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martinf 02-28-2010 07:14 AM

Undersized Joists and a solution...I think
I am renovating a room on the main floor of a two story house built in the 1920s. The room is 19x 17. The second floor is supported by 2x10s spaced 16 inches (or less) apart. The wood is true 2x10 rough sawn. The joist to the second floor run 191 and are supported by pockets in exterior brick walls at either end. Above the room is a sheet rocked bed room, 3 closets and a bathroom that has old style tile and a cast iron tub. The span tables that I have read suggest that this is not sufficient to support the second floor. I think the room at one time was separated by a supporting wall. I suspect this by viewing other homes in the same community. I think it likely the previous owner removed it 3-5 years ago based on neighbor conversations. There is some slight sag in the second floor, about a half inch. I am contemplating jacking then sistering 11 7/8 LVL to the joists going the full span. There are minimal obstructions and I think I can fit them in. Will this prevent future sag? I dont want to be just within code, I want to error on the side of overbuilt. I love this website, I am excited to be part of it!

Snav 02-28-2010 08:39 AM

Yes, sistering in a matching beam will give you more than enough strength. If you do it right what you'll have is a solid 4x10 instead of a 2x10 - a properly sistered joist has immense strength and you'll be surprised at how spring-free your room feels when you're done. (I'm not sure how bad it is for you - but for us our fishtank use to splash water just by walking in the room.)

To properly sister you want to get several wood-clamps (about one for every 16" of span) and a bottle of high-end wood glue and some carriage, lag or other heavy-duty nails/screws (preferrably screws - nails might pop out over time - I use deckmate screws, they have a firm grip and are much easier to drive in - if you use screws you'll be using a lot more of them.)

After you get your new span in place you should use a 2x4 on top of the jack-head/under the beam - so your jacking doesn't damage or tilt your joists) (if you're having to jack from the floor below - you'll want to rent a ceiling jack (my unofficial name for it) - it's a jack that's designed to hold a 4x4 vertically.)

Now - rather than do what I did the other day, I suggest you string together some 2x4's that's the span of your joist and see if you can get it in place. What might look simple is likely not to be so simple - it's easier to trial/error with something lighter first before you really try to hoist your super heavy beam up there.

I also suggest you examine all reasons for sag - other than the beams being undersized - how is the supports for them where they rest? Is there sag there, as well? If there's any sag on the girder or ledge that supports them then you also need to solve that problem.

I have both problems - my mudsill is what's carrying the weight, not the girder, and the joists are undersized so I'm fixing both things before I continue my rebuild by bringing the girder forward with a new piece of 4x6 and then attaching my new joists (which I'm sistering to the old ones) to that, instead.

Ron6519 02-28-2010 11:48 AM

I would have this change in structure checked out by a professional to give you a proper solution instead of a "best guess" scenario. The loads across the entire floor would need to be taken into consideration.

Snav 02-28-2010 05:08 PM

Yeah, you should check.

Doubling the existing joists is often a routine and acceptable measure but that might vary depending on where you live.

Scuba_Dave 02-28-2010 05:17 PM

Is it 19' 1" between the walls.....or are they 19' 1" long when resting on the brick ?
Between the walls is the span distance
What will support the new joists ?
Are you taking the ceiling down in the lower room to fit the 11 7/8" in ?

Most lumber stores can run the numbers
They need to know the span distance & loads (walls, where roofs land) on & above this floor

You might find that LVL's are overkill, but until the numbers are run its all guessing
I had an 18'+ span where I could have installed 2x12's
They ran the numbers & BCI40 I-joist were much better
So I went with BCI60's for an even stiffer floor

Tonglebeak 02-28-2010 05:29 PM

Correct me if I'm wrong, but aren't modern span tables entirely useless for old, hard, true dimensional wood? If my true 2x6 ceiling joists can span 16ft and provide very little bounce...but I'm no expert.

Snav 02-28-2010 05:50 PM

Yes, they're useless, really - but if your old ones are sagging then you'll still be increasing their strength considerably (maybe not completely doubling it, though)

To sister beams I've never had code-issues or problems with permit requirements. It's a fairly common thing and, at least in my area, they don't care what you do so long as you're not removing the existing structure.

Tonglebeak 02-28-2010 05:59 PM

As for sistering, would sistering the midspan third of the joist be as effective as sistering a whole piece?

With that said, correct me if I'm wrong, but if the sag is now part of the old wood's "memory," then won't the old wood win in a sistering contest (as in the new lumber would take the sagging shape of the old wood)?

Snav 02-28-2010 06:44 PM

If what you presume is correct and wood 'remembers' the sag and wants to sag - then sistering the middle third won't do anything.
I actually tried that under my house and it made no significant difference. The new wood should be of equal or greater strength to correct sag.

Using a smaller-than sister would be idea for repairing a broken, cracked or otherwise damaged beam, though.

Tonglebeak 02-28-2010 06:49 PM


Originally Posted by Snav (Post 407539)
If what you presume is correct and wood 'remembers' the sag and wants to sag - then sistering the middle third won't do anything.
I actually tried that under my house and it made no significant difference. The new wood should be of equal or greater strength to correct sag.

Using a smaller-than sister would be idea for repairing a broken, cracked or otherwise damaged beam, though.

Sorry, I got a little off-track. For the sistering 1/3 midspan part, I meant to add strength, not to remove sag. As for as the sag in memory, I was asking if that's what happens to new lumber the whole length.

Scuba_Dave 02-28-2010 06:50 PM

To "de-sag" something that has existed long term you need to slowly jack up the middle of the span
This can take quite some time
If you try to go to fast either you will break the wood, or lift the walls/side of the house up
Someone else tried to go too fast & ended up jacking the side of the house up

Once the sag has be eliminated then you can sister
It is possible that simply jacking up the center will get rid of the sag, then you can sister the joists
Just be prepared that this may take time

martinf 02-28-2010 07:06 PM

It is 19' 1" span with 4 inch bearing in the brick pockets.

magpie57 02-28-2010 07:26 PM

undersized joists
I agree that the sagging joists mightl have memory or are already prestressed . I think, my intuition would tell me not to jack it up and live with a small sag. Don't forget the blocking , two rows equal distance.

I love this site too. Great information and oppinions.


Snav 02-28-2010 07:35 PM

a small sag will only get worse overtime - and the longer you wait the more problems you have to fix in the future.

I'd suggest to figure out the proper joist size and feed in new ones - it's such a common issue to deal with it's almost routine for many home-repair pros.

Gary in WA 02-28-2010 09:54 PM

A lot of interesting answers. You need bolts at the ends to transfer loads when sistering to correct a sag. Read this from page 134 --- 147 and look at illustration 8-2-1:

Far as the old wood spanning more, I answered this before….. “All you need to do is ask! And some math....... Table #7, page 643:
Compare it to today’s wood span tables.

Flitch plates: page 655--------------then the spans and math per inch, starting on page 717. Three kinds of oak, page 651.

Are we having fun yet? Let’s put 50 men in one room, elbow room only, to test the floor: you find it.” From: How to sister for strength only.

Don’t try to jack up the sag in a floor more than a tiny bit, you won’t win, even over a period of time. The wood is permanently shaped to that design now and that is why you need a new joist next to it.

For span with an LVL:

Be safe, Gary

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