.


Bruce W. Maki, Editor.




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Compiled January 22, 2002