Cement Mortar Shower Curbs (using mud cap/radius bullnose -- A-4200)
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John P. Bridge, January 20, 2002
The topic here is shower curbs where standard wall tile trims are to be used. The most common trim is called a mud cap, a radius bullnose or, technically speaking, "A-4200." If this doesn't make a lot of sense to you it's because this is an extension of a previous article that explains what we are talking about, and you've somehow wandered in here without having gone there. You might want to have a look at the other article before you continue.
Shower curbs using standard wall tiles and their trim pieces have been around quite a while, and their appearance will surprise no one. Usually, the curb is wide enough across the top to include the trims, either bullnose or quarter-round (A-106) and a cut piece of tile between them. What I have done is eliminate that cut piece of tile. The curb depicted here is made by placing two mud cap pieces back-to-back. It is a narrow and "trim" looking curb, one that allows the maximum of usable space inside the shower.
The core of the curb is built similarly to the one covered previously, except that it contains only one 2x4 or 2x6 wood member (timber) laying on its edge. The shower pan material is then wrapped over its top and attached to the front of the member. The metal lath is shaped and attached and the curb is mudded. The top of the rough curb, after the mud is applied, is 3-3/8 inches across. With tiles attached to the front and back sides, the curb will measure 4 inches across.
In the photos above the curb has been formed and the sides have been tiled and leveled. The next step will be to add additional mortar to the top.
Thin set mortar is smeared on top of the rough curb to ensure that the new mortar will bond properly.
1/2 in. thick sticks are held above the tiles at the front and back sides. They will be used as guides when striking the excess mud away. The sticks in these photos happen to be discarded pieces of wood base shoe (quarter-round), but anything that is 1/2 in. thick can be used. The mortar is packed between the guides and then struck off with a flat trowel or a short straightedge, leaving the mortar 1/2 in. above the tops of the tiled sides.
The mud caps are now installed. Back butter thin set mortar onto the back of each piece and tap it into the mortar. Tap gently with the handle of your trowel, aligning the pieces with each other as you go. Try not to set the pieces too low into the mortar. If you have to remove a piece, the mortar will separate and come up with the piece. It will then be necessary to apply thin set to the mortar on the back of the trim piece and re-install it.
All the pieces have been installed and aligned, and we are ready for grout.
All that is needed now is someone to clean the bathroom floor where the grubby tile setter made his mess. Guess who that person will be.
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© Copyright 2002 by John P. Bridge
