A.Contributing editor Michael
Byrne, a tile-setter and consultant in Los Olivos, Calif., and
moderator of JLC Online's tile forum, responds: I use the
same approach on plumbing fixtures without a tile lip as I use
for plumbing fixtures with a lip.
Liquid waterproofing membranes that meet
ANSI 118.10 specifications can be applied to the surface of
many tile substrates, including cement-based backerboard,
plywood, and concrete. Waterproofing membrane should be applied
to setting-bed edges, too, and around any plumbing that
penetrates the substrate.
First, I install the setting bed (either a backerboard or
mud-bed substrate) so that there is a gap of at least 1/4 inch
between the lower edge of the setting bed and the top of the
Next, I apply a liquid waterproofing membrane — such as
Bonsal's B-6000 (800/738-1621,
www.bonsal.com) — to
the setting bed with either a trowel or a roller.
Around plumbing that penetrates the setting bed and along the
bed's exposed bottom edge (where it faces the top of the tub),
I use a brush; the membrane helps prevent wicking in these
areas (see photos, left).
Once the membrane on the completely waterproofed setting bed
has cured, I carefully caulk the gap between the bed and the
tub with a thick bead of 50-year silicone sealant (or a
compatible sealant specified by the manufacturer of the
And finally, when I set the tile, I always leave a 1/8-inch
(minimum) movement joint where the field of tiles meets the
tub; I fill the joint with a sealant color-matched to the
By the way, this is now the same approach I use when tiling a
shower built with a mortar-bed floor and backerboard
I install the backerboards so that their bottom edges are 1/4
inch to 1/2 inch above the top of the finished mortar-bed floor
(before tiling), apply the membrane system to both walls and
floor (with a break in the membrane at the gap), and fill the
joint between the walls and floor with a bead of compatible
sealant after the membrane has cured.