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Rusty 01-05-2004 03:39 PM

How to strengthen joists
I want to install a tile floor, but I got a thumbs down on the deflection calculator. My joists are 15' long, 2x10, 16"OC. What is the best method to strengthen the floor and how do I do it? I can access the joists(I'll just have to remove the insulation between them first)

Steven Hauser 01-05-2004 04:34 PM

Hi there Rusty,

Without seeing the floor I would suggest putting a post to split the 15' span down to 7.5 feet. This would be good for all ceramics and most stones.

The other idea is to double up and "sister your joists". This would be adding a 2x10 along the span of the joists.

This would be good for 12" ceramic or less but not stone.

The engineers will be along soon enough to correct any confusion I create.

Best wishes, Steven

bbcamp 01-05-2004 04:38 PM

Hi, Rusty!

If you play around with the deflectolator, you'll find that reducing you span by only 1 foot will give you a "thumbs up" for tile. So, can you use a closet in the lower room? Addiing a 2 foot deep closet inder the area to be tiled will strenghten your floor acceptably. You will need to frame the wall like a load bearing wall.

You can add a support beam down the center of the room. This is good if the lower room is a unfinished space, and you don't mind the support posts every 6 feet or so. Doubled 2x10s supported on 6 or 7 foot centers work well. You may need to consider the floor. If slab on grade, you're fine, but if dirt, you'll have to dig a footer.

Otherwise, adding a 2x10, or even a 2x8, sister will do the job, too. This needs to be done for each joist under the tiled area. The downside is handling those long joists, and removing any interfering utility, such as wiring, plumbing, or HVAC. Your floor will not be as stiff as the beam option, but will be equal to the closet option.

Take a look in your basement and let us know what you plan to do.


Sonnie Layne 01-05-2004 04:50 PM

I'd like to add a footnote to the sistering and/or the additional stringer/beam concept. They are both good options, but pull yourself a laser line or simple string line from end to end along those joists to make sure they're straight. Some adjustment may be necessary. We build our houses out of pine down here and it's not uncommon to find irregularities that can be corrected whilst beefing things up.

P.S. If ya gotta shim things up, use steel plating instead of cedar shims.

Rusty 01-05-2004 04:53 PM

Thank you very much for your attention and information. I need to add some more info for clarification, as I don't think I can do any of the suggestions. The floor area in question is 15' X 30'. It is a kitchen with my garage underneath. I can't sister the joists due to a tremendous amount of wires, cables, pipes, and HVAC running through and under the joists. The beam/support posts is also a problem as the posts would be in the middle of my garage. Is there another solution, or am I doomed to use fake flooring?

Steven Hauser 01-05-2004 04:58 PM

:shake: I don't think ceramic is going in here.

Sorry Steven

Sonnie Layne 01-05-2004 04:59 PM

Rusty. Is it a double car garage so that a one post could be arranged in the center? Darned, where is telepathy when you need it?? ;)

bbcamp 01-05-2004 05:12 PM

I have yet to see a garage that couldn't use a little storage area along the wall. :D

Rusty 01-05-2004 05:17 PM

If I don't have other options, it sounds like things aren't looking too good for tile. I don't think I could handle a post in the middle of the garage. I might have to settle for the fake tile laminate stuff. I just wish I hadn't wasted the $50 I paid to have the '70's show linoleum tested for asbestos because I was gonna pull it up for the tile installation. If I use the fake stuff, I'll just go over top.

bbcamp 01-05-2004 05:24 PM

If that's the last $50 you ever waste on your house, you're a lucky guy. :D

archy 01-05-2004 07:07 PM

You could add a steel beam or a microlam spaning from wall to wall. You will need to add posts or columns in the wall to support the beam. You may also need to add footings since you are now resupporting the floor. Another option is to add a "flange" to the bottom of the existing joists and add a plywood membrane to the flange creating a "kind of" box beam to stiffen the floor. A structural engineer for a couple of hours might not be a bad investment.

Mike2 01-06-2004 10:30 PM

What do you guys think about that method Robert Randal wrote up in the Journal of Light Construction, maybe 8 years ago or so? The one where he would glue and screw a 1" x 4" (or maybe 6") the full length of the underside of the each joist, maybe even pre-stress it mid-span. Claimed if I remember right that method would effectively reduce deflection equivalent to increasing to next larger lumber size (i.e. in this case taking a 2" x 10" to a 2" x 12").

I only remember the article, never did anything with it, nor have I ever seen any test data to support his claim.

bbcamp 01-07-2004 07:26 AM

Mike, for that to work, you would have to pre-stress the 1x4. This would mean that the floor has to be jacked up in the center, and the 1x4 glued and screwed and allowed to dry before the jacks can be removed. Then the glue and screws cannot creep even a little bit or else the whole exercise is useless.

On the other hand, this floor does not need much help...

Rusty, can you see any grade stamps on you joists? Maybe I could sharpen my pencil and do another calculation.

Billy 01-07-2004 10:13 AM

Can you bolt angle iron to the bottom of the joists, as shown here in the bottom photo (from Rob Z):


Sonnie Layne 01-07-2004 10:29 AM

Billy beat me to it, although I think Rob over-did it a bit.

Bob? Deflection capacity of angle iron? Or even flat steel.

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