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How should I reinforce this subfloor?

30K views 35 replies 6 participants last post by  NewHomeDIYGuy 
#1 · (Edited)
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!
 

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#4 ·
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.

Jaz
 
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#14 ·
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.
 
#19 ·
Deflection is inversely proportional to the moment of inertia. In other words, if you double the moment of inertia, you halve the deflection, and double the L/D value. Correctly sistering an identical sized joist to the original joist will double the L/D value. If you use a smaller board, you will not double the moment of inertia, so you will not double the L/D value.

You can compute the improvement this way. The moment of inertia for a rectangular solid lumber board is b*d^3/12, where b is the width, d is the depth, and ^3 means cubed. So a 1 inch wide, 8 inch deep beam has a moment of inertia of 1*8*8*8/12 = 42.7 in^4. A 1x6 beam has a moment of inertia of 1*6*6*6/12 = 18 in^4. The moments are additive, so the total moment of the combined system is about 61 in^4, representing a net improvement of about 42 percent. You perform this calculation based on the actual dimensions of the lumber you are using. It is not really necessary to sister the full length of the original joist, as the maximum deflection occurs in the center, as long as you position the sistered joist over the center, and get adequate lap on each side (typically 2 feet per side is minimum), you are OK. You need an adequate nailing pattern, typically 16d every six inches, staggered, is OK. This analysis assumes you are using the same modulus lumber (same species and grade), if not the calculation gets more complicated.
 
#21 ·
It is not really necessary to sister the full length of the original joist, as the maximum deflection occurs in the center, as long as you position the sistered joist over the center, and get adequate lap on each side (typically 2 feet per side is minimum), you are OK. You need an adequate nailing pattern, typically 16d every six inches, staggered, is OK. This analysis assumes you are using the same modulus lumber (same species and grade), if not the calculation gets more complicated.
Thanks again Dan! That's great to know, so I guess the wise thing is to stick with sistering 2x8's so I'll get about half the deflection. The only other question I have for you is what exactly do you mean by "lap" on each side, is that another word for overlap from the center, so minimum is 2' on each side from the middle? In my case, with a 10' span, that means just a 4' beam centered on the span? I realize you said that's the minimum, but is there a point of diminishing returns where it's not worth it? If I went to within say 18" from each end point (so a 7' long 2x8), would that provide still a near halving of the deflection? Thanks!
 
#22 ·
So my issue is obviously the joist deflection lengthwise itself, and the only fix for that is sistering the joist or running another joist..
I understand the problem now, and I think the best solution is to sister the two joists on the sides of the bathroom (underneath). While adding another joist down the middle would work, it seems like sistering is the best option, as it'll add strength
It is not really necessary to sister the full length of the original joist,
Gee, I think someone already said all of those things several days ago.

Jaz
 
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#5 · (Edited)
JazMan,

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:
 
#6 ·
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.

Jaz
 
#7 · (Edited)
Jazman,

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!
 
#9 ·
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.
 
#11 ·
Snav,

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.

Bud,

No I don't have a thread over there. I found the link to the calculator from a thread on this forum..
 
#12 ·
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.
 
#13 ·
Snav,

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.
 
#18 · (Edited)
I know - it does and can get very confusing.

I see your current situation as a severe sagging problem waiting to happen. While your existing subfloor is solid - take care of it now (meaning - you don't have water damage or warping)

However - your existing area isn't supported at all as it is - the 2x4's that are there are only supporting pipes - not the floor. Quite a sturdy pipe-support, though :)

I'll just tell you my experience and what I actually had to deal with - which is why I suggest you do that extra work:

My house is crap - was crap - literally falling in on itself when we bought it. In our master bathroom we had severe water damage. When I finally got around to fixing it (gutted down to the beams and studs) I found that the previous own cut through one of the floor joists in two places to do plumbing (the joist that ran under the head of the tub, toilet, and sink) - leaving most of the 14' joist attached to the subfloor and just 'hanging' there.

The floor - being unsupported in that area (about 30" wide or so) began to sag - the more it sagged the more pressure was put on the plumbing causing more leaks. ETC.

If that support was left fully in tact the span of flooring there would have been reasonably supported - it wouldn't have sagged and it's likely that some other problems would not have happened - which would have saved me time and money.

Now - we had other issues going on, too - in the end I had to feed in all new beams under the master bath, level the floor off, and lay out all new sub/under/rock so I could do porcelain tiles.

Hell of a lot of work? Yep

Worth it - absolutely. It is done well, it is done right - and it won't be failing anytime soon meaning that the obscene amount of time I spent laying tile and al lthe other work on top of that won't be in vain and for nothing.

Now - my first bathroom: I did not address this problem (which was pretty much the same thing) - and I regretted it 3 years later when I had to peel up the tile after they were 'coming up' (the thinset was separating from the underlayment and the tile) and reinforce the structural members like I did in the master bath. . . nothing SUCKS worse that *repeating and repairing* work you've already busted your ass to do.

If you take the time to do it now - trust me - you won't HAVE to fuss with it later on.

If you STILL don't want to do it (which I understand - I really do) - then I would strongly suggest you hang your 'ladder rung' like support joists vertically like they're suppose to be - don't lay them flat. At least then they will give solid and adequate strength and support.
 
#15 ·
Daniel,

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..
 
#17 ·
Ok.. after looking at the joists, it looks like I have room to sister the joists.. :thumbup: One sister will have to go on the other side of the joist where the plumbing won't get in the way, but that should work a-ok. Now the next question is.. what would be a good size so I get the deflection reduction I need? Is there a calculator that factors in sistered joists or a rough number to go by?

My spans are 10' 2x8's, so if I ran the sisters say 8' (1' shorter on each end) and used 2x8's, what would that put the deflection number at. Would 8' sistered 2x6's provide enough support as well? Only reason I'd consider a 2x6 is because of the tight space. Thanks a lot for everyone's help!
 
#20 · (Edited)
Thanks Snav.. I understand the problem now, and I think the best solution is to sister the two joists on the sides of the bathroom (underneath). While adding another joist down the middle would work, it seems like sistering is the best option, as it'll add strength AND be a "good" way of doing it. Fortunately, the joists don't seem to be sagging, so if I sister them now, hopefully that should alleviate any problems now.

I'm all for doing things "right," but I was concerned that I might have been "overdoing" it, based on different recommendations I've gotten. I was confusing joist deflection with subfloor deflection when I used that online calculator. I never thought about deflection along the length of the joists, just in between.. :wallbash: I have a friend that's a GC, and it seems like many GC's don't even factor in joist deflection, but just go by the subfloor strength. Hmm.. not very comforting, but that's why I came here in the first place. I'm new to house stuff, but I'm an avid DIYer, as I really enjoy learning about things and knowing how things work.
 
#23 ·
Jazman,

My apologies. I think in my searching for different answers and searching online other places I started confusing the two different types of deflection and missed your clear answer. If I sister the joists within 18" or even 2' from the end of each end of the 10' span, would that provide sufficient support?
 
#24 ·
We still don't know your exact numbers since we don't know the species or grade. However if I use the most commonly used species and grade, Southern Pine - Douglas Fir - Spruce-Pine-Fir grade #2, I find you are from 6" to 20" over spanned to meet minimums. So, you're pretty close. This is based on 50/20 live load/dead load specs I use for ceramic installations.

BTW, you may be thinking "how did this house pass inspection of the building inspector?"

The floor as is meets minimums based on 40/10 load criteria for regular floor coverings. We're just not into doing things the worst allowed, and needs to be stiffer for tiles.

So yes, I believe if you sister the middle 2/3 of the span, you'll be good. I have no way to tell you what the new deflection number will be though. Glue, screw and then bolt them well.

If you're interested you can buy or make a gauge to conduct your own test. Instructions available on request. :laughing:

Jaz
 
#25 ·
Thanks again Jazman. I posted in a previous post that I couldn't find any info regarding the species of the wood joists.. I found a stamp on one of the joists, but it's completely illegible, so I can't even run it through google.. Either way, it sounds like sistering is the way to go, and that should provide enough support (regardless of the original joist species).

My next question was going to be "how do you sister joists?" :whistling2: I apologize for the never ending questions.. Is gluing the joists together on top of screwing/bolting them together a good idea? I'm just asking because it's always been my understanding that different materials expand at different rates, but perhaps if the expansion rates are very similar (both wood and perhaps pine in this case) I should be a-ok?

When you say "bolting" the joists together, do you mean lagbolts from one joist into the other (like the kind that are used for mounting a flat screen tv mount into wood studs), or do you mean drilling a hole and using a bolt with washers and a nut on the other side?
 
#27 ·
Yes drill holes and use 4" lag bolts with washers and nuts. The screws are fine but 1/2" diameter bolts really do the job.

I also think it's a good idea to jack up the joists about 1/8" before laminating and fastening so both pieces of lumber share the existing load when the jack is removed.

Jaz
 
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