# Sistering Bouncy Floor Joists



## TJ_in_IL (Aug 24, 2009)

Vote "1" for the plywood and glue/screws. If I recall, that was done once on an episode of Holmes on Homes. Seems to have worked out well.


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## hyunelan2 (Aug 14, 2007)

This was also done on the current project of "This Old House" - Auburndale project. It was early this season so I don't remember all that clearly, but I'm pretty sure they used plywood+glue and nails to strengthen up some sagging joists. Maybe google can tell you more about that project.


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## Octopus (Dec 3, 2010)

You are fine with the additional 2x8. 3 - 16d nails in a vertical row @ 16" on center. No screws, no plywood, no glue. Solid blocking between joists ( same dimension as the joists themselves ) is a must @ half the distance of the span


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## mrgins (Jan 19, 2009)

I'd go with plywood. Easier to install.


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## Daniel Holzman (Mar 10, 2009)

Standard 2x8 lumber with true dimensions 1.5 inches wide by 7.25 inches deep, with a modulus of elasticity of 1.1 million (a reasonable estimate) would have an L/D ratio of approximately 270 at a 12 foot span. This is only a little less than the recommended ratio of 360 for wooden floors. For large format floor tiles, 720 is recommended.

You can increase the L/D ratio to 540 by sistering on another 2x8. Use of glue is not required, however you do need to use an adequate number of nails to connect the two joists. Glue would not hurt, simply not necessary.

Sistering on two 3/4 inch thick pieces of plywood will increase the width to 3 inches, and will be slightly stiffer than using another 2x8 because plywood typically has a higher modulus of elasticity than SPF lumber. I doubt you would notice the difference between the two techniques.

In either case, as mentioned, blocking is typically required by code, and is certainly recommended, as it allows multiple joists to work together and effectively stiffens then entire floor.


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## mrgins (Jan 19, 2009)

Glue greatly increases the holding strength.
Plywood strips don't have to be doubled up on each side, one on each side with staggered joints is sufficient.
Plywood strips don't need to be supported on the top plates at each end, are more stable, and lighter to handle.


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## Daniel Holzman (Mar 10, 2009)

Mrgins, a few points of structural clarification.

1. _Glue greatly increases the holding strength_. The only reason you need to connect two sistered joists is if you do not run one of the joists over the bearing plates, in other words you cut them short. This can be done either with plywood or solid lumber. In that case, it is necessary to use sufficient nails, glue, bolts or some combination thereof to transfer horizontal shear between the joists so they both perform equal work. This is easily done using an adequate number of nails, screws, or bolts. Glueing does not hurt, but is completely unnecessary if adequate nails are used. Increasing the horizontal shear capacity beyond the required minimum does not make the joists stiffer or stronger.

2. _Plywood strips don't need to be supported on the top plates at each end. _See comment 1, you don't need to support solid lumber on either end, the key is to use adequate fasteners to achieve the required horizontal shear capacity.

3. _Plywood strips are more stable_. I have no idea what you mean by this comment. Both plywood and solid lumber are equally stable if properly installed.

4. _Plywood is lighter_. Plywood has approximately the same density as solid lumber. A pair of 3/4 inch thick, full depth plywood sandwiches weighs the same as a single solid lumber piece of the same dimensions. Of course a 3/4 inch thick piece of plywood is half the weight of a 1-1/2 inch thick piece of lumber of equal depth, but the advantage of easier handling is counterbalanced by the fact that you have to attach two pieces rather than only one.


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## mrgins (Jan 19, 2009)

It's interesting that you are a civil engineer, as it was a friend who is a civil engineer who told me of the additional strength/stiffness in using glue.
Plywood is more stable in that it doesn't expand or contract as much as lumber, and it also does not have the same defects as lumber that would make it twist or act with the existing joist to create some kind of movement.
Regardless of what you say about not needing to be placed on the plates, as a contractor, I feel I would gain more by doing so, even though I know what you're saying. Someone else could argue that, since the weakest point in the existing joist is the center, then just an 8' strip of plywood in the center would be sufficient.
Since we're talking to a DIYer, I'm sure he'd rather easily handle a strip 8' plywood, one at a time, than have to find a helper to install a 16' length of lumber.


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## SimpleFix (Sep 2, 2010)

Thank you for the input, everyone. I also located a thread where 1/8" steel strips screwed (every 4" to 8" with 3" wood screws) to the bottom of the joists can supposedly help with the bouncy floors. 

http://ths.gardenweb.com/forums/load/repair/msg0915531723856.html

It helps by counteracting the elastical difference in distance between the top and bottom of the wood joist when a load is applied to it. When the joist bows with a load, the top will be slightly shorter and the bottom will be slightly longer. The steel tries to prevent the bottom from getting longer. A few people on the thread who tried it had good things to say.

Thanks.


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## artmark (Oct 24, 2010)

I asked a similiar question about a month ago and the response was plywood would not help? so again I have a 1/2" hole 1.5" from the joist edge if I sistered that joist with plywood would it help? be the same as sistering with another joist?


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## Jackofall1 (Dec 5, 2010)

2. _Plywood strips don't need to be supported on the top plates at each end. _See comment 1, you don't need to support solid lumber on either end, the key is to use adequate fasteners to achieve the required *horizontal shear capacity.

*Doesn't shear occur in the vertical plane???


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## Bird Doo Head (Dec 8, 2010)

*I Wonder If I wasted My Energy!*

Hi Everyone!
I'm new here. I was searching around for some sistering information and found this great forum. (Then I spent a couple of hours looking at all kinds of great information.) I'm not trying to HiJack this thread, but I have very related questions that may help with the original posting. If I did hijack and violate protocol, I apologize to all!

I just sistered several joists in my house to prepare for ceramic tile. My house is maybe 100 years old and, apparently, built on a budget. The floor joists are nominal 2 x 6's and span 9 feet at various centers (mostly near 16" one at 26"). I nailed and glued sister 1 x 6 fir. I chose dimensional lumber because I "learned" that 3/4" plywood is supposedly only equal to 3/8" dimensional regarding vertical shear. The arguments for using dimensional lumber say 3/4" plywood with 5 layers had 3 laminations going the direction we want and 2 going the opposite, doing no work. (I learned this on the internet- We all know the internet never is wrong. Ever) If I take it further, in theory, we have 3/5ths of 3/4" working for us. That would be somewhere around 29/64th of an inch doing anything. (Actually, everywhere I read said 3/4" ply is equal to 3/8 solid)

Mr. Holzman mentioned using plywood instead. Now (too late) the light bulb in my head went on: Plywood Is An Engineered Product & All The Layers Are Laminated With Adhesive,thus forming a cohesive unit working against vertical shear. If I let my mind wander- I picture the grains of sand in concrete. They all go in every possible direction, but when glued together with the portland cement in the mix, they act as one, strong unit. It makes sense that plywood (glued veneers) does the same thing.

To make sure I understand correctly, for the rest of the house can I use 3/4" plywood to sister the joists? This sounds much more logical than dimensional, especially mating new-to-old sizes. I can custom cut.

Should I use 2 layers for maximum stiffness, or is that wasteful? My house has lots of conduit on one side of the joists. Can I put them both on the same side of the joists to eliminate lots of re-piping?

Thank you all for the great information on this site and for any clarifications!
Paul


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## Aggie67 (Dec 20, 2008)

Jackofall1 said:


> 2. _Plywood strips don't need to be supported on the top plates at each end. _See comment 1, you don't need to support solid lumber on either end, the key is to use adequate fasteners to achieve the required *horizontal shear capacity.*
> 
> Doesn't shear occur in the vertical plane???


 
Both planes.

Here's a visual of the shear that Daniel is describing. Take a chunk of 1 inch plywood, 2"x6", with the plys laid flat. Steam the heck out of it so the glue softens. Take it, and lay it on a coffee can and press down the ends, so the strip takes the shape of the perimeter of the round can. Basically an upside down U. You will see the ends of the strip deform and no longer be a square cut. The ply at the can's surface stays put, but the outer plies (because they are the same length as the ply on the can) will slide and pull those square end cuts into a uniform raked edge. Those outer plies, pulling against the glue of the ply beneath it, are experiencing horizontal shear. I'll try to do with the keyboard:

Start:

==============
-------------------------
==============
-------------------------
==============

Finish:

............========
.........-----------------
......===========
...----------------------
==============

The only way to get from Start to Finish is for the plys to slide. The resisting force of the glue is resisting the horizontal shear. 

Now forget that it's plywood. A bending 2-by joist undergoes the same thing. A sistered joist or slab of plywood needs enough fasteners to overcome the horizontal shear in the original loaded joist. There's a way to calculate it out, but my oven is beeping.


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## Tigerloose (Dec 5, 2010)

Years ago I built a loft in a barn. I crafted 4"x10"x16' from T1-11 scraps. Mostly door cut outs and fall off. I spaced them 5' on center and used more scraps for the floor sheathing. I stored stuff up there for twenty years. When I sold the orchard I wanted to remove the loft for fear that someone would overload it but the new guy insisted on keeping it.
Plywood is strong material but gosh it was a lot of work ripping and nailing[no glue]. 
Use lumber, nail the snot out of it and be done with it.


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## Daniel Holzman (Mar 10, 2009)

Yes there is both vertical shear and horizontal shear in a beam loaded with uniform vertical load such as live load on flooring. However, in virtually every typical circumstance in residential construction, the limiting design factor is the bending moment induced in the beam by the load, NOT the vertical shear.

Vertical shear is normally greatest at the supports, so if there were ever to be a need to increase vertical shear capacity, you would generally need to do it at the supports. However, as I noted, it is EXTREMELY rare that vertical shear is a limiting factor in the performance of joists and wooden beams in houses.

The reason that sistering a joist with either plywood or solid wood reduces the "bouncy" feeling of the floor is that adding material to the joist increases the moment of inertia of the joist, which reduces the deflection under a given load. You can also reduce deflection (improve stiffness) by shortening the span of a joist, i.e. adding an intermediate transverse beam, replacing the joist with a material of higher modulus of elasticity (substitute steel beam for wooden beam), or increase the moment of inertia by adding a steel plate or strip to the bottom of the joist (the straps referred to is one example of how to do this).

If you add a strap to the bottom of the joist, it is essential that you get the strap to work together (composite action) with the joist. To do this, you need to connect the two members together using nails, glue, bolts or similar fasteners, in order to overcome the horizontal shear that develops between the two pieces. Calculation of the horizontal shear is somewhat complex, and I am not going to present the formula and theory on a DIY site. That is why structural engineers go to school.

If you use an adequate number of fasteners, the two elements work together, and the composite moment of inertia is greater than the original moment of inertia, so you get a stiffer joist. If you use an insufficient number of fasteners, the composite beam is likely to break at the connection point.

PLYWOOD: Plywood is a complex material, which as was pointed out has every other ply oriented 90 degrees to the previous ply. The properties of plywood are difficult to compute mathematically, however the Forest Products Laboratory in Madison, WI has published formulas and guidelines for the computation of the strength of plywood when loaded parallel to the face plies, and perpendicular to the face plies. The publication Plywood Design Specification January 1997, published by the Engineered Wood Association (APA), discusses at length the appropriate strength and stiffness parameters for plywood. This is a lengthy document, not light reading, however if you want to know how a plywood-lumber composite beam is going to perform, this is the document you need to consult. It is inaccurate to state that plywood is equivalent to a solid wood beam of half the width of the plywood, in fact computing the equivalent size solid lumber beam to a 3/4 inch thick piece of plywood is not simple, and depends on the type of loading, the orientation of the load, the grade of plywood, and the method of attachment of the plywood to the solid lumber.


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## jklingel (Dec 21, 2008)

Aggie67 said:


> Both planes.
> Here's a visual of the shear that Daniel is describing.


 So the plywood is essentially solid wood. Correct? Or did I fumble? j Never mind. Dan's reply came up after I posted this, so I'll ask a different question. In general, if plywood is used as a stiffening member with the face grain parallel to the floor joist, does it add approx the same stiffening as a same-thickness piece of solid wood, given that each are of the same length and located at the same place along the floor joist?


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## jackpine (Sep 3, 2008)

I hate when I am searching out info on a project, knowing what each alternative is and trying to choose the cheapest, easiest, and most effective, how I never ever really get an answer.


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## mrgins (Jan 19, 2009)

I'm sure by now, the OP has done what he thought was best and, hopefully, it has worked. Obviously in this case we're looking at stiffness rather than strength. I think, as a carpenter, it goes against my grain to install a full length 2x8 without resting it on the plates at each end. It makes no difference in this case, but i guess I'm being anal about the way it looks. The point I really meant to get across was that Plywood, even though you have to rip it (not really a big deal is it?) can be handled in 8' lengths, doesn't risk splitting as it dries out, can be cut and placed easier if there are wires/pipes in the way, and is lighter and more maneuverable in a tight space. And yes, because it's 8' lengths, you'd probably need to double up the thickness, although if the joists are only 12', it's possible applying plywood to the middle 8' would be enough, and maybe you could get away with just one layer. Another way is to strap the underside perpendicular to the joists, but I doubt it would be as effective as sistering the joists.


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## jackpine (Sep 3, 2008)

So here is MY deal. Home was built in 1970. Upstairs floor joists 14' span, he used 2x8s 16" on center. I have the downstairs ceiling out right now and I am going to fix the bounce this weekend. So I research. At first I was going to sister every other joist with 2x8 and be done with it. Now I am considering the plywood and glue option as possibly the best option. I am going to go purchase the materials later this afternoon, so I need to make a decision. I have all the tools and help for each option, just want to choose the one that is best.


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## mrgins (Jan 19, 2009)

Well you know my answer to this! Either way is good, but do you have to circumnavigate a lot of pipes or wires?


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## Bird Doo Head (Dec 8, 2010)

Mr. Jackpine-
I don't know if this will reach yo in time, but here are the methods I used on my very old house with plenty of bounce & a few cracked joists:
I am not an engineer, nor a carpenter. So take this as "Might Be Crazy"

Background: Joists are 2x6, about 100 years old. Each has shrunk to about 1-3/4 x 5-1/8" (I would have expected actual 2 x 6). Centers are 16, 23, 26- Kind of randomly. Span is 9 feet.

Mehtod One: Sister 2x6 on one side. Wash!, glue & nail. Glue is wood glue, not construction adhesive for a bond stronger than the wood. Nails are 16d. Tow rows, staggered by 2" horizontally. Spaced 8" apart. Rows are 1" from top & bottom edge of the wood. Sisters land on main beam & outside wall. (House has no top plate).
Installed cross bracing in entire house- both sides of main beam at span center. I could not do solid blocking due to gas, hydronic, domestic piping and air handler ducts, plus conduit & more stuff in the way. It looks like a factory down here.
Result: No bounce at all. A couple of rooms are ceramic tiled with no tile nor grout cracking. (One room, I cheated & did a floating installation, like a sand bed- but used Ditra for less room-to-room height offset.)

Method Two: Plywood Sisters on one side of the joists. The plywood is subfloor rated, not sheathing. This has less voids (so I'm told). I did this on the second floor. Installation was: *Wash the wood! Glue (wood glue) Nail as above. The subfloor was 1x12 planks. It is now subfloor rated plywood; glued & screwed. This helps spread the load across many joists (I hope).
The sisters land on the ledges at each end. (House is balloon framed.)
Result: No bounce nor squeak at all. None.
Side Note: These are 2 x4 oak joists. The span was up to 16 feet.(Yep-16 feet) I also added a beam-in-soffit below & a bearing wall below- to a footing in the basement. The longest span is now 6 feet. I can't say if the sisters or shortening the span helped more. Still no where near by-the-book, but the house has been here over 100 years.

If I had to do it again:
I found the plywood much easier to install. I did have problems with the 2x6 when I tipped them in place. I had to rip them to the existing joist size & bevel one corner- then bang them in with block & sledge.

If I had a brain in my head: In Europe, it is common to lay a 2x4 flat on the bottom of each joist, then generously screw it into the joist. This creates a kind-sorta I Beam. If the subfloor is also well secured to each joist, you get a wood I Beam. But, blocking is required to prevent twisting. I learned this from my Made-In-Germany father who was an engineer. I did this in a crawl space house I owned over 25 years ago. It worked great! 
I don't know why I did not think of it for my basement accessible joists here. (Could not do it on the 2nd floor without destroying beautiful plasterwork below.)

I hope this reaches you in time & helps you decide.
Again, I'm no pro but these are my results.
Paul


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## mrgins (Jan 19, 2009)

Bird Doo Head;1024965 I could not do solid blocking due to gas said:


> Sounds like a thorough job. Just out of interest, if you had no room for solid bracing, how did you manage to get a one=piece 2x joist in there?


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## Bird Doo Head (Dec 8, 2010)

*It Wasn't Fun!*



mrgins said:


> Sounds like a thorough job. Just out of interest, if you had no room for solid bracing, how did you manage to get a one=piece 2x joist in there?


I guess I should have added that detail. Oops!

For things perpendicular to the joists (only NM cable & some communications wire in my case); it was remove; attach new wood; re-drill & replace. Boring, but simple enough. 

Some joists had nothing attached to (parallel with) one face & stuff on the other. Those were the lucky ones. I used the empty face.

For each & every joist with stuff on both sides, one at a time I removed anything attached to the existing joist's face. That meant removing conduit straps, gas lines, etc. If I had enough free-play, I'd use temp tie line to hold the offending item away from the face enough to get the sister in (at an angle) so I could bang it upright. If I couldn't get clearance, down it came!

For the conduits, it was simple to re-offset to match the new location- adding to compensate as needed. What a time consumer that was! (I was an industrial electrician for many years. Conduit's pretty intuitive for me. Most days, anyway!)

For gas, I re-rolled the end fittings, replacing 90's with 1/8th bend pairs (with nipples between) as necessary. Fortunately, I only had a couple. The hydronic is all copper, so it was simple to cut & re-work. Again- Time consuming.

The solid blocking problem is that, with so much mechanical attached to the long faces of the joists, I'd be notching forever. On some, it's stacked quite a bit, so I would not have much end left on the blocks. I also have some with, as odd as it sounds, plumbing running along the bottom of the subfloor half way between joists. Goofy, huh?

There actually a couple that I could not even get cross bracing in. For those, I wiggled in 1 x 3 toward the top & one toward the bottom to span between the joists. I doubt they do any good at all. (Flex?)

In retrospect, I wish my father's advice was in my mind. I would have tried the bottom 2x4 flat system for all but the cracked joists. (I wish his advice was in my mind for lots & lots of things I've done over the years!)


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## mae-ling (Dec 9, 2011)

2008 Flashback thread
http://www.diychatroom.com/f5/osb-stronger-than-plywood-23523/

Adding a 2x4 along the bottom to make a T increases strength to 1.777777% in a 2x10 existing joist. 
Sistering it doubles the strength. But adding the 2x4 is a lot easier. Does nothing for straightness though.


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## Rocket98 (Sep 14, 2012)

Octopus said:


> You are fine with the additional 2x8. 3 - 16d nails in a vertical row @ 16" on center. No screws, no plywood, no glue. Solid blocking between joists ( same dimension as the joists themselves ) is a must @ half the distance of the span


 X2. I prefer this Kiss method


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## Bird Doo Head (Dec 8, 2010)

mae-ling said:


> 2008 Flashback thread
> http://www.diychatroom.com/f5/osb-stronger-than-plywood-23523/
> 
> Adding a 2x4 along the bottom to make a T increases strength to 1.777777% in a 2x10 existing joist.
> Sistering it doubles the strength. But adding the 2x4 is a lot easier. Does nothing for straightness though.


Thanks for the data Mae-Ling. That's interesting to know. 
Is the formula something a BDH like me can calculate? I curious about 2x4 on the bottom of 2x6's. That's what is in my 'grand estate'. (Code for: 'Little House Of Big Projects")


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## mae-ling (Dec 9, 2011)

Take total when added (5.5+1.5=7) divided by height of height of old joist (5.5) then cube it.
7 divided by 5.5= 1.2727272727272 now cube that 1.272727272727272 X 1.272727272727272 X 1.2727272727272 = 2.061607813673927.
Just over twice as strong, basically the same as doubling it up.

You have sorta made a 2x8 ( actually just slightly under) I punched 2x8 spruce into the span calculator and it says your good for about 12'.


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## Bird Doo Head (Dec 8, 2010)

*Thank You*

Thank you for the formula, Mae Ling. I enjoy the science behind just about everything. This entire series of entries is full of "save this" information, thanks to all of you. Personally, I learned a lot.
By now, I hope the original poster has a good start on his or her project.
Paul


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