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Wolverine
06-07-2001, 06:30 PM
<< I also posted this message on the infotile board >>

Forgive my terminology in advance. I am looking to install some tile (originally planned to do marble) in a bathroom that had been carpeted. I just inspected under the house. The house was built in 1958.

The substrate beams look to be about 1 3/4" X 7 1/2". There is about 15 1/2" average space between the beams, measured on center. The frame of the house is supported underneath by cinder block columns. The bathroom itself is in one corner of the house (in other words, one end of the support beams rests on, or is attached to, the concrete foundation right next to where the tile is going), and I'm looking to tile approximately a 6' X 12' room (minus a 3'X3' shower). About six feet from the foundation, pieces of wood, also approx. 1 3/4" X 7 1/2", have been inserted between the long joists. The subfloor, underneath the asbestos, appears to be at least 2 inches thick, and the pieces are in two layers, the top one of which has interlocking adjacent pieces -- I don't know the significance of that, but it is all I could tell looking around the air vent. From underneath the house, it appeared that the bottom layer was laid diagonally (only a portion was visible through insulation that I did not feel like tampering with to disturb the brown recluces. I have already covered up the asbestos.

Is the marble tile completely out of the question (the newlywed wife will be very disappointed)? Is ceramic tile on top of 1/4" wonderboard even an option?

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Bud Cline
06-07-2001, 06:54 PM
Wolverine,

What you describe sounds like a typical "pier and beam" with a double subfloor of that era. I'm not clear on the asbestos but actually I don't want to know.

Your double subfloor if that's what it is may be your saving grace here and you may be well on your way.

John will say you need a mud base this will give you more rigidity than cement board. I say; go for the cement board but if your transitions to other rooms will allow it I would use 1/2" cement board.

As far as the marble I have a big question. Does your floor bounce like a trampoline when you walk?

Wolverine
06-07-2001, 07:44 PM
There is a layer of vinyl applied directly to the substrate, that I am assuming contains asbestos adhesive because of the age of the house; I don't know for sure, but I didn't want to find out or fool with it so I covered it up with 1/4" luan plywood.

The floor does not "bounce like a trampoline" in that part of the house. In the living room (e.g. middle of the house, not so close to the foundation), the floor does seem to flex some, especially if I walk hard on my heels or jump), but there is none of that in the bathroom.

Thanks for the response. The wife will be disappointed if the marble doesn't get to go in (we've actually already bought it -- but it was on sale at HD and we can take it back). Not as sad as she'd be if the joists gave way, though! Unfortunately, I don't have "Idiot" coverage on my homeowner's insurance.








[Edited by Wolverine on 06-07-2001 at 09:49 PM]

LDavis
06-07-2001, 07:53 PM
Wolverine, the support structure you describe has more than enough "beef" to support a ceramic installation. I'll echo
Bud and suggest a cement backer unit nailed or screwed down over a bed of thinset. The 1/4" backerboard should be adequate if transition height differences matter.

A good idea to include a waterproofing membrane on top of the backerboard in a bathroom situation.

Wolverine
06-07-2001, 07:59 PM
Thanks, LDavis. Does it sound like enough "beef" to support the marble?

Bud Cline
06-07-2001, 10:01 PM
Wolverine,

Sounds like your ok, or you were until you mentioned the luan. The cement board needs to be installed with both thinset and mechanical fasteners.

The thinset may be a problem with the luan as the moisture from the thinset will delaminate the luan. Luan has prooven to be a poor flooring underlayment, even the "underlayment rated" luans have had problems. But I'll tell ya what, since you've already done it and that money is spent. If you securely screw the cement board with screws every six inches along the sides and not more than eight inches apart in each direction in the field, I think you'll be ok. Don't forget the thinset and don't cheap-out on the thinset cost either.

I don't think marble is very forgiving when a floor flexes. There are formulas for determining if your floor is suitable if you want to get into that. I'd still be a little concerned if that bathroom floor flexes noticeably.

Marble usually requires a special thinset (and usually white) because it will wick the grey color noticeably. And it will too. You should also use latex additive to mix your grout with, NOT WATER.

I'd feel better with a little input from these other guys. It might be I just don't want your wife dissapointed since you've said that twice now.

What about it gentlemen???

John Bridge
06-08-2001, 05:42 AM
For what it's worth, the TCA recommends twice the stiffness for marble as it does for ceramic tile. L/720 as opposed to L/360 for ceramic.

From what I've seen, you would seem to have no worry at all with ceramic. Why push your luck?

kalford
06-08-2001, 07:46 AM
Wolverine,
I don't care for the Luan but since it's already there....
I agree with all that my "learned" colleagues have said except for one thing.I would use Mapei white mastic to set the cement board on.Using the smooth side of your trowel, spread the mastic "mashing" it into the grain.This will improve the bond. Spread it with a 1/4" square notch trowel and screw the cement board with 1 1/4" cement board screws every 6 inches.The cost is a little more but the adhesion is better on the wood.Be sure to allow at least 24 hours for the mastic to cure.
Like Bud said, don't go cheap on the thinset.Latex is your friend.

[Edited by kalford on 06-08-2001 at 09:48 AM]

chip
06-08-2001, 12:17 PM
Mastic though it is an adhesive, isn't the reccomended product for leveling coat for backer board.

To work as a leveling coat it is filling in all the voids of the substrate, using a wider notched trowel,1/4x1/4 or so will provide enough "thin set" to provide a good bed of mortar to work as the prescribed leveling bed.

And once again the TCA reccomends the leveling coat be a dry set mortar and for a dry set or latex modified setting material to be used to set your tile.

Wolverine if you live east of the mississippi, email me and I will tell you the only cement board manufacturer who makes and warrants their product installed in this method.

kalford
06-08-2001, 03:34 PM
Yeah,Dave G. reminded me of that awhile back.
In most cases it is being used as an adhesive not as a backer leveler.While I am aware of TCA's recommendations I don't ALWAYS agree.The method I described is a time tested and proven one not only by myself but by some very reputable and experienced tile setters.Right Derek!? ZERO failures so far.Right Derek!?

I do agree with latex thinsets and that is all I use.I don't see the point in using a product(like Laticrete)that you have to buy expensive additives to just to bring it SLIGHTLY above minimum standards when you can get products like TEC that double those standards with water and save money in the process.

If big "D" sees this he'll probably just turn off the computer and go to bed.

[Edited by kalford on 06-08-2001 at 05:46 PM]

John Bridge
06-08-2001, 04:36 PM
But Keith,

Why are you recommending mastic in this instance -- the luan?

I think Art is right, inasmuch as the mastic won't support the backer board as well.

And Dave G. will tell you that the backer is not supposed to be bonded to the subfloor to begin with, but I don't agree with that either. If I were going to use it, I would bond it with multi-purpose -- any brand.

LDavis
06-08-2001, 04:40 PM
One more "plea" to go ceramic. I personally don't feel comfortable setting marble on a pier/beam structure. I've done it, but never over less than 1 - 1 1/2" of lath reinforced mud bed. In both cases, the subfloor was 2x12 joists 16" oc with 3/4" EGP plywood. Longest span was 11 feet. Ceramic will be easier to maintain in a bath, and will look better than marble thats "having problems". Good Luck!

kalford
06-08-2001, 08:02 PM
John,
Several reasons.It's ready to go,easy to spread,(in my opinion)bonds with the grain of the wood better and is extremely strong.I agree that it doesn't have much in the way of compressive strength but with less than 1/4" going down then mashing that by pulling the backer down tight with screws,there isn't enough space between the backer and the luan/plywood to warrant any concerns.

Don't get me wrong.I think latex thinset is the best thang since sliced bread.I just prefer the mastic.I do believe that some applications,floor resembles a rollercoaster etc.,call for thinset beyond a doubt.

As for not wanting to bond the backer to the plywood,if that were true why use adhesive "and" nails/screws in the first place?

Like I said,No failures using the mastic thus far.

DEREK!!! HELP ME OUT HERE!! I'M RUNNIN OUT OF STUFF TO SAY!!!

Tom
06-09-2001, 01:37 AM
I'm probably in over my head here, but I'd sure like to clear something up. 1/4" backer board has similar compressive and flexural strenghts to 1/2', meaning it won't expand, contract, crush, or if properly bonded to the floor, pull apart any more than 1/2" backer. 1/4" has the advantage of fastening to the floor with less screw/nail lenght(less chance of "swimming"), making it a better choice for floors. These same properties, with latex thinset, seem to make the floor much stiffer. I'd appreciate any thoughts on it in case I'm wrong.Thanks

chip
06-09-2001, 05:57 AM
Let me try this one more time.

The reason you don't use mastic as a leveling coat for back-
board is the same reasons you don't use mastic for the majority of your tile installations.

Before you blow this off, think about all the installations that you have done and chosen thin set over mastic.

Were any of these installations involving setting a tile that was 36" x 60"? You have a rigid piece of concrete board that isn't going to contour to a large dip in your
substrate let alone all the small ones.

Now to the fun part, when you use a non modified dry set mortar, you will not adhere to the plywood. But you will fill all the voids in the substrate and adhere to the cement backer board.

Yes you will have nails/screws through the board into the substrate, and they will move with the expansion and contraction of the substrate.

But that one nail/screw head moving will not create the same type of fracture in the thin set as the entire piece of plywood. So the stresses on your tile installation are reduced to the nail/screw head and not the entire cement backer board that is adhered to the plywood with a latex or multi purpose thin set.

Once again let me say that my company loves to sell multi- purpose thin set and reccomend it for most all installtions.
But we will not knowingly sell you latex or multi- purpose thin set for use with our cement backer board as a leveling coat.

Keith,

We also agree with your statement in regards to buying and using liquid latex. We, and all of competitors put water in our latex/milk. you have water in your area. We don't know how much latex/milk every one will use on every job, but we do know that the polymer (dry latex powder) is in the bag.
You have water, we have polymer lets mix the 2 and have a product that will out proform the latex additive.

Not only are you buying latex, but you are buying water. And it is isn't even Perier! Or Evian or any of those trendy waters that Bri, drinks down at the local micro brew.

Sorry Bri, I couldn't help myself.

Art

John Bridge
06-09-2001, 06:32 AM
I guess my plan to get most of the really technical stuff over to the "Hangout" ain't gonna, work, huh? Oh well . . .

First of all I want to welcome Tom.

Tom, you sound like you have done the work or have done a lot of reasearch. Why don't you tell us a little about yourself?

I don't agree that the 1/4 in. and 1/2 in. backer boards have the same compressive strength. Seems to me the half in. will have twice as much, since floor stiffness generally relates to section (thickness). It's sort of like: Is 3/4 in. plywood stiffer than 5/8 in. plywood? Or is a 4 in. concrete slab stronger than a 2 in. concrete slab?

I'm sorry, Keith. I just can't buy the mastic idea. The fact that it's easier to spread doesn't recommend it to me, and I think multi-purpose thin set bonds as well. I know it will last longer.

The other problem with mastic is that it has to air dry, and I don't know any tile installers who have the time to sit around and wait for that to happen before grouting and getting paid. Consequently, most mastic jobs are not completely dry, which means the bond isn't there, and neither is the marginal "support" you mention.

Art, I've had my butt kicked all over the Internet for my views on bonding the backer board to the subfloor. Properly dried plywood isn't going to move much inside a house. The main problem with placing tile over a plywood floor always was and will continue to be deflection. Bonding the various elements together will improve stiffness and decrease deflection where live load is concerned.

Oh, and I didn't know you folks made backer board.

And Bri has gone to drinking water now?

Can we please somehow get this over to the Hangout?

kalford
06-09-2001, 08:17 AM
I hope we didn't scare Wolverine off with all this debate.
John,We may disagree about the mastic but we definitley agree on the bonding issue.Besides,the whole idea behind Latex modified thinset is to provide a "flexible bond".

I'll save the "dry-set" to .......................??????

chip
06-09-2001, 08:41 AM
I moved this discussion to "cement backer on floor" at the proffesional hang out.

See you there.

Art