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Old 07-02-2010, 10:01 PM   #1
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sistering/sandwiching vs reducing centers


103 year old wood, hard as a rock, 2x6 24"OC. Some span 14', others span 16'. The joists that were never touched since the house was built, are incredibly strong. The joists that were "modified" (put on that crappy wall in my other topic) have a nice amount of deflection. Granted, making the wall strong stiffened those joists up quite a bit, but they still have a good bit of give (enough that will crack drywall ceiling).

So, seeing as how these are true 2x6, and modern 2x6 is 1.5x5.25, I was wondering what would be the best thing for me to do.

a) Simply sister the joists
b) Sandwich the joists (might be a better idea if I try to level out a small amount of sag, not sure how well this will work)
c) Reduce the centers from 24" to 12"

The subfloor is the original T&G hardwood floor, and then there's a hardwood floor on top of that.

I realize modern 2x6s aren't designed to span such distances, but I have no headroom to lose at all.

If the sistering or sandwiching idea is best, I will try my best to sister them all the way to the bearing points. But if I cannot, how long should I make the sisters? Also, when sistering, should I push the new joists against the floor, or keep them flush with the bottoms of the old joists (since there's going to be 1/2 inch or so to spare)?

Block bridging would probably help I guess, but before I cover these back up, I want to make sure everything is sturdy and that I won't have a cracked ceiling again.

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Old 07-04-2010, 09:25 PM   #2
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sistering/sandwiching vs reducing centers


"If the sistering or sandwiching idea is best, I will try my best to sister them all the way to the bearing points. But if I cannot, how long should I make the sisters?" ---------- Only shorter than the joist depth from the end: Sorry, still looking for that bookmark!

"Also, when sistering, should I push the new joists against the floor, or keep them flush with the bottoms of the old joists (since there's going to be 1/2 inch or so to spare)?" ----- Depends on supporting the full load or not: Page #136: http://books.google.com/books?id=1uY...lywood&f=false

“Block bridging would probably help I guess, but before I cover these back up, I want to make sure everything is sturdy and that I won't have a cracked ceiling again.” ----- http://www.finehomebuilding.com/PDF/Free/021184090.pdf

Be safe, Gary

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Old 07-05-2010, 12:49 AM   #3
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sistering/sandwiching vs reducing centers


From the sound of it this was an attic which was turned into living space. 2x6 on 24" centers on a 14' span was never designed to carry live load. It was to hold the weight of a plaster ceiling and that's all.
Having a double 3/4" thick floor will help with floor deflection between the 24" span and a bit of load bearing ability. At the same time it is adding to the weight being supported by the joists.
First if needed jack the center up to level out the joists.
I would suggest sistering and sandwiching 3/4" plywood on edge between from bearing wall to bearing wall. Place the 8' rip of plywood in the center of the spand and add 3' pieces on each end. The plywood will add much needed rigidity. I would nail together then bolt with carrage bolts staggered up and down not less than 3' apart.
Since the weight is on the floor above push new framing to the top. Since there is some deflection this may not happen for the entire run.
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Old 07-05-2010, 10:25 AM   #4
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sistering/sandwiching vs reducing centers


Gary, thanks for the info and links. You always back your statements up

Nailer, I read somewhere that plywood does no good (something to do with the grains going in the wrong direction). Am I incorrect here? The 16' span doesn't deflect worth beans (yes, believe it or not), just the part that was hacked away in a remodel done years ago.
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Old 07-05-2010, 05:06 PM   #5
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http://www.finehomebuilding.com/how-...-up-beams.aspx

Be safe, Gary
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Old 07-05-2010, 05:54 PM   #6
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sistering/sandwiching vs reducing centers


Thanks Gary, looks like solid lumber is the best way to go then. I never knew about metal straps (just read that pdf you linked). One thing I don't understand is the solid blocking. It shows to only nail one side per block. That doesn't make sense.

Quote:
Originally Posted by Nailer View Post
From the sound of it this was an attic which was turned into living space. 2x6 on 24" centers on a 14' span was never designed to carry live load. It was to hold the weight of a plaster ceiling and that's all.
I don't think that's true. The attic joists are true 2x4s, so this was definitely a 2nd floor when the house was built ages ago. It's all about fixing what the remodelers broke years ago at this point, when they built a 3-stud wall to hold up half of the 2nd floor (with one of those studs being cut and toenailed into itself). >

I think I'm going to call my grandfather for the header part. He did this carpentry work for over 30 years, I have got to ask him more of these questions lol.

Thanks again guys.

Last edited by Tonglebeak; 07-05-2010 at 06:07 PM.
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Old 07-05-2010, 08:01 PM   #7
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sistering/sandwiching vs reducing centers


The link in #5 seems to say that the modulus of elasticity, E, for plywood used vertically is twice that for solid wood so plywood will give you twice the stiffness for the same thickness.

Coulda' fooled me. He may be assuming average wood has an E of one million PSI and average plywood has an E of 2 MPSI.

Last edited by Yoyizit; 07-05-2010 at 08:43 PM.
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Old 07-22-2010, 10:37 PM   #8
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sistering/sandwiching vs reducing centers


Quote:
Originally Posted by Yoyizit View Post
The link in #5 seems to say that the modulus of elasticity, E, for plywood used vertically is twice that for solid wood so plywood will give you twice the stiffness for the same thickness.

Coulda' fooled me. He may be assuming average wood has an E of one million PSI and average plywood has an E of 2 MPSI.
How could plywood give twice the stiffness if half of the grain is going the wrong way?
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Old 07-22-2010, 11:03 PM   #9
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sistering/sandwiching vs reducing centers


Stiffness of plywood is effectively independent of grain direction due to construction techniques for the material. For a full discussion, see http://g-lumber.com/pdf/APA_Plywood_...cification.pdf

table 3 on page 19, where the modulus of elasticity for plywood is shown as the same regardless of grain direction.
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Old 07-23-2010, 03:13 PM   #10
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sistering/sandwiching vs reducing centers


Could you simply glue and screw a 2x4 to the underside of each joist? Only 1.5", but you increase the capacity of each beam tremendously.

I had an issue with storage in my garage. I solved it by making 20' long 2x6 beams with a 1.5" steel L piece epoxied to the bottom. It's bouncy, but I'm only using it for storage - not dancing!
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Old 07-23-2010, 10:30 PM   #11
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sistering/sandwiching vs reducing centers


With such a limited space I think your going to need a steel flitch plate on each.
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Old 07-24-2010, 10:19 PM   #12
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sistering/sandwiching vs reducing centers


Quote:
Originally Posted by xxPaulCPxx View Post
Could you simply glue and screw a 2x4 to the underside of each joist? Only 1.5", but you increase the capacity of each beam tremendously.

I had an issue with storage in my garage. I solved it by making 20' long 2x6 beams with a 1.5" steel L piece epoxied to the bottom. It's bouncy, but I'm only using it for storage - not dancing!
Thanks Daniel, that clears that up.

As for screwing another board, I thought of that, but every inch does count.

I was looking through the link GBR posted about Simpson Strong-Tie Coiled Straps. Just how effective would those be? That looks like a great option with limited headroom, and I see there are 18 and 16 gauge versions made as well.

Basically what I'm asking is: would it be the equivalent of sistering, or adding depth, or what? Thanks
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Old 07-25-2010, 02:14 AM   #13
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sistering/sandwiching vs reducing centers


That would be adding depth, but I'm not sure how much. A bead of PL Premium down the center before you screwed it up using the SD9 screws would do it.
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Old 07-25-2010, 09:14 PM   #14
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Hmm, Mr. Holzmann, have any idea?

Would there be much of a difference between the 16, 18, and 20 gauges?
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Old 07-26-2010, 01:55 AM   #15
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sistering/sandwiching vs reducing centers


Personally I wouldn't do it without at least 3/16 steel layer there unless an engineer specifically gave me the thumbs up. Of course, the cost of the engineer would immediately eat up the cost difference anyway.

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