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Unread 12-30-2003, 06:30 PM   #1
DavidLila
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Subfloor advice

Help!

I would like to tile by small bathroom (it is 5 x 6 feet not including the tub). Currently the floor consists of 1"x4" planks running on the diagonally over the joists (which I think are 16" OC but are not accessable unless I rip out the basement ceiling). Anyway, over the diagonal planks are 3/4"x3.5" planks running perpendicular to the joists. Thus, in total I have 1.75" on wood on top the joists. Can I lay 1/4" backer board on top of this and then tile with 12 x 12 porcelin tiles? Or do I have to rip up the subfloor and replace it with 5/8" exterior plywood? (I can't add plywood over this as it would raise the profile of the floor way to high).


One additional problem is that the planks run under the tub which I would like to avoid moving.

One more piece of information: the house was built in 1908 which may help explain the construction.

Thanks. David
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Unread 12-30-2003, 07:02 PM   #2
John Bridge
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Hi David and Lila, Welcome to the forums.

No.



I think you'll have to remove the top layer and replace it with plywood. The strip flooring is just too unstable for a tile installation. You don't have to go under the tub.

Over the new plywood you could use Ditra by Schluter Systems, which consumes an eighth inch in height. Or you can use 1/4 in. cement backer board.
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Unread 12-30-2003, 09:45 PM   #3
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Subfloor - more information

John:

Thanks for the rapid reply!

I went home and measured the joists to calculate the deflection (is that the correct term?). The joists are 16" on the center but are only 2x6" (1.5 x 5.5") and 12 feet long. Your calculator says this won't hold tile. Am I in trouble?

Anyway, despite 3 people in the flooring department at Home Depot telling me just to put 1/4" backer board over the planks I feel much better with your advice and will rip out the planks and replace them with 5/8" exterior grade plywood. (That is, if the joist problem mentioned above doesn't sink the whole project).

Thanks. David
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Unread 12-30-2003, 10:02 PM   #4
cx
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Welcome aboard, David.

Very bad news if you have correctly measured that floor structure. A 12 foot span with 2x6 joists is more appropriate for a residential ceiling structure. That floor should feel awfully bouncy with even a moderate jump test.

You sure you don't have some mid-span support under there somewhere? If not, you're gonna hafta do something to provide more support. What's below this floor structure?
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Unread 12-31-2003, 08:27 AM   #5
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Learned something in life,You can't believe every thing some of them people at H.D.tell's,haha JMO> Mark
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Unread 12-31-2003, 11:50 AM   #6
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I think there must be a beam under there somewhere. A 12-foot run of 2x6s would feel like a trampoline when you walked across it.
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Unread 01-02-2004, 02:14 PM   #7
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Subfloor

John and CX:

Thanks for your replies.

Underneath the bathroom floor is a finished basement which was added, I believe, sometime in the 70's.

Anyway, at this point I will have to do some more exploring in the crawlspace to look for beams. I suspect you're correct about the beam for two reasons. First, the floor in the bathroom has no noticeable bounce to it (even with a good jump). Second, on the other side of the house there is the same measly 2x6 joist construction but it is supported by (small) beams and supports running perpendicular every 6 or so feet. Anyway, if you can bear with me for a couple days more I do some digging around and get back to you tomorrow.

Thanks.

David
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Unread 01-02-2004, 06:30 PM   #8
John Bridge
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You bet. We've got the time.
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Unread 01-03-2004, 09:26 AM   #9
rat4spd
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Quote:
Originally posted by John Bridge
I think there must be a beam under there somewhere. A 12-foot run of 2x6s would feel like a trampoline when you walked across it.
.....I can attest to that. I have 2x6 oak 24"OC with 12' spans....very bouncy. By the way, getting hold of 2x6 douglas fir is a pain.
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Unread 01-03-2004, 01:24 PM   #10
bbcamp
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Douglas fir isn't the only option, you know. What can you get?
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Unread 01-03-2004, 02:00 PM   #11
rat4spd
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Not to hijack the thread, but what is a viable option to sister them. I haven't checked the small local yards yet, just the boxes. I threw this around with you guys a while back. It just hasn't been a priority because I'm not setting tile.

My apologies to the poster.

Dave Ames
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Unread 01-03-2004, 02:19 PM   #12
bbcamp
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Southern Yellow Pine is as good as Doug. Fir.

Really, almost any wood sold for framing is about equal.
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Unread 01-03-2004, 07:00 PM   #13
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"Southern Yellow Pine is as good as Doug. Fir.

Really, almost any wood sold for framing is about equal."

Not true. There are different ratings for structural framing. Doug fir is rated for longer spans depending on the rating (i.e. select structural vs stud grade). A lumber yard will have different graded lumber but the big box stores typically do not. I would use lvl's (paralam or microlam) to sister the joists
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Unread 01-04-2004, 09:08 AM   #14
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Archy, you are correct, of course. However, I'm usually interested in stiffness only. SYP and DF are about the same, and most other framing lumber species are within 20%. There are many other wood species that are not good at all for framing, but are stiffer, and a bunch that are stronger, but not as stiff. For sistering a joist that doesn't meet L/360 just for tile purposes, almost any readily available framing-grade lumber will get you good enough results. I just wanted to point out that there is more than one way to skin a cat.
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Unread 01-05-2004, 12:15 PM   #15
DavidLila
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More Subfloor details

Thanks for everyone's interest in this topic. Your advice is great.

In terms of the joists, as best as I can determine the 2x6 joists are supported at about 8 feet out by one of the finished basement walls. If this is true (and I believe it is) then the defection is o.k. for tiles.

However, just to be safe, and because I noticed a dip in the center joist in the bathroom, I now plan to rip out the old subfoor planks and sister all the joists up as much as possible. I believe I will be able to fit in 6 to 7 foot sister joists of 2x6 material. Unfortunately, I will have to cut out some notches for piping and wiring that runs through the current joists (and thus probably accounts for some of the sagging in the middle of the bathroom floor). As I understand, it is a bad idea to cut notches or holes in joists as it weakens then and can lead to sagging.

In summary, if I can put 2x6 sister joists in underneath the bathroom area (which is 6 feet long by 5 feet wide), do you think this will add enough stiffness to support tile?

If this is possible, then I will install 3/4" exterior plywood over the new sister joists, followed by 1/4" backer board.

One final question however. If I do get to the point of installing the plywood and backerboard as described above, when I lay down the thinset over the plywood do the 1/8" gaps between the plywood get filled in with thinset or not?

Thanks again.
David.
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