Mosaic Tile Backers
What to use for a backer is determined by how the mosaic will be displayed. For example, you could use a piece
of sanded plywood as a backer for a small mosaic plaque or trivet, but you could NOT use plywood as backer for
a mosaic floor because wood is compressible. Similarly, you could NOT use plywood as a backer for an outdoor
mosaic or wet mosaic because plywood warps over time due to changes in humidity.
We have some additional recommendations for choosing a backer based on
how the mosaic will be displayed.
Concrete Backer Board
The best all-around mosaic backer is concrete backer board, such as used in bathrooms as the subfloor for tile floors.
Concrete backer board is cheap, cuts easily, comes in 1/4" and 1/2" thick sheets and is available at most any building
material store. Drywall can be removed easily and replaced with a sheet of concrete backer board. The 1/2" thickness
is recommended for floors, and we only use the 1/4" size when laminating it to plywood.
Durable, Strong, Inflexible and Clean
Mosaic backings should be durable, strong, inflexible and clean.
Durable means resistant to water and moisture. Over the years, humidity in the air can turn plywood and particle board into
splinters. Long before this happens, the tiles will begin popping off as the plywood warps. MDF and particle board swell and
disintegrate with humidity as well. Avoid using wood products of any type outdoors. If you have to use wood outdoors, then use marine
plywood, and make sure you paint the undersides and side edges with multiple coats of exterior paint and seal the finished mosaic.
All that being said, plywood can be used for dry indoor mosaics such as small plaques, picture frames and mirrors. If you make
an indoor mosaic picture on plywood larger than 2 feet wide, then you should still paint the underside and sides to keep humidity from
warping the plywood over time.
Mosaic materials are heavy, no matter if you are using stone, ceramic or glass mosaic tile. Grout is concrete. The weight of even one
square foot of mosaic can be heavy. This means that not only your backer must be strong but also your points of attachment. (Never hang
a mosaic from a single nail like a painting.) Mosaics made on thin panes of glass are also not recommended, especially glass table
Glass table tops can sometimes be replaced with a sheet of concrete backer board if the table is strong enough to support the
weight. (Never create a safety problem by gluing heavy mosaic materials to a glass table top.) The table should also
have a wide enough base so that the mosaic doesn't make it "top heavy" or otherwise unstable. The table should also
support the concrete backer in the center and not just on the edges. If your table does not have support in the center,
consider bolting or welding a piece of angle-iron across the center.
Grout and mosaic tile are inflexible. If they are mounted to a backer that flexes even slightly over time, the grout and tile will crack
and pop off. Mosaic backings for floors should also be incompressible. This is why concrete backer board is used beneath tile floors instead
Note that plastic and thin sheet metal fail all three of the above criteria, but especially the requirement to be inflexible. Plastic and sheet metal
also pose additional problems with bonding securely to glue and grout.
Walls and floors made from concrete, stone or masonry are obvious choices for mosaics, but even these ready-made backers should be scoured
to remove paint, sealants and dirt, and you may need to plaster them smooth with mortar before starting the mosaic.
It never hurts to take a wire brush and scour a surface before you attach tiles to it. Nothing is worse than putting days or weeks into a project
only to have it not hold up very well. It only takes a little dust or a little grease or a little paint to cause tiles to pop off within a few years
or even months. How do you know that concrete wall doesn't have an invisible pore sealer on it? You don't. Take a wire brush and scour it!
If your surface is large, such as for a mural, then use a power tool called an angle grinder with a rotary wire brush. (Wear gloves and a face shield,
especially if you haven't used one before, because things like angle grinders and belt sanders can take the hide right off you.) You can rent power
tools at places like Home Depot, or you can ask your favorite handyman or handywoman to scour the wall for you. The point is that this type of surface
prep can be done with not much effort and is worth doing because it might determine whether or not your mosaic lasts more than a year or two.