Most homeowners expect cement board to be the final surface before tile installation begins. home owner Britney Smarts A bathroom remodel features one more step that replaces the entire shower before the first tile is set. Instead of reaching for thinset, she covers every seam, screw hole, corner, edge and plumbing opening with a thick waterproof membrane that transforms the gray cement board into a bright red surface.
Color is only temporary. Once the tile covers the walls, this waterproof layer disappears from view, although it continues to protect the shower behind each tile for years. The result looks like fresh paint, but its work has nothing to do with appearance.
Each seam receives a first coat
The waterproof membrane goes where the two cement board panels meet. The brush pushes the material into the joint before spreading it to both sides, covering one of the most common places where moisture can get behind the tile.
Rather than treating the seam as a simple gap, this step turns two separate panels into one continuous waterproof surface.
Screw holes should not be exposed
Each fastener that holds the cement board to the studs creates a small opening in the surface. It gets the same waterproof coating as the screw head seam.
By the end of this step, the wall begins to fill with bright red circles and lines that mark every place where water can get under the tile.
Small gaps are filled before the tile
Not every cement board joint closes completely. The small spaces between the panels receive another layer of waterproof membrane before the shower moves to the next stage.
A thick coating bridges shallow gaps rather than leaving them open beneath the finished tile.
A thick coating covers more than a surface
The membrane spreads well beyond visible cracks. Brushing both sides creates a larger waterproof area rather than just sealing a narrow opening.
This extensive coverage helps to tie neighboring sections into one continuous layer.
The ceiling becomes part of the waterproof system
Waterproofing continues at the sealing joint, inside corners, window edges, horizontal seams and every exposed screw head. Gray cement boards begin to disappear beneath the bright red bands that connect one section to another.
At this point, waterproofing reaches almost everywhere where two materials meet.
Window edges receive similar protection
The narrow space between the cement board and the window frame deserves careful attention before tile installation begins. The brush moves the membrane into the edge rather than stopping at the face of the wall.
Windows present more corners and transitions than flat walls, making these details important parts of the waterproof layer.
Plumbing openings remain sealed
Even the larger holes for the shower valve get a full coat around the exposed cement board. The waterproof membrane reaches the cut edge instead of ending on a flat surface.
Although the trim plate hides this area laterally, the waterproofing remains underneath.
The entire shower turns bright red
After coating every seam, screw hole, corner, edge, and opening, the shower looks completely different from the gray cement board that was installed just a short time ago. Bright red waterproofing now outlines nearly every joint on the walls.
Once the tile covers the shower, none of this work is visible. The color disappears forever, but the waterproof membrane continues to protect the wall behind the finished tile, making it one of the last hidden layers installed before the transformation is complete.
