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SimpleFix 12-03-2010 08:23 AM

Sistering Bouncy Floor Joists
My 1985 house was unfortunately built with 2x8 floor joists over a 12' span between the basement wall and the steel beam. The result is a bouncy floor when people are walking. I've read several sources about how to sister the joists to make them more rigid (including posts on this site), but none seem to agree. Many mention sistering them with another standard 2x8, but other say to sister two layers of 3/4" plywood ripped to 7 1/4" on it's long dimension, with the cuts offset between the two layers. All say to use screws and liberal amounts of glue. Do you have any suggestions from your experience? I'd like to avoid losing 2" more of my basement ceiling height by sistering them with 2x10s and sistering them with LVL is probably prohibitively expensive. When I would perform this change, I would also probably replace the cross blocking with solid blocking to get even more stability.

TJ_in_IL 12-03-2010 08:27 AM

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.

hyunelan2 12-03-2010 09:01 AM

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.

Octopus 12-03-2010 10:47 AM

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

mrgins 12-03-2010 12:24 PM

I'd go with plywood. Easier to install.

Daniel Holzman 12-03-2010 01:05 PM

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.

mrgins 12-03-2010 07:21 PM

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.

Daniel Holzman 12-03-2010 09:19 PM

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.

mrgins 12-04-2010 03:35 PM

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.

SimpleFix 12-06-2010 09:17 AM

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.

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.


artmark 12-08-2010 10:46 AM

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?

Jackofall1 12-08-2010 11:21 AM

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???

Bird Doo Head 12-08-2010 04:15 PM

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!

Aggie67 12-08-2010 04:56 PM


Originally Posted by Jackofall1 (Post 547947)
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:





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.

Tigerloose 12-08-2010 06:52 PM

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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