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NewHomeDIYGuy 11-28-2011 10:34 AM

How should I reinforce this subfloor?
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I'm looking to reinforce the subfloor for a bathroom that is 31" wide by 6' long. I'm going to use porcelain tile, and the current subfloor is 3/4" OSB, joists are 2x8's with 24" OC spacing w/ 10' spans.

I plugged the info into a deflection calculator and it came back with this:
"This translates to a deflection of L / 245. Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Sheet Vinyl or wood."

I snapped a few pictures underneath of the subfloor from the basement, and am not sure what the "correct" way to reinforce the subfloor is, all I know is that it needs some reinforcing. I'd love if I could just add 2x4's underneath the subfloor and between the joists, and screw the existing subfloor into the 2x4 braces, and that would provide enough support.

Ok, here are a couple pictures of what I mean. For whatever reason, the previous owner installed some 2x4's between the joists on the bottom of the joists, and the insulation (I'm guessing as sound proofing) was put there by the previous owner as well.

First picture is looking down the joists, and second is perpendicular to the joists. The bathroom runs lengthwise with the joists. The toilet and vanity will be at opposite ends of the room, and you can see the drain for the toilet in the labeled picture. I'm hoping that if I install some 2x4's underneath the subfloor perpendicular to the joists the subfloor will be reinforced enough. I was thinking that perhaps if the subfloor needs more support/braces need to be stronger, I could put a 2x4 flat against the top and bottom of the joist, and then connect them with 2x4's in a couple spots like an I shape? Unfortunately I can't put 2x4's or 2x6's vertically (which would provide more support) because the plumbing is in the way.

The other question is whether I would need to add a layer of plywood to the existing 3/4" OSB. Thanks!

Sephora 11-28-2011 11:36 AM

If you have already started a thred on this then you should refer to the original least that is how things seems to go around here. FYI

NewHomeDIYGuy 11-28-2011 11:41 AM

Ok.. my apologies if I wasn't supposed to start another thread. I created another thread before with a very different topic in the flooring section, which probably led people to not respond, so I thought a new topic would be a good idea.. :icon_confused:

JazMan 11-28-2011 12:59 PM

Hi NewHomeDIYGuy,

First of all, can you give us all the specs for your joists figures? You said 2x8 at 24" oc spanning 10 ft. Did you measure the span from face to face of the supports or the actual length of the joists? How about the species and grade of the joists? Species and grade makes a BIG difference on the results.

It also appears you are getting confused with the deflection rating you got at JB"s site of L245. That rating has nothing to do with your subfloor sheathing rating. only the deflection of the joists. The subfloor deflection is something altogether different. BTW, although it's scary to me, a single 3/4" OSB at 24" oc does meet the minimums as a base for ceramic tile installations. You have to add a membrane or concrete backer of course.


NewHomeDIYGuy 11-28-2011 01:07 PM


Wow, I guess I got things mixed up when I was searching online and through the forums. Thanks a lot for that clarification! My plan is/was to lay down 1/4" durock w/ thinset on the OSB, and then of course thinset and the porcelain tile, but after doing some searching, I thought I didn't meet the minimum strength for the subfloor.. I guess I was looking at the wrong thing..

Regarding the joists.. I measured the distance between the large steel support beams where the 2x8 joists rest. The span between the two beams is 10'. I didn't look at the species and grade of the wood for the joists, but I'll do that when I get home.

Regarding the subfloor.. that's good to know that 3/4" OSB at least meets the minimum reqs for ceramic tile. The subfloor actually feels quite solid (perhaps since it's such a small room), but I really like to do things right the first time, which is why I got concerned that the subfloor strength might not be enough after doing some searching.

I guess at this point adding a couple 2x4's between the joists against the bottom of the subfloor and screwing the subfloor into the 2x4's certainly couldn't hurt..? At least if I did that in a couple spots for the small room, it should provide a little more rigidity for the floor, and not require me to lay down another layer of plywood.

Perhaps I was a little overworried about the subfloor strength. :thumbup:

JazMan 11-28-2011 02:36 PM

There is no 1/4" Durock, you must mean 5/16". Durock's instructions are among the worst ever. They don't wanna recommend what the max joists spacing is safe.

You can stiffen the joists several ways.
1. Shorten the span by building a supporting wall underneath, or with a beam.
2. Sister the joists. The sisters do not have to be the same size lumber.
3. Glue & fasten lumber to the bottom narrow edge of the joists.

To help with the between-the-joists deflection, install blocking as you stated and or apply more plywood under the subfloor. Or of course you can install more ply over the OSB, which is what most people do.

The between-the-joists deflection is more critical than the joists's deflection as the curvature is greater.

I'll wait for the species and grade info, hope you find it.


NewHomeDIYGuy 11-28-2011 02:44 PM


My mistake.. I got 1/4" wonderboard. My understanding is that wonderboard and durock are very similar, just that durock is the name brand vs. the "generic" per say? The local big box store didn't have 1/4" durock, so I picked up wonderboard instead. I originally got 1/2" durock, but it seems like 1/4" should match up better with the existing 3/4" hardwood outside of the bathroom after the thinset layers and tile. I considered Hardiboard, but I've never used it before, and am familiar w/ durock/wonderboard, so wanted to stick w/ what I knew/have used before..

That's good to know regarding the subfloor reinforcing. Worst case then hopefully I won't have to add another layer of plywood or OSB, and I can just add some blocking underneath the subfloor to provide more support.

I'll still check and see if I can tell what kind of joists I have when I get home and post here. Thanks again!

NewHomeDIYGuy 11-29-2011 08:30 AM


No luck on the wood species. I found some stamping on the wood but they're illegible and googling turned up no results. I guess I will proceed by just adding bracing between the joists with 2x4's. Thanks for your help.

Snav 11-29-2011 08:46 AM

Assessing from the pictures: there's inadequate support under your floor - in fact - I'm surprised that's 'acceptable' - you said the bathroom is 31" wide . . .that sewage line is, what - 5" wide . . . so that means your beams are spaced over 20" apart. Their lateral supports - laying sideways. . .are an illusion of support.

Where's the adequate support? I know it's a tight spot - no tub or shower . . . but still - seems horribly unstable.

So what I would do - won't make you happy but it would please the dickens out of me:

Remove all existing plumbing.
Remove their 2x4 lateral supports.
Feed in a new additional beam - could be slightly off to the side of the existing location for the toilet flange.
Reset new pipes - with whatever means necessary.

There - full stability. . . .you can tile with whatever you want after that.

Bud Cline 11-29-2011 09:02 AM

After going to JB's to get your deflection values do you also have this same thread running over there? Only makes sense that you would.

NewHomeDIYGuy 11-29-2011 09:04 AM


Moving/removing the existing plumbing to run another joist seems like a whole helluva lot of work, and I wonder if it's really necessary. You said the lateral supports are an illusion of support for the joists? How is that so? They should provide SOME support and prevent deflection to some degree. Like I was saying above, I was thinking that perhaps I could put 2x4's against the subfloor and between the joists, and perhaps connecting the two 2x4's would provide more support. If the subfloor strength is an issue, I could certainly add a layer of plywood as well if need be.


No I don't have a thread over there. I found the link to the calculator from a thread on this forum..

Snav 11-29-2011 01:26 PM

You asked for opinions on the support-problem. I gave you my opinion and told you what I would actually do. Hell of a lot of work? Improving homes that have serious flaws is a hell of a lot of work - but it's hard work that's worth it.

NewHomeDIYGuy 11-29-2011 04:12 PM


Thanks for the info and your solution, I just wonder if it's truly necessary/worth the effort over laying an additional layer of plywood over top or bracing the subfloor. I've asked for opinions, and I just get wildly different ones from different folks which is why I'm having a hard time is all. Some say I should be fine just putting the durock and tiling over, and some say I need to reinforce the subfloor.

Daniel Holzman 11-29-2011 05:35 PM

I think you are confused about the different deflection criteria. So let me try and unsnaggle the issues.

1. Deflection of the joists occurs due to dead load and live load imposed by the weight of the tile, subfloor, and framing (the dead part), plus live load due to people walking on the floor, and movable items like furniture. Typically you run a deflection computation based on the total load on the floor, which is composed of the sum of the dead load and live load. The value used depends on the use of the room, typically a bathroom is considered living space and is assigned 40 psf live load, plus 10 psf dead load. You need to know the exact size of the joists, the clear span, the species and grade of the wood to determine the maximum deflection, which typically occurs at the center of the joist. You can use deflection tables to determine the deflection, then you divide the maximum deflection (usually given in inches) by the clear span (also use inches), which determines the L/D ratio (length divided by deflection). The minimum allowable for tile is typically 240 for small format tile, 360 for large format tile, and 720 for natural stone.

If your joists fail the deflection test, you cannot stiffen them by applying bridging between the joists, or using 2x4 cross bracing. Bridging reduces sideways movement, but does not increase the L/D ratio. Neither does use of backer board, and generally the plywood subfloor is ignored when computing the stiffness of the joists. The only effective way to stiffen the joists is to sister on another joist, reduce the span by installing an intermediate beam, install deeper or stiffer joists, or install additional lumber or steel onto each joist to increase its moment of inertia. By the way, stiffness and strength are related, but ARE NOT the same. Your joists may be strong enough to support the load without breaking, but can still undergo too much deflection to prevent tile cracking.

2. If the joists are stiff enough, you then check the stiffness of the subfloor. This is usually done by a simple chart that tells you for a given joist spacing how thick your subfloor needs to be. Check the Schluter website (they make Ditramat) for an excellent discussion about the required stiffness of the subfloor. Note that subfloor stiffness is controlled by the spacing between the joists, and the type of subfloor material you use, and has nothing to do with the stiffness of the joist itself. If the subfloor is inadequately stiff, you correct that by adding a layer of appropriate material, typically OSB or plywood. In rare cases, it may be necessary to install additional joists between existing joists to reduce the spacing, but this is rare.

NewHomeDIYGuy 11-29-2011 06:29 PM


Thanks for the thorough explanation, that makes it perfectly clear. So my issue is obviously the joist deflection lengthwise itself, and the only fix for that is sistering the joist or running another joist.. Without sistering the joist, I run the risk of too much deflection and ultimately some cracked tiles/grout down the road.. Great.. Perhaps then the better option just might be hardwood flooring, or I roll the dice without adding additional support for the joists.. Thanks for the clarification Daniel. I guess I have the hardest problem to fix..

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