Lethe River of Forgetting

Backers for Outdoor Garden Mosaics

Outdoor mosaics must be made on concrete or stone or masonry, but that doesn’t mean you have to pour a concrete slab or do some other form of heavy construction.

Flagstones (flat paving stones) and concrete stepping stones are readily available at building material stores and lawn and garden centers. The flagstones are great if you want a natural irregular shapes, and the molded stepping stones are great for square and rectangular shapes.

Of course, thinset mortar must be used to attach the tiles, but that isn’t difficult to do, especially if you lay your mosaic up in advance on mosaic mounting tape or mounting paper.

If that seems complicated, it isn’t. I wrote some instructions for using packing tape and contact paper to lay up a mosaic design.

The last section of this article explains why you shouldn’t use plywood, Hardibacker, or (sometimes) even concrete backer board.

Flagstone Mosaic Backer
Flagstones for use as mosaic backers are available at lawn and garden centers. The “Lethe” mosaic pictured at the beginning of this article was made on a similar piece of flagstone.


Flat irregularly-shaped flagstones are perfect for making a mosaic “relic” such as fragment of an ancient Roman floor mosaic.

The only potential problem is for the stone to break if it is very large. This problem can be solved by cementing a piece of expanded metal or several plies of hardware cloth (metal mesh) to the backside of the flagstone.

You can even laminate several flagstones together to make a thicker “tombstone” that could stand vertically without breaking. This would be done simply by putting thinset mortar between them to make a sandwich. If possible, I would include a few plies of hardware cloth in that sandwich, but the thinset by itself is probably strong enough.

Flagstone Reinforced With Expanded Metal
Flagstone reinforced with expanded metal on the back side. Note that this would need a second coat of thinset mortar to totally encase the steel to prevent rusting. I would probably cover the entire stone in thinset to make the color consistent.

Tip for selecting flagstones:

Avoid thinner stones and types of stone that tend to break easily. (Of course that means slate is probably a poor choice.) You can get a feel for what varieties break easily by paying attention to the piles of flagstone at the lawn and garden center.

Mosaic Stepping Stone Bases
Mosaic Stepping Stone Bases made from concrete are available at building material stores such as Lowes and Home Depot.

Stepping Stone Bases

If you prefer a square base with straight edges, these are perfect, although they also come in circles and rectangles and other shapes. I prefer square because you can make different mosaics on different stones and then put them together in a “crazy quilt” pattern similar to what Victor Kobayashi did for his mosaic patio.

Mosaic Stepping Stone Patio
Mosaic Stepping Stone Patio by Victor Kobayashi. Each square in the quilt was its own design made at leisure. In this way, the project advanced as time allowed instead of being one master project executed on a schedule with contractors and little opportunity for real art.
Cindy's Garden Mosaic
Cindy’s Garden Mosaic makes great use of contrasting colors in intense hues.

A Case Study

Cindy from Wisconsin made this very colorful mosaic for her garden using a piece of Hardibacker as the base. This presents two problems: The vulnerability of the Hardibacker to mold and moisture being the first problem, and how to support the mosaic.

One way to minimize the risks of mold and freezing rain would be to seal the sides and back with multiple applications of a tile and grout sealer, but outdoor paint might be even more effective since paint is an actual coating. I would also install the mosaic in a way that allowed moisture to drain away from the backer.

There are many ways to install and support a mosaic such as this, and the simplest way might be merely to lean it against a fence with the bottom of the mosaic resting on a couple of bricks to ensure that it wasn’t sitting in damp soil. Other options include stacking bricks or fieldstone to form a support the mosaic could lean against.

BUT, all of these would require some clever use of vegetation or stones or something to make the mosaic look integrated with the garden and less like a board with nothing behind it. That is why I prefer flagstones as bases for steles and “tombstones” and other freestanding vertical mosaics.

What Not To Use And Why

Plywood, Hardibacker, and even concrete backer board each have problems that prevent them from being used as backers for outdoor mosaics.

Plywood cannot be sealed well enough for it to be used as a backer for an outdoor mosaic. Humidity will eventually make it warp and crack grout and pop tiles off. By eventually, I mean surprisingly soon unless you live in a desert.

Hardibacker is great for indoors, but it does contain cellulose fibers that would be vulnerable to mold and moisture wicking. (When installed indoors, the sides and bottoms aren’t exposed to moisture, and that is why Hardibacker is a great backer for inside tile work but not for outdoors.)

Regular concrete backer board is durable outdoors if sealed to prevent penetration by freezing rain, but its edges are crumbly, and it will sag and warp if not supported.







50 responses to “Backers for Outdoor Garden Mosaics”

  1. Jane Avatar

    Hi Joe ,
    Thanks for all the great advice … I’ve enjoyed getting your emails!
    I use fiberglass forms with a 100% silicone glue and a silicone grout. It is hard to apply the grout , but it allows for a durable outdoor sculpture without the weight of concrete.
    Just a thought!

    1. Joe Moorman Avatar
      Joe Moorman


      Thanks for letting us know. I’m sure your comment will definitely help people who want to use silicone. You did point out something that I think is key: If you use silicone to attach tiles, then the “grout” that you use needs to be silicone and not traditional grout. The reason is simple: traditional grout isn’t flexible and will crack when the silicone used to mount the tile allows the tile to move by a tiny amount. I suppose the fiberglass base would be prone to flexing a tiny amount, and so a silicone “grout” probably makes more sense for those.


  2. Rachael Avatar

    Thanks for all of the information! What backer would you recommend for house numbers – I was going to use concrete board and seal, but now I worry about the edges. Do you have a recommendation? Many thanks!

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Rachel,

      I would cut them from the Schluter Kerdi-Board using a jigsaw.

  3. peg Avatar

    Can I use porcelain tiles for a base for a rock mosaic?

    1. Joe Moorman Avatar
      Joe Moorman

      Yes, you can use a large 12 inch or 18 inch porcelain tile as a base for an outdoor mosaic. The only issue might be the tile breaking and producing a sharp edge, and so you would want to make sure the mosaic was placed on level soil so that it was supported equally and didn’t beak at some unsupported place.

  4. Paul Avatar

    I want to make an outdoor mosaic about 1200mm x 900mm that will fit on the side of a timber structure.

    I was going to use some concrete backer board but you seem to suggest it might blow out on the edges.

    What would you suggest to use?

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Paul,

      I have used concrete backer board and reinforced the edges with thinset mortar. I used two coats applied with two days in between to create a hard shoulder around the backer board. You could also have a welder fabricate a simple frame from 3/4″ angle iron. You could also use light weight Wedi board or Schluter-KERDI-BOARD, but you would need to make some sort of frame or shoulder for these as well.

      I hope this helps,

      1. Paul Avatar

        Many thanks Joe, I’ll give the thinnest a try first.

        Also, on a slightly different note, I’ve read that you shouldn’t use standard ceramic tiles outdoors as the frost could damage them but I’m having trouble finding exterior grade tiles in a decent range of colours. What can I use that will look vibrant but also last longer than a couple of years?

        Thanks again

        1. Joe Moorman Avatar
          Joe Moorman

          Hi Paul,

          Glass works well outside, and porcelain does too. We are launching a new line of polished porcelain in the next few days.

          I hope this helps!

      2. Pranoti Avatar

        Hello, I want to do something similar.. can you share any pics about how to add the thinset shoulder to the board.

        1. Joe Moorman Avatar
          Joe Moorman

          That is a last-resort option. It’s better to use that same mortar to attach some long strip or series of strips that act as a frame.

    2. Claire Lemmon Avatar
      Claire Lemmon

      Hi Joe
      Thanks so much for sharing your expertise! I am working with my daughter’s school on a series of large exterior mosaics – they were originally going to be put straight onto the wall, but now, due to a variety of reasons, they want to mount them on sheets which will then be screwed into the wall…I’ve agreed to research this part of the process and wondered what your advice might be? The wall is 6 by 3 metres, so each backer would need to be approx 1.5 metres by 3 metres…..I am assuming that the concrete backer you suggest above, might work but wanted to double check, as the panels will be large. With many thanks!
      P/S the school has very little money for the project, so the idea of creating iron frames is unfortunately, not possible.

      1. Brittani A Avatar
        Brittani A

        Hi Joe! I’m planning a similar project and was wondering if you can share what you ended up using for the backing and anything helpful that you learned in the process, I’m a first timer. Thank you in advance!

        1. Rachele Avatar

          Hello, I was wondering if any of you two received an answer. I am starting a very similar project and I am having a difficult time finding a solution. Thank you so much in advance for any feedback.

          1. Joe Moorman Avatar
            Joe Moorman

            Emailing is always the fastest way to get an answer from us. The pandemic is making it difficult to monitor comments.

  5. Mercy Avatar

    I have a wrought iron table which I would like to cover with mosaic….. this is my first time trying to do mosaic. The top has small holes due to rust…. Could you help me? Thank you

    1. Joe Moorman Avatar
      Joe Moorman

      You would definitely want to replace the top before investing the time and materials in a mosaic. Check with a local welder if you are committed to that particular table.

  6. Lanie Spoljaric Avatar
    Lanie Spoljaric

    Hello! I want to make a mosaic bench with a groups of kids to donate to their school. I had originally thought that we would pour concrete into a mold, but would you recommend something easier??

    We are looking to mosaic a bench that could fit about 4 kids, so maybe 5 feet long….

    I would really love to help the kids do this, so any tips you have would be most helpful!

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Lanie,
      The best approach for a durable bench would be to buy a concrete bench, lay up the tile on mounting tape, and then mount the tiles using thinset mortar.
      Here are my instructions for laying up a mosaic design using mounting tape.
      Here are my instructions for using thinset mortar.
      I hope this helps.

  7. Cindy Avatar

    Hello! My first project will be a free-standing house sign, about 18″x24″ for use outdoors in the Pacific Northwest (dampness and occasional light frost). In reading entries above, I’ve learned about KERDI-BOARD, Wedi Board, and that use of standard ceramic tile may not be advisable? We’ll likely build a frame from cedar. Your thoughts? Thank you!

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Cindy,
      Since you aren’t getting really hard freezes, the ceramic mosaic might be durable if sealed with multiple applications of a tile and grout sealer. But, freezing temps normally destroy ceramic tile fairly rapidly.

  8. Guy Nason Avatar
    Guy Nason

    I’m looking to make a planter box. What would you say is the best way to make the planter box

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Guy,

      Whether you use brick, stone, or concrete blocks, make sure you use type S mortar to make it water tight.


  9. Fran Avatar

    Greetings! I am planning to make a few mosaic totems for my yard using 6in pvc pipe. I would like to use glass on the first one, since I have a lot thanks to a stained glass artist friend. Am interested in any tips you might have, particularly adhesives and cutting so glass has not sharp edges sticking out. I live in coastal California, hence lots of sun, some fog, little rain. Thanks, Fran

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Fran,

      Conventional mosaic adhesives and thinset do not adhere to plastics such as PVC. What I would do is encase the PVC pipe in a “sleeve” by wrapping it with fiberglass mesh and thinset. I would scuff the PVC pipe with 80 or 50-grit sandpaper (or pieces torn from a belt sander’s belt because that lasts a lot longer) and tightly bind the mesh to the pipe with baling wire before plastering on a couple of coats of thinset. That should make a durable base for the sculpture that is still relatively lightweight. I would probably leave the bottom 2 feet of the PVC pipe bare so I could install the finished totem pole in a hole.

      You would use thinset mortar to attach the tiles or stained glass, and I would use the Diamond File to smooth any sharp edges.

      Please send us pics!

  10. Pranoti Avatar

    Hello Joe,

    I have been running in circles to find the right kind of backer and sealing the edges for a project I want to make. I came across your blog and feeling that I landed in the right place finally.
    I want to make a shell and tile mosaic wall in my backyard. I already have a wooden retaining wall that I can use as support but not sure what backer to use and how to seal the backer from all sides from rain and shine. I live in Georgia so we get plenty rain. I didn’t know how I could send you a picture of the wall. It’s a backyard retaining wall that I want to cover.

    1. Joe Moorman Avatar
      Joe Moorman

      Wood is not an acceptable backer for outdoor and wet mosaic. and so would make a reinforces concrete slab in a mold and lean it up against the retaining wall. I would mount the mosaic to that slab using thinset mortar.

      Thinset is easy to use if you lay up the mosaic so that it is face-mounted on Mosaic Tape:

      You use this method to lay it up on tape:

      The product description of the Notched Trowel explains how to spread the right amount of thinset:https://mosaicartsupply.com/shop/stainless-steel-notched-trowel-for-thinset/

  11. Marie Avatar

    Hello Joe! I have read your recommendation for using thinset for outdoor mosaics. I have 3 large rusted metal disks (old farm equipment) that i would like to mosaic with glass tiles, and that i would like to install outside. Is thinset the only adhesive that will work or could i use waterproof epoxy?
    Thank you so much for your help.

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Marie,
      I think that epoxy would be the best. I would take a rotary wire brush and scour to bare steel in the place where the mosaic will go.
      I hope this helps!

  12. Jennie Underhill Avatar
    Jennie Underhill

    Hi Joe,
    I am wanting to mosaic the top of three wrought iron glass topped tables but want them for outside. Can I mosaic to glass ? I was planning to use wet room grout as adhesive and special outdoor mosaic black grout to seal. They are a nest of side tables, so not that big.
    any advice would be greatly appreciated.

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Jennie,

      Outdoor mosaic are made using thinset mortar not adhesive. You could make tops for the tables solid bases for mosaic with a couple of successive layers of fiberglass mesh and thinset wrapped around foam-core board, but you would have to make sure you aren’t overloading the metal legs and supports. Or making the table top heavy. Keep the weight down! Tip: Don’t leave wrinkles in the mesh. Wrinkles mean the mortar will have to be thicker to cover them and level the surface. Here are instructions for making your own foam core tile backers with mortar and mesh.

      1. Pranoti Savadi Avatar
        Pranoti Savadi

        Hello Joe, liked the article you shared for making outdoor backer board with thinset mortar and fiberglass mesh. I have been planning to make an outdoor mosaic with the shells I have collected over the years !!!
        I was looking for a backer board that I could mount on my wooden retaining wall and that would be waterproof from all sides and hold the weight of mortar and shells.
        Please advice if the described process here-https://blog.mosaicartsupply.com/making-foam-core-mosaic-backers-for-outdoor-projects/ will be good for me. Do I need to change anything to fit my requirements.

  13. Kay Wheeler Avatar
    Kay Wheeler

    Hi, I am trying to mosaic a shovel handle, but having trouble with The curves. At present I am using gorilla glue, then holding in place until it sets as you can imagine this is a very slow process.
    Any suggestions

    1. Joe Moorman Avatar
      Joe Moorman

      You might apply the glue and let it get a little tacky before pressing in the tile, but I should state that I don’t use that type of glue because it isn’t archival, and so I’m not familiar with how it cures.

  14. Frank Cote Avatar
    Frank Cote

    I’ve read about all the issues with plywood backers boards. What I want to do is make a Mosaic table for my wife. I can buy the legs but should I pour a concrete top for the table then Mosaic over that?

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Frank,

      Given that there are products like foam-core backer boards and concrete backer boards, I don’t think it would be necessary or desirable to pour a thin slab of concrete and try to use that. The weight would be excessive.

      1. Frank Cote Avatar
        Frank Cote

        Thanks Joe, but it’s my understanding that the foam core boards and backer boards need to be supported by something.
        So what do you recommend?
        Would a couple layers of backer board bonded with thinset be strong enough? I never tried that but I always wondered.


        1. Joe Moorman Avatar
          Joe Moorman

          Hi Frank,
          In most situations, you would need a frame or something. For one thing, you want the legs to anchored to the top securely and not hinge when a transverse force is applied to the edge of the table, such as someone pushing the table to slide it over the ground. All the normal structural concerns of tables still apply. The mosaic only makes them more relevant due to the extra weight.
          I hope this helps some.

  15. Aly Avatar

    I’m curious if a porcelain tile sealed before and after making the mosaic would work? I would be using them more as garden decor rather than stepping stones, propped up and or hanging in my garden.

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Aly,

      That sounds good. Never seal any surface that needs adhesive or glue to adhere.

  16. Mark Schade Avatar
    Mark Schade

    Planning to use concrete board. I plan to use thin set with glass tile then grout. I will seal the grout. This will cover all edges, save the edge that is against the brick wall. Would I want to seal the concrete board on the backside?

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Mark,
      That might be advisable if you live somewhere with hard freezes.

  17. Joslyn Fritz Avatar
    Joslyn Fritz

    Can you assist me with the following questions, please? 1. Some tesserae (mirror and vitreous tile) have fallen off of the exterior backer board edges on two projects. Is there anything that I can try to attach those tesserae back to these soft edges? The hard shoulder option is not a possibility now, right? I did not know about creating hard shoulders around the edges until now but will definitely do so for my next project. I live in a harsh climate (Saskatoon, Saskatchewan) but only put them out for the summer. The Thinset worked fine for the frypans I covered and the metal water can I did. 2. About Thinset, does it do bad? Should I be buying a big bag of new stuff annually?

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Joslyn,
      I’m not certain what you mean by soft shoulder, but I think you are referring to the side edges of the backer and not the front of the backer. If that backer is plywood, I’m not sure anything will be a lasting fix. I have kept thinset for years sealed in a plastic bucket, but artists report that leaving a bag in a humid environment without AC can cause the thinset to go bad in less than a year.

      1. Joslyn F Avatar
        Joslyn F

        Thank you, Joe. You are correct, I am referring to the side edges of the cement backer. My husband helped me build wooden frames for each project (4) by drilling through the cement backer (idea from a YouTube video I found) so that we can attach the wire to hang these heavy pieces. I’m trying to salvage those 2 mosaics and removed as much as I could from the raw edges of the cement backer. I put a layer of thinset and am going to do a thin second layer of it tonight. My plan is then to reattach the tiles that fell off and that I pulled off (vitreous on one and mirror on the other). I want the edges to look as pretty as the front of the mosaic looks!

        I appreciate what you shared. I am a novice having fun playing with beautiful tesserae of all types, experimenting with the way lines of tiles flow around the edges of images, learning how to cut glass (a skill that must take lots of practice), etc. I have no dedicated space for my work but make do using part of our laundry room, plus the garage in the summer. One day I will take a class to learn more about the technical aspects of this art. For now, I admire my creations and dream of going to Ravenna one day to take a class there like my friend did. She has lived abroad for many years and like me, appreciates street art wherever she goes. I am grateful that she introduced me to this ancient art when she lived in little old Saskatoon.

  18. Francesca Monachino Avatar
    Francesca Monachino

    Hello – – I have some colored stones – different, small sizes, and pieces from broken plates, mugs. I was thinking of making stepping stones or decorative plaques for outside. ( I liked the example of the ‘quilt’ look of different stepping stones.) I have leftover 2 x 8 inch tiles used for a bathroom floor AND 2 x 6 inch tiles from a backsplash. Questions: Could I use the 2 x 8 inch floor tiles for backing for outdoor stepping project? Does it matter if i use shining side or bottom side? Should I just get the concrete from HD or Lowes? Also, different project, could i use 2 x 6 inch back splash tiles for indoor trivets, etc for backing with glass mosaic pieces -(not for stepping on). ( lots of projects – just getting started) Thank you Francesca

    1. Natalija Moss Avatar
      Natalija Moss

      2″ x 8″ seems small (narrow) for a stepping stone. If you use them, definitely mosaic to the rough back side. Make sure it’s bare and dry — no mold release or dirt or dust.

      For mosaic trivet backers I jigsawed some plywood into circles and squares. I think you want them to fit the underside of a pot, right? 2″ x 6″ is going to be too small for those.

      I would use “thinset mortar” instead of normal concrete. Thinset is adhesive. We’ve used both HD’s “Versabond” brand and Lowe’s “Mapei” brand with good results. Ace or Home Hardware probably have good thinset also. We don’t currently recommend one brand over another.

      Please e-mail us at help@mosaicartsupply.com with some photos of the project, or if you have additional questions!

      1. francesca Avatar

        Thank you for your help. You were correct. I was mostly considering the size and shape of the design not the use ( ie size of pot or step)


    I am a beginner with mosaic. I want to mosaic my new metal mailbox with 1/2″ x 1/2 x 1/8″ recycled glass tiles that I bought from your website. Because of the curved shape of the mailbox (standard type) I imagine using the mosaic mounting tape is the way to go. Any recommendations you could give would be greatly appreciated.

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