How do I strengthen my floor joists? [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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05-30-2006, 07:42 PM
Really cool forum. I've gotten lots of great information here. I'm planning on installing 18" travertine but deflection rates only L480. I have 2x8's, 16"oc, 10' spans mostly. The joists are exposed underneath in my unfinished basement. Can I just sister some 2x6's at the middle of the spans? I thought I would screw them even with the bottom of the 2x8's. I'd rather not, although I could put up some more supports from the floor if I had to. Any advice is appreciated.

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05-30-2006, 07:56 PM
I don't know exactly, but the "Deflecto" implies that if you use 2x8 to sister, you'd be good...not sure if a 2x6 would be enough, maybe. Use both glue and nails or screws to attach the sisters.

05-30-2006, 08:34 PM
Lotsa ways to skin this cat:

1. Sister some more 2x8's.

2. Add a sheet of inch and an eighth Sturdi-Floor which is t&g. This stuff is so awesomely rigid.

3. Head off the joists every 4 feet or so, with a perpendicular 2x8 which ties the joists together.

Combinations of the above.

05-31-2006, 09:19 PM
If I sister the joists with 2x 8's, do I have to place them the length of the span? Some joists I can do this, but with others it's not as practical with existing ductwork, electrical runs, etc. Does the new 2x 8 need to be supported? I'm guessing no. The option for sturdi floor t&g won't work, because I don't have the height, and because I'm not sure that will help with my deflection problem. But what about this other option, heading off the joists with a 2x8. That sounds good. Will that work? THANKS for any reply!
There are more questions where those came from, lots to do.

05-31-2006, 10:35 PM
Andy, you said something above in response to Scotter's post that might become the show-stopper. For stone tile (any natural stone incl. travertine) you need a two layer plywood sub-floor, 1 1/4" thick. Can you do that? If not, probably best to begin thinking about a travertine porcelain look-a-like. They do exist.

Now then, that plywood sub-floor requirement is above and beyond the L/720 deflection requirements for the joist members. As far as yours are concerned you don't need too much more beef to get you up to the L/720 mark. I'd question that adding blocking every 4' or so will get you to L/720 for a uniform distributed load. Can you sister a 2x6 to the bottom of every joist for the middle 2/3 rds of the span, let's call it 6.5 feet? Or alternatively, can you reduce the span to 8 feet by framing in a partition in the basement creating a room perhaps for storage or whatever?

06-01-2006, 07:25 PM
I have 1/8 planking on the joists. I was going to use 1/2" or 5/8" ply on top of that with ditra. As far as strengthening the joists, I can get to them all if I replace some drywall. Will 2'x6's sistered 6.5 feet minimum get me past L720? I can sister 2x8's in some the length of the span. I have this night mare about my floor falling in. I might be able to reduce the span with a wall, that's a good idea too. Thanks for the help!

06-01-2006, 10:04 PM
My brother in-law is an engineer and I recently had this problem with an install, he suggested installing 2x4 laid flat and glued and screwed to the bottom of the existing joists, said it essentially doubled the 2x8's. Was an easy install and no problems. As all joist bays are not wide open this option made it easy.

06-02-2006, 08:49 AM
I like the sound of the 2x4 or 2x8 across the bottom of the joists. Does this work? I can't figure how it would strengthen the joists. By transferring stress? Every 4 feet, eh? I think I'll use a combination of these techniques like Scooter suggested. Thanks for the advice, please offer more if anyone has it.

06-02-2006, 09:12 AM
Andy, if you can reduce the span with a wall, that's the best approach in terms of work and effect.

06-02-2006, 09:50 AM
Hi Andy
If you apply your 2X8 for the full span of the joists, so that they are supported at each end that will definitely work. What that does is give you the same results as 2x8's spaced 8" o.c. Also adding a wall below to add support will work well as it is reducing your span.


06-02-2006, 03:33 PM
Unless you have expansive soils like we do in Colorado, then you have to float the walls and that will give you no help at all. Not sure they have that problem in the RED state.

06-03-2006, 10:41 AM
We our in the final stages of house design and engineering. The house is a day light basement/ walkout basement. The main floor will have travertine in the kitchen/hallways/bath and great room. The floor joists are 14" TJI/2100 on 24" centers. The longest one is 17'. Will this support the stone floor? I tried the calculator and it seems more for solid 2x?
The flooring calls for 1 1/8" sheathing.



06-03-2006, 01:11 PM
Fred --

The magical mystical deflecto only works for solid lumber joists. For engineered joists you either need to talk to the manufacturer or look in the liberry, where data for many of them are posted.

And as it sounds like you are at the beginning of your own project, you should have your own thread. Maybe one of the moderators could split this post off to one for you.

06-03-2006, 01:16 PM
Tell the architect that you want natural stone flooring, and where you want it. My guess is that the 24" on center may be a problem, even with that thickness of subflooring. Also note that stone installations normally require two layers, offset so you don't get lifting of the edges from flex. If you are trying to obtain no transitional height differences between rooms with different materials, then they can engineer the trusses to be installed at different heights to account for that. Stone needs a minimum of L/720 - they will know this. They normally build for L/480 or so which is okay for ceramic, but not natural stone. Neither really likes the 24" on-center.

06-03-2006, 07:47 PM
Okay, I'll get after this. Not sure exactly how it will work out for sure. I'll let you know when I get to the sub floor. Is there some documentation or literature Mike, to help me understand how a 2x4 glued and screwed across the bottom of the joists will DOUBLE the joist strength? This sounds awesome if it's true. Of course I only need about 35 percent more strength.
I did read about someone installing plywood over the bottom of the joists and I know cross members and blocking will help. There is some good stuff in the liberry too and I found and several good articles from JLC. THANKS

06-03-2006, 08:42 PM
Andy ---

I'm no structural engineer, but I have read dozens of threads involving reducing the deflection of joist system, and the addition of 2 x 4s has never been mentioned before. So I'm very skeptical.

The usual suggestions are to either sister the joists, add new full-length joists between the existing joists, or adding either a structural wall or beam below and perpendicular to the joists to reduce the span.

06-04-2006, 08:35 AM
Hi Andy,
I'm no structural engineer either but I haven't heard of a 2x4 under the joist as a method of deflection reduction.

I had to add major sistering on some my 2x10s especially the ones that span 14 feet. In my case I added 3/4" plywood double thickness to both sides of each joist. It was a lot of work and I used alot of PL400 and nails. A power nailer was a god send for this job.

If you do more research on the net you'll find that most sistering projects use lumber that is glued and bolted to the main joist. I think my combo of plywood, liberal use of PL400, and many, many nails should be strong enough.

Note: Since my plywood was only 8' long, I staggered the seams between each layer and on each side.

Here's a pic.

Good Luck

06-04-2006, 10:35 PM
Jeff and Andy thanks for the fast reply's

When you saw I need two layers are you referring to a plywood sub floor with a backer board/ cement board below the travertine?



06-05-2006, 05:19 PM
The TCA guidelines for stone tile requires two layers of plywood, then a decoupling layer (something like DItra or a cbu).

06-05-2006, 06:00 PM
I think Andy was referring to the plywood that he used to sister his joists. He glued and screwed a layer of 3/4" ply to both sides of each joist.

06-21-2006, 12:49 AM

I've been reading this forum and several others trying to educate myself on sistering joists. I have a 5 1/2' x 6 1/2' stair riser where I'm replacing the floor tile. In a moment of poor judgment I decided to use a 14 pound sledge to demolish the existing floor tile. Unfortunately, I split 3 out of 6 of the 6 1/2' long 2" x 8" joists in the process.

After reading many posts I decided to get the job done and sister the split joists using a single 2" x 8" sister per split joist, 3 evenly spaced 5/16" lag bolts with concentric washers and construction adhesive.

The new 2" x 8" joist was cut to 60" so it would fit and is not the same length as the original joist (i.e., it's not attached at both ends). Also, I only bolted the sisters together and did not use nails to join. Should I add nails to strengthen?

Last question, is it necessary to nail/screw the subfloor to the newly sistered joist? The subfloor seems very solid at the joists without much deflection. I determined this by asking my 200 lb brother to bounce up and down on the joist (not very scientific but it seems a much improved).

Anybody mind giving me a few pointers. This is something I've never done before so comments are most welcome. Thanks Scott