• How to Build Cement Mortar Shower Curbs (Shower Dams)
- John P. Bridge (December, 2001)
A surprising number of novice tile setters are building their own showers using the cement backer board (CBU) method. When I wrote Ceramic Tile Setting (McGraw-Hill, 1992), I was convinced that full ceramic tile showers were beyond the capabilities of “weekend warriors,” but I’ve since changed my mind.
Through our online advice forum, my associates and I have coached both men and women through ceramic tile shower projects (some of them quite elaborate) with very satisfactory results. I’m now convinced that almost anyone with normal manual dexterity and a basic understanding of hand tools can do the work. If you would like step-by-step technical guidance on building a shower, or on any tile or stone project for that matter, log on to the John Bridge Forum.
While there seems to be an abundance of information on the web and elsewhere concerning mortar bed shower floors, the curb or dam of the shower has been overlooked. I intend to remedy that. A shower is not a shower unless there is some means of keeping the water inside. Included in this article are thumbnail photos, and if you click them you’ll be taken to the full-size versions.
Ontario Tile has published a very usable article on constructing mortar bed shower floors. Please read the article if you are not familiar with the techniques involved in building shower floors. Only then will you be able to digest the information I am about to give you.
It must be noted that while the article is quite superb, I don’t agree with everything it says. (No one in the industry will be surprised by this!) For one thing, CPE shower pan liner material is made by only one company in the U.S. and is very difficult to acquire. PVC, on the other hand, is made by a number of companies and is available everywhere. I’ve been using PVC for nearly 30 years, and it had been around for a number of years when I entered the tile trade. I have not known the material to fail, save in instances of improper installation.
• The Cement Mortar Curb
Shower curbs have been built from portland cement mortar for as long as showers have been around. There is no better way to build a curb. Using cement mortar to complete your shower curb is necessary , even when you’ve selected the cement backer board method for the shower walls. This is because you cannot nail through the top and inside of the curb to attach the backer (CBU). Nail holes in that portion of the shower pan would cause the shower to leak. Reinforced cement mortar can be more or less draped over the curb and allowed to cure with attachments at the front (outside) of the curb only.
We will begin by pre-shaping galvanized metal lath to fit snugly over the top and sides of the curb. Metal lath can be purchased from tile supply stores, masonry supplies, and from home centers and lumber yards. It is sold in sheets that measure 26 in. wide by 8 feet in length. You will need only one sheet to complete the curb. Tin snips are needed to cut the lath. You will also need a hammer or a stapler, a flat masonry trowel, a pointer or margin trowel, a straight-edge (which can be a 1×4 board) and a level. A helper is a definite asset.
It is easy to cut yourself quite severely on metal lath, but I consider a little blood on the floor to be the true mark of a tile setter or “mud man.” Since this is your first time, however, no one will laugh at you if you wear gloves.
Working across the sheet of lath, cut a piece that when properly shaped will extend up the front of the curb, across the top and to within an inch of the floor on the inside of the shower. Using your straight-edge as a guide, bend the lath into the shape of a “U” so that it will fit snuggly over the curb and hug the curb’s sides. Over-bending will ensure the snug fit.
Carefully ease the bent lath down over the shower curb, being very careful not to snag the cut ends of the lath on the pan material (PVC or CPE). Your curb may be longer than the 26 inches the lath covers, so you might have to cut and shape another piece. (No, that is not blood on the floor.)
Install the second shaped piece of lath to overlap the first piece by at least two inches. More won’t hurt. Remember that the inside section of the lath must remain tight against the shower pan material of its own accord.
When you have covered the curb from end to end you can attach the lath with roofing nails, drywall nails or staples. Nail ONLY through the front (outside) of the curb. When the lath has been secured to the front of the curb and you are satisfied with its shape, you are ready to begin the mortar (mud) process. (Playing in the mud is my favorite part.)
The mortar you’ll be using is the same as that used to lay bricks, except that you’ll mix it a little stiffer than bricklayers do. You can purchase pre-sacked mortar at home centers and building supplies, or you can make your own from masonry cement and sand. Masonry cement, sometimes called masonry mix, is portland cement with lime added. The lime gives the cement “cling.”
It will be much easier for you to buy the pre-mixed mortar (with sand) and just add water. A sack or two will do the curb, depending on its size. Mortar can be mixed in a wheelbarrow, using a garden hoe or a shovel to do the mixing. Small amounts can also be mixed in a bucket, although that is definitely not the preferred way to do it.
Mix your mortar thoroughly, let is rest (slake) for five to ten minutes, and then re-mix it.
Using your margin trowel or pointer, spread about an inch of mortar on the top of the curb. It doesn’t have to be neat or level; just get it on there. The mortar will be used to steady the straight-edge as you complete the inside and outside surfaces of the curb.
Now, lay the straight-edge on top of the mud and push it down so that suction is created. You will be mudding the front/outside of the curb first, so align the front edge of the board accordingly.
If your shower has a doorway, you will want the front of the curb to agree with the shower walls. Remember to account for the thickness of the tiles that will be installed on the curb later.
If your shower is open and has no doorway, you will want to align the curb with the back wall. You can measure from the back wall to both ends of the straight-edge to get the board parallel. Set your straight-edge so that the front of the curb will be about 3/4 in. thick. This is not critical and will depend on whether your shower has a doorway that needs to be considered. Less than 1/2 in. of mortar is not advisable, though.
Push the mortar firmly into the front of the curb. Fill the entire length under the straight-edge.
You can strike the excess mortar off by scraping it with the edge of the flat trowel. Don’t try to take it all at once, since this might cause the mortar to pull away from the lath. Holding the butt of the trowel against the floor will aid you in keeping the side of the curb “plumb” or vertical.
Just take your time and scrape gently.
This is not the final shaping of the curb. That will occur after the mortar has had a chance to partially set, so you don’t have to do a perfect job at this time.
When one side of the curb has been completed, slide the board to the other side, get it parallel and repeat the procedure. In showers with doorways (jambs), the width of the curb will agree with the width of the jamb walls alongside the opening. On open showers, the curb will be about 5 in. wide “in the rough” — without tile.
When both sides have been completed, you can slide the board off the top and take a break. Let the mortar begin to get hard. This might take an hour or so. If the temperature is extremely hot, it’ll only take a half hour to forty-five minutes. During this period you can work on other parts of the shower, or you can just sit there and admire your handiwork. You, my friend, are now a “mud man,” regardless of your sex. Congratulations!
I would not advise heading down to the corner pub at this time. Been there. Done that.
When the mortar has partially set, you can fine tune the rough curb by scraping it with your straight-edge. You can also lay the straightedge on top again and slice bits of mortar off with the flat trowel or pointer. Be very careful not to scrape along the shower pan material inside the shower. Make sure both sides are parallel with one another and that the curb is aligned properly, either with the back wall or with both sides of the doorway. Use your level to get the sides of the curb plumb.
You need not worry about the top of the curb at this time. You will tile the sides and then deal with the top by bonding additional mortar to it. Some tile setters do complete the top of the curb. However, this entails the use of more than one straight-edge and usually the use of more than two sets of hands. It’s really not worth it.
You can now complete your shower walls while the curb hardens, or you can begin tiling the curb itself. I usually do the curb after the walls, just to keep from kicking the curb tiles loose while I’m working.
Set the inside of the curb first. Use a board to hold the tiles up while they set. Level the board and shim it before you begin. Butter thin set on the backs of the individual pieces of tile, and press them firmly against the side of the curb. Adjustments can be made by shimming individual pieces of tile as you go. Scraps of cardboard, tile spacers, slivers of wood and plastic wedges can be used to support the tiles and keep them aligned. Lay the level along the tops of the tiles frequently to make sure everything is straight and level from one end of the curb to the other. The level can also be used as a straight-edge to check the “in and out” straightness of the tiles. When you are satisfied with the inside, go to the outside and continue tiling.
Shower curbs are not level from inside to outside. The top of the curb is tilted slightly toward the inside of the shower. This allows water to run back in the shower instead of out on the bathroom floor. A typical curb will be sloped about an eighth of an inch, which represents about a half a bubble on the level. You will install the tiles on the front of your curb to allow for this slope or “pitch.”
Install the tiles to the outside of the curb in the same manner as the inside. Level, shim, and align the tiles, and keep them parallel with the inside tiles. Remember that the outside tiles are approximately an eighth of an inch (half a bubble) higher than the inside row. I cannot over emphasize the importance of this issue. It has been the undoing of more than one shower job.
Now that both sides of the curb are tiled, you can use the top edge of the tiled surfaces to guide you as you smooth off the top of the curb. Smear a little thin set on the dry mortar and add more mortar to the curb, raking the new mortar off flush with the tops of the sides. If you are going to be using radius bullnose for the top of your curb, you will need to raise the mud about a half inch above the tops of the tiles. You can hold half-inch pieces of wood on top of the tiled edges to gauge the mortar. (The radius mud cap curb is covered in an addendum to this article.)
Completing the top of the curb should now be relatively simple. Apply thin set to the backs of the bullnose pieces and set them into place. Use your level to keep things straight and true. Masking tape aids in holding the pieces in place while they set.
All that is needed now is a little grout, but there’s no hurry for that. Let’s head for the pub.
Note: Our curb was constructed using 2×4 framing members (timbers) stacked one on top of the other to about 3 high. The core of shower curbs can also be constructed from bricks and/or pieces of concrete blocks (CMUs). The pan material is brought up and over the rough curb in the manner depicted above, except that it can’t be fastened at all. The pre-bent lath holds the material in place while the curb is further “roughed in.”
If you like, proceed to Radius Shower Curbs