11 Reasons Your Mosaic Backer Decisions Are Critical

How do you decide to use the mosaic backers or bases that you use? In just about all cases, the durability and often the aesthetics of a mosaic are dependent on the decisions you make about which backer to use and how you prepare it. Here are the reasons why your choices are critical:

  1. Moisture will degrade a backer. You need to avoid moisture. Moisture is good for your skin and plants, but not for your mosaics. If the mosaic is a practice piece that doesn't need to be durable, moisture may not matter. However, moisture movement in and out of backers will wreak havoc on both the backer and the tiles it supports. For some materials like wood, moisture will lead to decay. It also leads to expansion and contraction that ultimately break adhesive bonds and grout and will allow even more moisture issues.
  2. Warping of a backer will break adhesives, grout and maybe tiles. Materials that are susceptible to warping need to be prepared before being tiled. That may be as simple as sealing the backer with a glue-water wash on all sides and edges. And it may be as complex as adding structural supports that eliminate the possibility.
  3. Flexing of a backer may immediately pop tiles and, if not, will eventually cause adhesives and grout to break down. Rigidity has to be achieved either through structural supports, thicker material, coating the backer with rigid material like thinset, or different backer materials.
  4. Adhesion of tiles to a backer can be compromised if the backer is incompatible with the chosen adhesive. In some cases, the backer may not adhere to the chosen adhesive i.e. thinset and metal, and in other cases, the backer may be degraded by the adhesive i.e. certain foams with certain glues.
  5. Hanging and installation requirements may be unachievable if they are not installed before tiles are applied. In some cases, a solution may be removal of select tiles, installation of the hanging system, and replacement of tiles.
  6. Dimensional requirements may be unachievable or require an expensive alternative. This probably occurs most often when a piece is to be framed and the sum of tile thickness and backer thickness is greater than the rabbet in the frame molding can accept. In that case, the edge of the backer will be visible behind the frame. Other potential problems occur with back-splashes, floors, and walls that need to have the same thicknesses as the surrounding surfaces.
  7. Structural movement and vibration can degrade adhesives, grouts and tiles. Thermal expansion and contraction are common, if not certain, for exterior walls. Both floors and walls are subject to shifting soils and may be subject to vibrations. If a mosaic is applied directly to the building it becomes subject to the same movements. Mosaics can be separated from these forces by using an uncoupling system if the decision is made early on.
  8. Manipulation of a backer's form, shape and size are affected by the material chosen. Choosing a material that is more difficult to manipulate may not prevent you from getting the job done but can make the job much more difficult than necessary. For example, if a project needs a moisture resistant backer, both concrete board and foam core boards may be acceptable, but cutting concrete board is significantly more difficult.
  9. Weight can be significant with mosaics and sometimes leads to big problems. Options for managing the weight of a mosaic can include the weight of the backer material, making the mosaic in sections (even 1 tile at a time), or putting the mosaic on mesh and applying it to its permanent substrate.
  10. Exposed edges of the backer may not support the chosen aesthetic. This probably occurs more with found objects i.e. tables with rounded edges. Once the tile is applied, making a modification to the edges may not be achievable, so the treatment of edges should be considered before the mosaic is applied.
  11. The backer may be a part of the aesthetic you are creating. Examples include free-form shapes, 3-dimensional backers, found objects, walls and floors.


  • Thank you so much…AGAIN. This info is exactly what I’m researching for a huge exterior mosaic.

    I’m talking with the tech guys at the foam substrate companies. I need reassurance before I leap. You’ve been very helpful. I appreciate you sharing your deep knowledge of mosaics to keep the rest of us out of trouble. You’re a gift!

  • I mosaics are done in a cement stuff very informative

  • You are such an awesome instructor!!!

  • Hi Anna! Thanks for your question below. For foam core backers, there are specific hanging kits you can buy that won’t pull through. They are here: https://skeew.biz/collections/mosaic-hanging-systems/products/mosaic-hanger-skeewbacker-qin – I don’t know about a hard foam board for wall insulation; but it’s possible these hangers would work there as well. Never tried that board. We have several “How To’s” on our foam core boards here: https://skeew.biz/blogs/how-tos?page=4 – hope you’ll check them out.

    Lou Ann Weeks
  • Learned so much.. I have a question— If I used a hard foam board like for wall insulation.. how could I accommodate the hanging system? Should I coat it first with quickset so I doesn’t rip thru the foam??
    Do you have a lesson on using foam boards ?
    Thank you for sharing your knowledge
    you’re a great teacher 🌹


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