Floor by the door chore
November 18, 2011 10:29 AM   Subscribe

Do I absolutely need cement board under my tiles? What happens if I don't use it?

I'm about to install this type of tile in our bathroom, and the Lowes video I watched that covered tile installation recommended using cement board (aka DUROCK) on top of the subfloor. This is a 50 year old bathroom and the floor is pretty small, about four foot by five foot. But it is also a bit uneven so I was going to use some leveler first. My concern is once I've used the leveler, and then added the cement board, and then put the tile on top of that, the floor is going to be about 3/4 to an inch higher than it was. Also an inch higher than the floor in the connecting hall way, making the need for some sort of transition. I'll also need to remove and cut off a piece of the door, so it can open.

The transition and door issues can probably be avoided if I skip the cement board, but if I do, how likely is the grout to crack? The cement board actually seems more flexible to me than the plywood it will cover. If I'm already putting down cement in the form of leveler, is the board still necessary to prevent future cracking?
posted by Toekneesan to Home & Garden (18 answers total) 4 users marked this as a favorite
If you don't use cement board, then it is more likely that tile or grout will crack. I've seen many cruddy tile jobs where tiles were installed directly over subfloor and the tiles have cracked in half, grout has cracked or popped out of the joints, etc.

The board is attached with screws and adhesive to your subfloor - you are creating a laminate, not relying on the properties of cement board by itself. You should also check carefully to check that your subfloor is stiff enough - sometimes a second layer of plywood or OSB is required (again, laminated to the current subfloor with screws and adhesive) to achieve the required thickness.

There are also thinner, plastic replacements for cement board underneath tile (e.g. Scluter. They cost more, but you save in thickness and also in install time.

This is a job to do right the first time, because you really don't want all your nice tiles or grout to crack.
posted by ssg at 10:44 AM on November 18, 2011 [3 favorites]

I know it's frustrating and the inclination is to "just get on with it already!", but yes you need the durock. It provides stability, resistence to expansion and contraction, and the correct substrate for the adhesives and grout you will be using. Also, don't forget to seal the seams. You want to do this right. It's not a huge amount of work, but the whole project is enough that you don't want to have to come back and do it again, or suffer the frustration of looking at the cracks (and potential water damage) ever time you spend a couple minutes...um...sitting there looking at it.

You will make up the elevation difference with a piece of marble threshold that you can get from wherever you are buying your tile from. Good luck!

(I always mention that I am an architect and builder in these threads, just so you know I'm not talking completely out of my butt. And also to indicate that this background has given me ample opportunities to do things the incorrect way, only to realize that sometimes it's not worth trying to save a buck and an hour.)
posted by nickjadlowe at 10:45 AM on November 18, 2011 [4 favorites]

Watch a couple of episodes of Holmes Inspections or the older Holmes On Homes and you'll why you want cement board AND Schluter: cracked floors, water damage, mold, costs of fixing things...
posted by Hairy Lobster at 10:52 AM on November 18, 2011

You do need something to prevent deflection, you will get cracking if you dont. Water on the plywood will also cause cracking by the expanding and shrinking. Although cement board is very common, there are other products on the market. I personally used Schluter / Ditra which prevents deflection,is waterproof, easy'ish to install and is 7mm. I am sure there are many other products.
posted by njk at 10:56 AM on November 18, 2011

If you have leveled the entire floor with the cement leveler you will be fine without the sheetrock. You just need something waterproof and stable under there - which concrete is.
posted by zeoslap at 11:42 AM on November 18, 2011

(and by sheetrock i mean cementboard)
posted by zeoslap at 11:44 AM on November 18, 2011

Seconding zeoslap. Used to do ceramic tile with my dad and family. We basically made our own leveler out of sand and cement and tiled over that. It's a pretty old school way to do it but it makes floors last.

First, we'd staple down thick wire mesh to give the mud something to grab onto, then we'd mix and dump the mud on the floor and level the floor with a straight edge. Let it dry and then tile on top of it.
posted by Loto at 11:46 AM on November 18, 2011

njk is the closest so far.

Whether or not you will need cement board depends on the amount of deflection in the existing sub-floor. Here is a PDF that explains how to determine the maximum amount of deflection permissible based on the span that you are covering and the type of tile being used. Most residential floors will have at least the minimum amount of deflection hence the instance that Durock be used. If for some reason you have very solidly built subfloors you may not need to put down cement board. Just be sure that when you are figuring out the expected load you take into account the weight of a full tub of water.
posted by Bango Skank at 11:59 AM on November 18, 2011

The leveler that the OP is using is a pour-in-place leveler (as in, it pours with the consistency of pancake batter - NOT a mortar-bed-type leveler that Loto describes). I have used it many times. The beauty of it is that it will level from, say a 3/8" thickness at the low point, to almost nothing at the high point. In other words, it's formulated so it can pour very thin and still function. The downside is that, as a tile substrate, this thinness does not give you protection against deflection or expansion/contraction of the sub-floor below. You simply wouldn't want to tile right over it....that's just asking for trouble.

With so many DIY projects, there is always that balance of trying to do it quick/easy/inexpensively or some combination of those things that also includes doing it sufficiently and correctly. With tile, you are best advised (imo) to play it kinda safe. If it goes wrong, it goes really wrong and your solution is to tear it out and redo it. Which is the worst type of solution. This is just one of those times when you don't really want to take a shortcut...even if there was some potentially arguable scenario that would give you hope that you could get away with it.
posted by nickjadlowe at 12:21 PM on November 18, 2011

The floor area is small, and the tiles he's using are also small so the worst that will happen is that the grout might chip out a bit but a nice flexible grout will make that issue essentially moot.

I'm not arguing that using the cement board/scluter isn't 'better' just that he'll be just fine doing it as planned - and yeah you don't want the leveler to get too thin in any one spot but over such a small area it seems unlikely the variance is going to be much at all.
posted by zeoslap at 12:53 PM on November 18, 2011

Whether or not you will need cement board depends on the amount of deflection in the existing sub-floor.

No, not at all. You want a floor that meets the deflection requirements (this mostly has to do with the joists underneath) and you don't want a lot of deflection between joists (so you need a thick enough subfloor, installed properly). The cement board or Schluter is not there to prevent deflection, but to prevent expansion problems due to temperature changes, moisture, etc.
posted by ssg at 1:06 PM on November 18, 2011

Do you really want to find out the hard way that you needed cement board or Schluter?

Not worth it. Use the backer board and play it safe.

*speaks from hard won experience*
posted by BlueHorse at 2:00 PM on November 18, 2011

Nthing Schluter Ditra, it was recommended on the excellent John Bridge Tile forums, and it allowed us to omit the cement board and reduce the thickness of the required plywood.

The real problem is that tile and grout are outstanding under compressive stress and pretty much suck at everything else. The "underlayment" is the single most important factor to a durable tile job I can tell you that from experience because the previous guy who owned my house was a full-time tile and granite contractor, and yet the grout in our kitchen is cracking like crazy because he didn't shore up the cheap, thin particle board subfloor. You can feel it flex under your feet. Whereas the bathroom where we, first time tilers, replaced the particle board with OSB ("plywood") and put down Ditra XL has no cracks and feels like stone underfoot. Do it right and you can be proud of it forever.
posted by wnissen at 7:13 PM on November 18, 2011

The previous owner of my house installed tile in the bathroom, without cement board, and the tiles have lifted and the grout is cracked. My floors are old & bouncy, YMMV.
posted by theora55 at 5:50 AM on November 19, 2011

What kind of floor is in there now? Don't you gain enough depth by removing the old floor?

Usually, there is subfloor over the joists. Then there is the finish flooring on top of that- 3/4 inch of hardwood, 3/4 inch of carpet+padding, 3/4 inch of durock and tile. So what I'm wondering is if there isn't another layer of floor that you can remove. Perhaps the previous floor was tile, and they used 1/2 inch plywood as a backer. If you remove that, you will have room for the durock backer.

Failing that, they do have stuff you can use. Forget the name, but it is some kind of relatively thick mesh that gets embedded into the mortar that acts as tensile reinforcement and some kind of water barrier. They use it for shower floors where the pitch would make using a solid backer difficult.
posted by gjc at 6:41 AM on November 20, 2011

What was there was a thin sheet of vinyl flooring applied with adhesive caulk. That's gone and I'm down to the plywood subfloor below. I'm pretty sure it's 1/2 plywood.

I've located a local Schluter dealer, I'm going there tomorrow investigate. The selling point of Schluter Ditra seems to be the uncoupling factor. But does it also provide sufficient protection from deflection?
posted by Toekneesan at 7:08 AM on November 20, 2011

> But does it also provide sufficient protection from deflection?

Regular Ditra does not in all cases, but Ditra XL does.

What I would do is pick an unobtrusive spot (maybe under a vanity, or something that will be covered by a threshold) and drill a hole such that you could check the floor thickness. If you have particle board subfloor (like I did) you are screwed. No tile can safely go above particle board, due to its expansion when exposed to water.

Assuming you have OSB or plywood that is in reasonable shape, then the key is the joist spacing. Hopefully you can look carefully and see the fasteners (nails or screws) used to fasten the subfloor to the joists. If not you'll need a bigger hole to stick a dental mirror down there.

If it's 19.2 in or less, you can use regular Ditra (only 3 mm/ 1/8 in), which is cheaper to boot. If so, congratulations! If it's 24 in (which, as I understand it, is more common), then you have a couple options. One, if you have 3/4 in (nominal, 23/32 in for real) you could add a layer of 3/8 in (again, nominal, so 11/32 in) and use regular Ditra. In their manual, that's "24 in (610 mm) o.c. joist spacing, double layer OSB or plywood subfloor" I did this, but it was just too high for my taste. So, I went with option Two: Stick with one layer of subfloor, and use Ditra XL (which is 7 mm / 5/16 in). This option is more expensive, but easier in terms of the subfloor, and it's thinner. It's referred to as "24 in (610 mm) o.c. joist spacing, single layer OSB or plywood subfloor."

In any case, you'll need a level surface underneath the Ditra, it can only do so much. But those are the deflection requirements, in my amateur understanding.
posted by wnissen at 9:07 AM on November 20, 2011

Another option would be to pull the subfloor up, and add blocks between the joists so that the subfloor doesn't have as much span to cover, and to connect adjacent joists so that a footfall that is directly on a joist isn't absorbed by just that one joist, but is spread horizontally and laterally. (Creating a grid of squares for the subfloor to span.) You can do this with joist hangers. Stronger and easier than trying to toenail them into place. Glue and screw everything, and you'll have a plenty stiff floor.

If you have access from the basement, this is easier and you probably don't have to pull up the subfloor. In that case, instead of blocking, it might be easier to just sister the existing joists, depending on how the building is laid out.

With the floor sufficiently stiffened like that, you won't need that extra layer of backer board, and can just lay the tiles right down on the subfloor with a (appropriate for the task) flexible mastic. Then grout with a flexible grout and you should be good. I think they make pre-mixed stuff, or have latex-like additives that you add to regular grout.

I would also second wnissen's advice to carefully drill a hole to make sure you really are down to the subfloor. You can also check the transition between the bathroom and the hallway. I don't know what finish floor is in the hallway, but you should be able to see if the plywood in the bathroom extends out underneath that floor, or if it stops.

Another thing I thought of for leveling the floor would be to get a straightedge of some kind and pull it across the floor looking for high spots. It might pay off to knock down the high spots with a belt sander. However, if the unevenness is wavy and matches up with the joists (high spot over a joist and low spots in between), you probably need to investigate replacing the subfloor. You might be able to get away with the same thickness by using a newer, stiffer material. I've seen OSB that is much stiffer than regular plywood. And again, you can kill two birds with one stone by moving to say, a 5/8 subfloor and laying the tile directly on that, instead of keeping the old subfloor and having to add way more than 1/8 of material to stiffen and even it up.
posted by gjc at 7:31 AM on November 21, 2011

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