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2x6 floor joists vs 2x8

justingbearing's picture

Hello,

What would be better to "stiffen" a floor: sister another 2x6 or to put a 2x8 with the ends cut to a 2x6 and then use a hanger?

The floor is spongy with the current 2x6 @ 24 OC.

Thanks.

(post #67462, reply #1 of 38)

JB,
It may help to know the span. Sistering has its applications, but can not correct all situations. It will stiffen the floor to a point, but also add weight. What about a mid span beam?
Mark

(post #67462, reply #2 of 38)

Thanks for the reply.

I'm looking at 10 feet between walls that is being spanned.

(post #67462, reply #3 of 38)

If possible, your best bet is to add more 2x6 so that you end up with 2x6 @ 12" oc.


If that's not possible, a mid span beam would be the next best option. But there's alot of work associated with this method such as adding posts, piers, etc.


Sistering the 2x6 would be my next option.


Adding 2x8 with a notch cut on the end effectively creates a "2x6" the 2" cut on the bottom reduces the 2x8s strength.

(post #67462, reply #4 of 38)

If it's a basement ceiling ( 1st floor ) I'd sister w/ 2x6, and run solid blocking the length of the floor. Also a " strongback" would help ( 2x4 running lengthwise,flat to ceiling w/ another screwed in an L shape to side of it) tremendously in my exp.

(post #67462, reply #5 of 38)

I would use 3/4 plywood "glued and nailed on both sides with seams on opposite ends" to sister up the 2x6, if a beam at midspan will not work.

(post #67462, reply #12 of 38)

Thanks.

"seams on opposite ends" Sorry, I don't understand. What seams?

(post #67462, reply #6 of 38)

In my experience usually 2x6 don't span more than 9 feet (new const.)


But if it's already there, you don't have the option.  Sistering will help some, but I would try the strongback......i've had good experiences with these too.


Don't cut down the 2x8's---

When in doubt, get a bigger hammer!

(post #67462, reply #10 of 38)

A lot of excellent ideas, thanks.

(post #67462, reply #7 of 38)

My calculation says that sistering another 2x6 will reduce the deflection by 50%, while sistering a 2x8 will reduce it by 72%. You may have to attach the sisters rigidly to the original joists (i.e. glue and screws) to get the full advantage.

Here are the calculations. A single 2x6 is taken to have a stiffness value of 1 and a deflection value of 1, with no units defined. The absolute stiffness and deflection numbers are not calculated, only relative values.

Stiffness varies directly with the width of the joist. Sistering another 2x6 doubles the width, and therefore the stiffness. Deflection varies as the inverse of stiffness, so doubling the stiffness cuts the deflection in half, for a 50% reduction.

Stiffness varies as the cube of the (vertical) depth of the joist. The depth of a 2x8, relative to a 2x6, is 7.5 / 5.5 = 1.3636. The cube of 1.3636 is 2.5357, so the stiffness value of the 2x8 by itself is 2.5357 times the stiffness value of the 2x6. Add that to the stiffness of the original 2x6 and you get a total stiffness of 3.5357. Again, deflection varies as the inverse of stiffness, so the deflection of a 2x6 and 2.8 sistered together is 1 / 3.5357 = .2828, relative to the deflection of the 2x6 alone, for a 71.72% reduction.

Boss Hog has instructed us that the "feel" of a floor depends on its resonant frequency as well as the the actual deflection. Stiffening the floor with 2x6's will raise the frequency, and 2x8's will raise it even more, but I don't know a way to calculate whether either one will raise the frequency enough get rid of the spongy feeling.

(post #67462, reply #8 of 38)

It is hard to tell whether you are asking sister 2x6 OR replace with 2x8 OR sister 2x8. of the three, the 2x8 sister is best. Replacing with a notched 2x8 will gain very little for the trouble

 

 


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We did the best we could...

(post #67462, reply #11 of 38)

Can't replace the 2x6s w/ 2x8s; would be nice but not possible w/o raising the whole upper floor.

(post #67462, reply #20 of 38)

Well, by notching the bottom out of the ends of a 2x8, or even a 2x10 you couild replace in the same locations, but like I said, there would be a lot of pain for the trouble when it is easier and better to sister with what you have

 

 


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Taunton University of
Knowledge FHB Campus at Breaktime.
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We did the best we could...

(post #67462, reply #13 of 38)

Gotta disagree that that the notched 2X8 provides little gain. IMO, the notched 2X8 is second only to a full 2X8  and far superior to a sistered 2X6.


Notching a 2X8 down to 2X6 at the ends DOES reduce the ultimate shear strength of the joist to exactly that of a 2X6. However shear strength is not the issue here, bending moment  (deflection) is. In this case the notched 2X8 will provide almost exactly the same stiffness as an un-notched 2x8, especially if the vertical face of the notch bears firmly against the concrete foundation or  whatever is below.


Chris


 

(post #67462, reply #14 of 38)

I agree with you, Chris.  I'm always surprised by the smart old carpenters who don't seem to get this principle.  I heard Tommy Silva say it once when i used to watch This Old House, that if you notch the ends of the joists you're reducing the strength of the joist to that dimension.  How often do joists fail in shear, versus showing excessive deflection?  Not often.


 

 

 

(post #67462, reply #16 of 38)

I guess you and chrisb have never seen a joist never split at the notch.

(post #67462, reply #17 of 38)

Yup, I have, but not a sistered notched joist.  A bouncy floor means the joist needs more meat towards the middle of the span.  The ends of the span are much less important.  I've sistered bouncy joists and rafters without the sister even bearing at the ends, and it's worked perfectly.


A notched joist splits because a stress point is created.  If you angle or curve the notch you spread out the stress point and splitting is reduced or eliminated.


Have you ever noticed how top-chord-bearing trusses have very little material at the ends of the span?  Or how the beams under steel bridges are often joined at mid-span?  Or how sometimes joist hangers are spec'd to be installed upside down?  All examples of the two major forces going on in the beam or joist, shear and bending.  Notching the ends of the joists reduces the shear strength, but a 2x6 has plenty of material in shear for most floor loads.  The majority of the bending stress is closer to the center of the span; that's where he needs the most material added.

 

 

(post #67462, reply #19 of 38)

Assuming that a notched joist's shear strength is reduced to that of a 2x6 and force is applied at the end where the notch is, there won't be much deflection. If the amount of weight needed to exceed the shear strength is applied to the center of the span, the member(s) would fail anyway. Splitting longitudinally at the notch is another issue.

"I cut this piece four times and it's still too short."

"I cut this piece four times and it's still too short."

(post #67462, reply #18 of 38)

By the way, I like your personal quote.  I once learned that a good engineer is lazy and cheap--they are supposed to find the easiest solution to the problem, using the least amount of resources.


 


 

 

 

(post #67462, reply #24 of 38)

If I notched a joist I would use a joist hanger to support.

(post #67462, reply #22 of 38)

This wise old carpenter said nothing about shear strength, and I don't think any others do either. We speak from plain old experience, having seen and repaired splitting joists that cause structural failure of the floor to the point that bounce is increased and splits continue further every time that boiunce or extreme load is added.

Once a split starts at the notch, the effective load bearing is that of the reduced joist size or less, depending on th edirection the split follows the grain.

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #67462, reply #25 of 38)

A grand piano and a room full of drunk dancers should split them. Like the hundred drunk dancers on the Chicago deck collapse.

(post #67462, reply #26 of 38)

Again, thanks for all the great ideas. As a rank amateur it is quite the learing experience.

Part of the problem I have is that "they" used an 18 foot 2x6 and a 6 foot 2x6 to span the 24 foot width. Sistered a 2' - 2x6 to the seam with a ton of 16s and called it good. There are interior walls but not under the seam. Added to that is the powder post beetle damage.

Basically, anything I do is going to help.

(post #67462, reply #28 of 38)

Now that full disclosure has been made...

You need to sister all of those joists with 2x6 AND run a beam with posts down the center under it all. Start by treating the existing for the beetles, and make sure that you are keeping excessive humidity and dampness out, the beetles like damp wood. boracare is what you want to google up for treating, unless you want to hire an extermoinatortor

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #67462, reply #33 of 38)

What I meant by seams at opposite ends was;


Plywood comes in 8' length's,  I would glue and nail the ripped plywood to one side and then on the other side of the joist I would start the plywood 4' off center from the other side so that the seams would not line up on both sides of the joist. (like a flinch beam) The wider you make the rip the stronger it will be and I would keep the grain going horizontal.  Think of a TJI, (that is just a 1/2" piece of OSB with a flange on both sides to keep it vertical), or a plywood box beam.


By sistering up the 2' oc 2x6  with 2x6's,  I would think the the strength of the floor would change to 2x6 1' oc .


one way that I have seen an engineer fix a cut floor truss was to gusset on both sides with 1/2" plywood


I am not an engineer just my thoughts


Edited 8/14/2005 6:42 pm ET by wyatt

(post #67462, reply #36 of 38)

thanks.

(post #67462, reply #35 of 38)

Beetle damage does not seem to have been covered by others, what is the extent, will the 2x6s hold bolts???


Also, if you have a drill press, the lowest cost, easist method has not net been mentioned.  Without a drill press, it is a pain to drill the needed shear holes.


Use a 1/8" or 3/16  1-1/2 wide piece of steel strap (or angle if clearane, etc) about 9 ft long(for the 10 ft span) .  Drill 4 each tight fit 1/4 inch dia holes on 2" spacing at each end of the strap, strap goes to within about an inch of the end supports (no need to be in bearing) ,  then continue the holes toward the middle, the spacing can increase by an inch each hole till you get to the center.  Bolt with 2" long 1/4" lag bolts to the bottom edge of the 2x6.  If you can slightly jack up the ;middle of the 2x6 while installing the bolts, it is even better.


About the same effect on stiffness increase as a sistered 2x8 (BTW, you only need to sister an 8 ft piece of 2x8 if glued, agree with previous poster on don't even need end support on a sistered 2x8.


 


 


 

(post #67462, reply #37 of 38)

pretty damaged by the beetles; really need some fresh wood somewhere.

(post #67462, reply #15 of 38)

>> ... the notched 2X8 will provide almost exactly the same stiffness as an un-notched 2x8 ...

In fact, you could cut the 2x8 six inches short at each end and still get a very large percentage of the stiffness advantage, as long as the sister is rigidly fastened to the original joist.

(post #67462, reply #23 of 38)

These guys were so narrowly focused that they didn't seem to notice the specific application re sistering

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #67462, reply #29 of 38)

These guys were so narrowly focused that they didn't seem to notice the specific application re sistering


OK, I'll agree with you there ;-)