Waterproofing ceilings is optional for showers and bathtub surrounds, but considered best
practice when tile will be installed on these surfaces. Waterproofing ceilings is required
in steam shower and steam room applications. This video demonstrates the use of the Schluter
-KERDI waterproofing membrane over solid backing. Here s how to do it.
Solid backing has been installed on the ceiling. In this case gypsum board is used.
The walls have already been made waterproof with the KERDI membrane.
Schluter -KERDI is bonded to the ceiling using unmodified thin-set mortar.
The mortar is mixed to a fairly fluid consistency, but still able to hold a notch.
A 1/4 inch x 3/16 inch V-notched trowel or the Schluter -KERDI-TROWEL, which features
a 1/8 inch x 1/8 inch square-notched design, is used to apply the mortar.
Clean any dust or other debris from the surface of the solid backing.
Dampen particularly dry and porous substrates in order to
help prevent premature drying or skinning of the thin-set mortar.
Apply unmodified thin-set mortar to the ceiling where the KERDI will be placed.
Apply a thin layer of the mortar with the flat side of the trowel to produce
solid contact with the surface and then comb additional mortar
using the notched side of the trowel. Apply the KERDI membrane to the ceiling.
Embed the KERDI in the thin-set mortar using the flat side of the trowel or
a drywall finishing knife to ensure full coverage and remove air pockets.
Lift up a corner of the membrane to check coverage. Proper installation
results in full contact between the fleece and thin-set mortar. Re-embed the membrane.
Depending on the size of the ceiling, it may be necessary to install multiple sheets of
KERDI to cover the entire surface. Waterproof integrity is maintained
at joints using 2 inch wide overlaps in conjunction with unmodified thin-set mortar.
KERDI sheets may be joined using a 2 inch wide lap seam.
Alternatively, KERDI sheets may be butted and covered with Schluter -KERDI-BAND waterproofing
strips centered over the joint. Since KERDI-BAND is one-half the
thickness of the KERDI membrane, build-up at seams is minimized using this method.
In this application, the use of KERDI-BAND will be demonstrated.
Apply unmodified thin-set mortar to the area where the banding is to be installed.
Embed the KERDI-BAND into the thin-set mortar. Waterproofing connections
Install Schluter -KERDI-KERECK preformed inside corners using unmodified thin-set mortar.
Apply the mortar to the area where the KERDI-KERECK is to be installed.
Embed the KERDI-KERECK into the thin-set mortar.
Holding one side of the corner in place while embedding the other side in the mortar helps keep
corner installation simple and easy. Repeat the process at the
remaining inside corners. The final step in waterproofing
the ceiling is to use KERDI-BAND to seal wall-to-ceiling transitions.
Measure and cut sections of the banding such that a 2 inch overlap is achieved at all junctions.
A 2 inch overlap can be achieved at KERDI-KERECK without continuing KERDI-BAND
all the way to the corners of the shower. This will limit build-up of the membrane in these
areas and make setting small tiles easier. The KERDI-BAND may be creased prior to
installation to help maintain tight corners and make subsequent tile setting as easy as possible.
Apply unmodified thin-set mortar to the area where the banding is to be installed.
Embed the KERDI-BAND into the thin-set mortar. Repeat this process at all remaining
joints ensuring that the minimum 2 inch overlap is achieved at all transitions.
Once the entire ceiling is covered with the KERDI membrane, including seams and
corners, the assembly is waterproof, vapor-tight, and ready for tile.