Cement Board Underlayment Question

[Mak Wilson]

I plan to use Durock backer board as underlayment for tile (in conjunction with plywood). My preference is to use 1/2". However, I downloaded Durock's install guide and got confused. It sounded as if they made two kinds, one for walls and one specifically for underlayment. They call the 1/2" and 5/8" board their "Standard Board" while the "Underlayment" version comes only in 5/16". Is there anything that makes the 1/2" board unsuitable for underlayment or is this just a matter of advertising the 5/16" as underlayment because it's thinner and will not raise the floor as much?

Secondly, convention suggests a gap of 1/8" between boards. However I can't seem to get a straight answer regarding the space that should be left between the backer board and the wall. 1/8"? 1/4"?

Help is a appreciated!

[RicodJour]

The Durock is bonded to the subfloor with thinset. It does not need the strength of the thicker board used for wall installations as there is no span, and as you noted the thinner board doesn't raise the floor as much. You can use the thicker board on the floor if you'd like - there's no harm in it.

You don't need a big gap. Whether it's new or old that you're butting up against, all joints between different wall and floor surfaces should be caulked. Don't try to grout it.

R

[Mak Wilson]

Thanks for the help Rico. As a reward for your assistance, I pile on a few more questions....

Regarding not needing a "big" gap between the wall and the cement board, I was hoping for a more concrete answer (no pun intended). I know with plywood you have have to allow for expansion. Is that the same thinking here with the cement board?

Regarding caulking the wall-to-backer board gaps, is it best to use

100% Silicone or is a tub/tile silicone better? (To be clear, I use thinset on the backerboard-to-backerboard gaps).

Finally, in true amature form, I don't know the difference between modified and non-modified thinset, but from what I've read I'm to use non-modified thinset to bond the backerboard to the plywood. Another source said to just use constructive adhesive. Any thoughts on that?

Thanks again for any help you can offer!

[RicodJour]

Lucky me.

There will be very little movement - nowhere near 1/4". You should think of it as more of flexing between different wall and floor planes. In a mud job they'd reinforce the corners with metal lathing and just continue up the wall. Since backerboard isn't as bulletproof as a mud job, you should allow for that little bit of movement.

I think you misunderstood me. The only place that needs to be caulked is the floor tile to wall joint...unless you're looking for a waterproof installation and then I'd go a totally different way

Modified thinset has latex additives. It allows for the materials to flex more - a good thing. I use it exclusively.

You're welcome.

R

[Mak Wilson]

I'm trying not to go too far off topic here, but the backer board is going over my entire bathroom floor in preparation for ceramic tile. I also plan to put the baker board under the tub primarily because it's the easiest way to keep the tub level with the rest of the bathroom. (Tub is new install). I assume you mean waterproof as in what you'd do with a shower floor or something like that, so no, there isn't a need to water proof. I'll leave a slight gap between backer board and wall

1/8"-1/16". I hadn't considered the reason for the gap is for the expansion of the wood wall studs. That was a good insight.

Oh, when the time comes shouldn't I also caulk where the tile meets the tub?

I'll probably go with modified then. Although, if you google on "modified" and "backer board" you might be surprised to find that there's some debate on this. I don't have the knowledge or field experience to sort it out.

I guess I'm ready to move forward from here. Thanks again. I really appreciate people like you taking the time to help out newbies like me.

[RicodJour]

When you say keep the tub level with the rest of the bathroom, what does that mean? The backerboard can be done either way - either under the tub or butting up to it. Doesn't really make a difference other than you don't have to worry about a precision cut for the backerboard if it extends under the edge of the tub.

Absolutely. Caulk all changes in wall planes, wall to floor transitions, around all edges of the tub inside and out of the wet area, etc.

There's debate on everything, but having a little leeway with flex between the floor and the backerboard can't hurt. The converse is not true. It also simplifies your job by not having to keep switching materials. If you're only doing one bathroom it's not a big deal.

The John Bridge Forums are another good place to get tiling information.

People like me? People like me?! Crap - here I thought I was unique.

We were all newbies at one point. Answering questions here is my way of trying to pass on some of the help and instruction I've received over the years.

Do a good job and have some fun with it.

R

[Mak Wilson]

No new questions this time around. I think you got me where I need to be...

Just means that when I tile the bathroom floor for the tile I'll go ahead and raise the tub floor too (as opposed to having the tub be a

1/2" lower which really wouldn't have bothered me that much but that's not the way I'm going with it). Put another way, the floor is already level across the stretch of the room including the floor that's under the tub (or rather were the tub was before I took it out).

Right. The tub itself rests on 4 evenly spaced square mounts so those are the important points to cover. Even so, I'll just put the BB under the entire tub. The board will extend out beyond the tub a half a foot before meeting up with another BB.

I figured as much and was just being clear. That's one of the reasons they call it TUB & TILE caulk.

That's one of the places they were debating the issue. I must say I wasn't really clear on the points of the arguement, but I'm satsified to go with the modified thinset. Thanks for helping me get off the fence.

That's what it's all about (not to mention saving a couple bucks and getting some time away from from my beloved wife). Thanks again.

[boden]

Durock is fine but just to complicate the issue look through the Schluter site. Ditra is perhaps a better material if you anticipate floor movement. I've used both in my home and have been happy with both. Home Depot has some Schluter products and can order the others.

Don't use construction adhesive to put Durock down, use a modified thinset mortar.

Boden

Mak Wils> I plan to use Durock backer board as underlayment for tile (in

[Mak Wilson]

I don't anticipate floor movement and I've already got the backer board on hand, so I'll pass on the Ditra for now, but thanks for the thought.

Agreed. The constructive adhesive is only for securing the tub itself to the (soon to be) Durock floor.

[RicodJour]

That's usually not necessary. Once it's all plumbed and tiled in, it's not going anywhere. If it's an acylic tub are you planning on using something under the floor of the tub to help take the weight?

R

[Mak Wilson]

No, I hadn't planned on it. The previous tub was cast iron and much much heavier and there was never an issue so I think the acyrlic should be okay. The construction adhesive is mentioned in the tub install specs as an alternative to a cement bed. It's not much trouble and I figure it can't hurt, although it won't be as much fun taking the tub out again should there ever be a need to do so.

[BobK207]

Mark-

the tub to help take the weight?

[BobK207]

Mark-

That cast iron tub (if it's not trashed) is worth soemthing to someone.

Put it on CraigsList, you might get some $'s for it , help out someone doing a restoration & dispose of it w/o trashing it. :)

cheers Bob

[RicodJour]

But smacking the crap out of a cast iron tub with a sledgehammer is a great way to get rid of some of those pent up aggressions!

The craigslist is a good way to get rid of all sorts of stuff. Pulled doors, windows, sinks, whatever. Let someone else reuse it instead of landing up in a dump somewhere.

R

[mb_mack]

Mac, Learned what I know working with my husband, professional tile installation, all applications, in Florida.

1) Use mastic, not thinset to set 1/2" durock (permabase). Butt sheets up, their slight variance will give you all tolerance required.

2) we typically use 1-1/4" coarse thread drywall screws about every 8

-10 inches to further secure the rock.

3) get out of the tech sheets and go to the local dealer who sells to the pros, (ie: Wholesale tile stores). If you see anyone in an orange apron RUN!!!

[BobK207]

Mark-

That cast iron tub (if it's not trashed) is worth soemthing to someone.

Put it on CraigsList, you might get some $'s for it , help out someone doing a restoration & dispose of it w/o trashing it. :)

cheers Bob

[RicodJour]

Why do you feel your way is better than what the manufacturer recommends? This from National Gypsum's web site:

Can I attach PermaBase directly to my floor or countertop? No. Using a 1/4" square notched trowel, apply a setting bed of latex-Portland cement mortar to the subfloor or base and immediately laminate the PermaBase, leaving a 1/8" space between adjoining pieces. Make sure you have a minimum of 3/4" exterior plywood/OSB as a subfloor or base and that the framing members don't exceed 19.2"o.c

Again from the NG web site: What type of fasteners do I use?

Either a 1 1/2" galvanized roofing nail or 1 1/4" cement backer board type screw are used with wood framing. If you are attaching the PermaBase to 20 gauge or heavier metal framing, use 1 5/8" S-12 cement backer board screws. DO NOT use typical drywall screws.

Notice the capital letters? Drywall screws are definitely not recommended.

I'd agree with you if you said to go to the pros for tile advice instead of a big box store. Regardless of where you buy the material, if you follow the manufacturer's installation instructions you'll rarely go wrong. If not...you're on your own.

R

[Mak Wilson]

I hadn't thought of that, but I think I'm okay. The tub, by design, is supported by 4 4" squares evenly spaced. The floor itself is very solid. Either way it's too late now. I already have the BB installed. Thanks for the feedback just the same. Although I'd say my biggest aggrivation with projects is over thinking things, it's good to hear all the considerations.

[Mak Wilson]

I busted it up something awful. Even if I had wanted to save it, I don't know how I would have gotten it down the steps or even through the door. I put the pieces in 4 or 5 heavy duty garbage bags and the garbage man refused to take them because they were too heavy. Anyway, I have another tub in the downstairs bathroom that's exactly the same but still totally in use. I can't imagine what the cost would be to ship something like that.

[RicodJour]

Crasigslist - they come get it.

Those cast iron tubs are damn heavy, huh? A 5' one weighs about 300#, yet one guy brings it up the stairs. It's the easiest way, believe it or not...just not necessarily the safest! First time I saw that I was flabbergasted.

R