backerboard vs. self leveling compound

[Guest]

I am preparing to tile over a 3/4" OSB T&G subfloor, using either 1/2" cement backerboard or a 1/2" thick layer of self leveling floor compound. The OSB floor is supported by wood I joists at 16"OC, engineered to L/480 specifications. The tile will be 12" & 18" Travertine marble. Would the cement board or self leveling compound be better for this situation? I would prefer the self leveling compound because there has been some edge swelling of the OSB, which will make a flat floor more difficult to achieve if using backerboard. If self leveling compound is OK for this application, what specific product would you recommend?

[Guest]

Re: backerboard vs. self leveling compound

Jim, run this by the crew over at http://www.johnbridge.com/vbulletin/index.php. Bunch of SLC guys there. Lot of the better SLC's would be fine for that application... assuming the floor is stiff enough, which it sounds like it is. Haven't done the calcs for I joists yet.

[Guest]

Re: backerboard vs. self leveling compound

Brian,

For 12" by 18" stone I wouldn't use SLC. So I want to ask your opinion. I have only used SLC on a limited basis, and this includes two large bathroom floors where the manufacturer's rep came to the jobsite and poured it with us to demonstrate it. Its best feature seems to be that it is fast and labor-saving. But its biggest weakness to me is that it doesn't result in a well-leveled floor. The corners are all high as is anyplace where anything juts into the floor space. This is the result of the high surface tension of this moderately viscous liquid (in the state that it is poured).

So from my experience I wouldn't use it if I wanted a first-rate level floor - I'd float that the traditional way. And 12" X 18" stone is a good example of a material that wants a level floor, a true plane.

The other thing that worries me is the results I see from places where I poured SLC on shed floors, over concrete slab, in my back yard. I did this just to observe, and I know the product isn't meant to be the final permanent walking surface. However what I observe is the the SLC fractures into many tiny sections, separated by hairline cracks. The concrete slab beneath is basically solid and monlithic, so the fracturing seems to be inherent in the SLC. I can't think of any reason not to downgrade the product as a result of this as well.

But I know that you have a lot of experience with the material- so maybe you have answers for my concerns.

[Guest]

Re: backerboard vs. self leveling compound

Hi Jim

The natural stone industry uses L/720 as the figure for deflection. Is there any way you can beef up the joists?

Riley

Did you use the primer on the concrete before the test pours of SLC?

I've used SLC about 30 (?) times, and I think in each case there was at least some small amount of cracking. The cracking is inevitable, I think, because of the high water content. The past few jobs where I used SLC I put Ditra over it to give some protection against the cracks.

[Guest]

Re: backerboard vs. self leveling compound

Jim

I forgot to add...go to www.johnbridge.com and post your SLC question directly for my friend Jim Buckley. he is a SLC manufacturer and will be able to give you the true scoop on your application.

[Guest]

Re: backerboard vs. self leveling compound

Riley, Jim (to whom rob refers) recommends diamond lathe under the SLC now, if I understand him correctly. I've had cracking, too, but as Rob said the bigger issue is deflection. Primer is a MUST with SLC's, but you pointed out the main benefit... it's easier for the average person than floating a mud floor. That's this lazy man's excuse anyway :).

[Guest]

Re: backerboard vs. self leveling compound

Jim,

Before you decide on backerboard or SLC you should look into solving your OSB issue. OSB is not a suitable substrat for backerbaord or SLC. Your L/480 is way below the recommended L/720 for natural stone. Backerboard is not going to give you a level enough floor over OSB with swelling edges. I don’t do backerboards on floors but troweling wet thin-set over OSB can’t be a good idea. Riley also makes a good point about the surface tension of SLC's. I have never actually measured the surface tension but I am sure it’s at least 1/8"-3/16". The highest point of your floor would require the minimum SLC thickness for it to produce a flat floor. I could see installing 5/8 min. plywood (screwed and glued) over your OSB to give you a suitable substrat for going with either of your 2 choices. I would install the plywood then put "Ditra" directly over the plywood. If sanding the swelling OSB edges before installing the plywood doesn’t produce a flat enough surface you could use an SLC over the new plywood and then install the Ditra.This would give you sound substrat for installing ceramic or porcelain. You would then have to bring your floor up to L/720 for a stone install.

Cisco...

[Guest]

Re: backerboard vs. self leveling compound

Thank you for the above comments/information. The extra layer of plywood is probably a good idea. That should help minimize floor deflection between joists. As for bringing the floor joists up to L/720, that may be more difficult, although I could sister a plywood gusset onto the sides of the I-joists to stiffen them. Any thoughts on this?

[Guest]

Re: backerboard vs. self leveling compound

Jim,

The real structural stregnth of I-joists is in the upper and lower cord the plywood center has no real strutural value so beafing it up with plywood gussets is not going to do the trick. One option would be to reduce the length of the overall span be installing a header (micro-lam or steel) and columns.

Cisco...

[Guest]

Re: backerboard vs. self leveling compound

Jim

If you added a plywood gusset on the sides of the upper and lower flanges of the I joists, a box beam would be the result. I think this would definately be stiffer than what is there right now.

Run this one by the engineers that frequent www.johnbridge.com and see what they have to say.