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Old 11-29-2011, 06:41 PM   #16
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How should I reinforce this subfloor?


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The minimum allowable for tile is typically 240 for small format tile,
I'll be sitting here quietly with my hand raised in the air in hopes of being called on to be the first to take issue with that notion.

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Old 11-29-2011, 07:00 PM   #17
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How should I reinforce this subfloor?


Ok.. after looking at the joists, it looks like I have room to sister the joists.. 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!
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Old 11-29-2011, 08:02 PM   #18
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How should I reinforce this subfloor?


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Originally Posted by NewHomeDIYGuy View Post
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.
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.
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Old 11-29-2011, 08:04 PM   #19
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How should I reinforce this subfloor?


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.
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Old 11-29-2011, 08:14 PM   #20
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How should I reinforce this subfloor?


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.. 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.

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Old 11-29-2011, 08:24 PM   #21
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How should I reinforce this subfloor?


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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!
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Old 11-30-2011, 01:09 PM   #22
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How should I reinforce this subfloor?


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So my issue is obviously the joist deflection lengthwise itself, and the only fix for that is sistering the joist or running another joist..
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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
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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.

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Old 11-30-2011, 01:16 PM   #23
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How should I reinforce this subfloor?


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?
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Old 11-30-2011, 01:57 PM   #24
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How should I reinforce this subfloor?


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.

Jaz
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Old 11-30-2011, 02:05 PM   #25
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How should I reinforce this subfloor?


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?" 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?
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Old 11-30-2011, 02:24 PM   #26
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Nuts and bolts.
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Old 11-30-2011, 09:17 PM   #27
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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.

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Old 11-30-2011, 10:05 PM   #28
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How should I reinforce this subfloor?


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Yes drill holes and use 4" lag bolts with washers and nuts.
Carriage bolts.
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Old 11-30-2011, 10:15 PM   #29
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Yes of course I choose the wrong name for the right bolts, Bud is right. It would be kinda hard to screw nuts onto lag bolts.

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Old 11-30-2011, 10:19 PM   #30
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Hey!!! There's our path to fame and fortune. Invent a hex nut for a lag bolt.

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