shower curb - 2x4 on edge [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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12-23-2016, 10:01 AM
Hi all,

I have 2x8 joists running about 6.5'' long spanning 24'' (max) in a small 36 sq foot bathroom. Part of the room is keeping it's existing subfloor, because it's tied in with the adjacent room. The remaining half has a new subfloor (old removed to updated pipes, etc).

I'm interesting in marble tiling and want to make sure it's feasible and what measures I can take to strengthen the structure (cross-blocking with hangers?, blocking underneath shower pan border?).

I'm also curious why many seem to recommend double wood subfloors. I had never heard of this until visiting the forum. What benefit does this offer? Is it better than using one single thicker sheet (7/8'' or 1'')? Is there a specific size of each plywood sheet that is recommended?

Thank you for the help!

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12-23-2016, 10:31 AM
Welcome, David. :)

The Marble Institute of America (MIA) requires a double layer of plywood subflooring regardless the joist spacing for any natural stone tile installation. The double layer is required so that there are no places where a subflooring joint extends all the way from the joist top through the surface.

I have 2x8 joists running about 6.5'' long spanning 24'' (max) in a small 36 sq foot bathroomA bit confusing there. Your joists have an unsupported span of only 6 1/2 feet (rather than inches)? And the 24 inches is the joist spacing, perhaps?

At least part of your new subflooring is inadequate, being oriented with the strength axis parallel to the joists rather than perpendicular. That needs to be corrected before you add another layer of nominal 1/2" exterior glue plywood over the entire floor.

My opinion; worth price charged.

12-23-2016, 11:45 AM
Thank you CX for the quick and informative reply.

Yes, you're right that the joist span is 24 inches and the joist span is 6.5 feet.

I will make sure the plywood's strength axis is perpendicular to the joists.

Is it worthwhile to add cross blocking to tie the joists together in certain places (ie. along the shower pan perimeter)?

Is there any concern of reusing the existing/old subfloor in conjunction with the new subfloor (which appear to be solid)?

Merry Christmas!

12-23-2016, 02:27 PM
You'd want blocking to support the unsupported joints, but any additional blocking won't buy you additional strength on a distributed does help with point loading, but not a distributed load which is held up by the joists. If you want to strengthen the joists themselves, you either need to shorten their span or add material by doubling/sistering them (which is harder to do when plumbing, wiring, ductwork, etc. is in the way).

If the old and new subflooring is the same thickness, replacing the old may not buy you anything UNLESS by putting in a new sheet, you can add T&G panels that will support the edges OR, if the existing sheets don't span at least three's not very strong to just hang the ply between one set of joists spanning only one bay - it will just be hanging by the fasteners verses being supported in the middle.

When you add the second layer, you want offsets and adequate screws. There's an article in the liberry (sic) that describes the proper method.

01-05-2017, 10:51 AM
Does the need for the double plywood subfloor change if whether CBU or mortar is used? The plan is to have a mortar bed to level the uneven floor.

Height of the bathroom and the adjacent room is a concern, so would prefer to leave out the second plywood layer if possible for minimizing height difference from room to room.

Houston Remodeler
01-05-2017, 12:04 PM

The plywoods are installed then the floor is flattened as desired.

01-05-2017, 12:47 PM
The plan is to have a mortar bed to level the uneven floor.If that is to be a reinforced mortar bed of a minimum 1 1/4" thickness with welded wire mesh in the vertical center, the MIA will, I believe, accept that over a single layer subfloor.

My opinion; worth price charged.

01-09-2017, 01:11 PM

My contractor is planning on building the shower curb with a single 2x4 on edge. It's a small shower in limited space. I'm more familiar with the curb base being built from 3 stacked 2x4s. Is there any concern that a single 2x4 isn't strong enough to support the weight of the shower enclosure? Thank you.

01-09-2017, 01:26 PM
David, it'll help if you'll keep all your project questions on one thread so folks can see what you're working on and what's been previously asked and answered. We can give it a more generic title any time you'd like to suggest one.

It'd help if we knew what type of shower receptor waterproofing system you intend to use, but in general that single 2x4 might be adequate if it's tall enough and if your contractor knows his business. Supporting the shower enclosure won't be a problem and you will not allow anybody to poke mechanical fasteners of any kind through the waterproofing of your curb.

My opinion; worth price charged.

01-09-2017, 02:18 PM
Sometimes having a narrow curb is desired, especially in a tight shower. I have built many showers with a ripped down 2x6 for a curb. I cut it down to about 5 inches. The pan liner goes over the wood and then I wrap it with lath and mud it. Makes for a strong curb.

01-10-2017, 02:41 AM
Thank you CX and Davy for the helpful responses.

01-17-2017, 12:23 PM
Is cement board on the curb considered a bad practice? Should I insist on on mud and lath for the curb?

My contractor is insisting on the 2x4 edgewise curb.

CX: Can you elaborate a little more on this comment? "a single 2x4 might be adequate if it's tall enough and if your contractor knows his business"

01-17-2017, 12:42 PM
1. It is not acceptable practice in a traditonal mud/liner/mud shower receptor construction.

2. It's not likely to be tall enough for some types of shower receptor construction, David and some tile contractors, in my experience, are not well versed in the other building trades, such as carpentry.

01-17-2017, 05:36 PM
Code calls for a minimum drop of 2" from the top of the curb to the drain...with a conventional shower pan construction, that's almost impossible to achieve with a 2x4 on edge.

Once the liner is wrapped over the 2x4, how would you install cbu without piercing the curb? Since the CBU isn't waterproof, how would you apply anything on top of it with it buried in the deckmud that would prevent moisture from wicking up and out of the curb? Classic situation where moisture gets in there, the wood swells, the tile/grout crack, and things fail.

01-18-2017, 01:07 PM
Starting to think about tile and grout.

Current proposal:
*1/8'' grout line
*unsanded grout (Bostick)
* carrara 3x6 tile on shower wall
* 1'' hexagon on shower floor
* 3'' hexagon on bathroom floor

I understand that 1/8'' grout line is at the midpoint between sanded and unsanded grout, but I know sanded grout is not recommended for marble.

Any feedback is appreciated!

01-18-2017, 02:06 PM
There are many sanded grouts out that contain a finer aggregate than older grout. Look around at what's available in your area. If it were me I'd either go smaller grout joint or go a different route. In my past experiences 1/8" and unsanded isn't the best combination.

01-18-2017, 03:31 PM
Sanded grout is both stronger and shrinks less...the size and shape of the 'sand' car vary considerably between brands and configuration of grout products. SOme sanded grout, the sand is much more rounded, and not as likely to scratch anything. Plus, it's easier to get into smaller joints.

01-18-2017, 04:06 PM
Any recommendations on specific sanded grout brands? Am I correct in understanding that sanded grout is fine for all the tile (inside shower and outside)?

Thank you rmckee84 and jadnashua

01-18-2017, 04:18 PM
fwiw.. . I had a small space and built my curb with a 2X4 on edge. The membrane was almost 2" above my drain, maybe 1/8-1/4" shy but my choice. The finished curb will easily be 2" above the drain. In my case the width of the door opening was only about 28" and I choose the nicest looking straight grain 2x4 I could find then secured it into the floor and both sides with framing screws and a deep countersink in the 2x4 with screws into my floor.

I used the PVC cement supplied with my pan liner and it glued the membrane more or less to the 2x4 so it wasn't sloppy and loose too much. Then formed some expanded metal lathe to go up and over into the mud bed and used roofing nails on the Outside Only to attach the lathe to the curb. Then I made a form inside and out to my rough height and filled the whole thing to a minimum thickness of about 5/8-3/4" with Type 'S' high strength mortar.

It was very sturdy and I think the narrow width of the opening helps make that possible. Probably not a good idea on a wide opening like 36" or more. Mine was very sturdy and a marble threshold 4" sets very nicely over the top of the mortar curb affixed with thinset. So far all is good.

We did Carrera 'like' 3x6 subway and I would prefer a 1/16 or 3/32 joint max and un-sanded grout. I like the narrow grout lines with marble and we did 3/32 on our floor (bath and shower) and 1/16 on all the walls, which are not yet done. Another reason to consider un-sanded is it will be less likely to scratch your stone.

01-18-2017, 05:13 PM
The industry recommendation for minimum thickness of a deckmud pan over a wooden subfloor (not a bonded one to a slab) is over an inch for each layer at the drain, and obviously, higher at the curb. At 3.5" on edge, you will not achieve the necessary height above the drain to meet the plumbing code. You might be able to do that using a sheet membrane where you don't need the second layer. Now, people also build curbless showers, and don't have 2" drop to the drain, but that's another story between you and your inspector.

01-18-2017, 07:47 PM

do you need rectified tile to have grout lines of 1/16'' or less?

01-18-2017, 08:12 PM
Industry standards call for 3x the maximum variation between tiles to be the smallest grout gap that will work reliably. That can be difficult even for some rectified tiles. The only way to tell what will work is to actually look at a box and measure. Keep in mind that accounts for not only the actual size, but how rectangular they are.

01-19-2017, 01:09 PM
Jim... I studied this site quite extensively and never saw that deck mud needed to be more than 1" at the drain flange for a pre-slope. I did almost 3/4" at the flange and 1"+ over the liner for 1-3/4" rough mud above subfloor. I did cheat a little I guess with the liner on my curb right at 3-1/2" high. My choice and no inspector to please but me and I'm not recommending you do the same. But with ~3/8 stone floor the drain is finished at ~2-1/8" and the curb with ~5/8" mortar plus ~3/4" threshold is 'finished' at almost 5" above the subfloor. So in order to flood the room you need more than 2-1/2" water. Is that not the intent of the codes ? IDK

David... in your first post you said you were going marble, natural or tile that looks like? Natural stone is machined & cut quite uniform and I thought that meant rectified automatically. I measured a lot of pieces with no more than ~.015" variation in the direction I need to layer, most more like <.010". So a 1/16" (.0625") is probably 4-5x the variation in the stones. I think my ability to lay them uniformly is maybe more than the variation and I go slow and carefully. I like the narrow lines and try to carefully match the grout color so it more or less fades into the field and not detract from the stone's beauty.

01-19-2017, 02:24 PM
Bill, there is no tile industry standard for shower floor pre-slopes, the pre-slope and waterproof liner in a traditional shower receptor being part of the plumbing. However, the thinnest mortar bed thickness for any floor application I'm aware of in the ceramic tile industry is 3/4 of an inch.

The minimum thickness per ANSI Standards for the final mud bed in a shower floor is 1 1/2 inches.

01-26-2017, 09:40 AM
The tile is all laid down. The shower walls came out nicely, but the contractor rushed the bathroom floor (3'' hexagon tile) and the grout lines are inconsistent everywhere. He didn't use spacers. In hindsight, I definitely should have brought this up to discuss since I noticed the spacing was already bad on the mesh sheets. Was I wrong to assume that the contractor would use spacers with the 3'' hex tile sheets?

How long does grout take to cure? I'm using Bostick sanded and unsanded grout. Thanks again!

01-26-2017, 10:30 AM
It really just depends, with a lot of sheets on the floor you just have to eyeball and split the difference because they are poorly mounted. I used wedges and spacers so they stay where I want them.

01-26-2017, 10:54 AM
Assuming a cement based grout, any cement product is rated at being 'fully' cured after 28-days (but in reality, it continues to cure for many years). From a functional thing about when you can apply a load or do much of anything over it (like maybe add a sealer), usually after a day, but some specify longer. The only way to know for sure is to read the spec sheet for the specific grout that was used. 3-days is probably safe for any, but some can be dealt with sooner.

02-02-2017, 04:10 PM
Entering the home stretch here. I picked up some Stonetech Bulletproof sealer for the stone and grout. The tile contractor recommended waiting until after the shower enclosure is installed before applying the sealer.

It seems more logical to put it on now before the glass goes in, but wanted to see if there what everyone thought.

When's the best time to put sealer on the stone and grout for a new tile installation?

02-02-2017, 04:36 PM
Personally I'd do it before the enclosure is installed. But that's just me.

02-02-2017, 06:18 PM
What Ryan said.

Did your tile contractor give any rationale for his recommendation?

02-07-2017, 12:01 PM
CX, the tile contractor said something about how it would be messy to apply it afterwards. The explanation didn't make sense, but it may have been a language barrier. I'll apply the sealer before the shower glass goes up.

Incredibly, this project is becoming more of a headache.

Over the weekend, a substantial amount of water overflowed from the toilet and got on the bathroom floor and into the adjoining rooms. We haven't had final inspection yet. I had not put the sealer on. I suspect that the water got through the stone and grout. What can I do to ensure everything gets dried out properly? I don't think it makes sense to seal the floor since I suspect the floor is wet underneath the tile. The tile itself looks the same as prior to the incident.

Second question, what's the best way to apply the sealer to the grout and stone (when I do apply it)? A basic sponge or some special applicator?

Thanks again everyone.

02-07-2017, 12:46 PM
Whatever sealer you chose should have application instructions.