Tag Archives: outdoor mosaic

Missing Tiles Detail View Mosaic Art

Why Did My Tiles Come Loose From Thinset?

Recently, artist Jackye Mills emailed me about a problem she was having with her first mosaic project, and it really caused me a lot of angst because the artwork was a strong design that was otherwise well executed. I hated the thought that a first-time mosaicist could do such a good job on something so ambitious only to lose the project due to a technical issue. Continue reading

stained glass shards in buckets

How to Make a Stepping Stone with Scrap Glass

If you work with stained glass over time you can end up with buckets full of scrap. When the pieces start getting too small and irregular, or if there’s just too much of it, you can used them in a stepping stone. This tutorial demonstrates how to make a stepping stone with an abstract pattern. You can also use tiles instead of stained glass scraps.

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Mosaic Patio Table with bench seats by Naomi Haas.

Glass or Ceramic Tile for Mosaic Patio Table?

Ceramic tile can be used for outdoor mosaic patio tables provided you live someplace warm year round, but otherwise glass tile should be used because it is impervious to moisture and freeze damage. There are other reasons to use glass tile explained later in this article.

Concrete Patio Table Set

Artist Naomi Haas recently completed a mosaic patio table set that included concrete benches, and I really liked it for several reasons. For starters, the table base and benches she had were sturdy and stable and appropriate for an outdoor mosaic (and not wood or rusted light-gauge metal or some of the other junk people email us about using).

Mosaic Bench 1 of 3

Mosaic Bench 1 of 3 uses a design of solid colors instead of being a copy of the other benches.

I also like the color scheme. Naomi colored the un-mosaiced surfaces black and used black grout to make the bright festive colors of the Mexican Talavera tile stand out. To make the concrete black, Naomi used a product called Flex Seal, but black spray paint could have been used. Another thing that draws me to this project is that Naomi made a different design for each bench instead of making them the same.

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Lisa Jones Patio Mosaic

Patio Mosaic Alternatives

Patios are excellent locations for mosaics, but the patio floor itself is not as good a surface for a mosaic as a surrounding wall or brick planter would be. The main reason is simple: metal patio furniture will crack and crush glass tile, and glass is the preferred material because it is frost proof, economical, and comes in many colors. Also, the floor of the patio is not as visible as a nearby vertical surface is likely to be, especially when furniture, grills, and the usual patio accessories are present.

Recently artist Lisa Jones emailed me some photos of her patio mosaic, and I was taken with it in spite of my preference for designs that use free-form placement of pieces of tile instead of gridded patterns of whole tile.

Lisa Jones Patio Mosaic right side

Lisa Jones Patio Mosaic right side continues with the garden theme and flower and bee motifs.

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Gaudi Mosaic Bench Freeze Damage

A few years ago, Karen J created a mosaic bench in her backyard using mining debris (large stones), cement, and chicken wire to form the base, which is similar the methods we recommend in our instructions for creating bases for outdoor mosaic sculptures. Karen modeled her bench after those made by the great mosaic architect Gaudi in Park Guell in sunny Barcelona, and she used brightly colored ceramic tile just as Gaudi had used. The problem is that Karen’s backyard is in Colorado, and so her mosaic experienced many long and hard freezes that a mosaic in Barcelona would never see.

Mosaic Bench after Antoni Gaudi

Mosaic Bench after Antoni Gaudi shows the ravages of freeze damage. Colorado winters are quite severe, but any temperature below freezing can crack and flake ceramic tile.

Ceramic Tile Is Vulnerable To Freeze Damage

Glass mosaic tile is non-porous, and so water cannot seep in and freeze and crack it, and so glass is preferred for outdoor use, as is porcelain tile for the same reason. On the other hand, ceramic tile tile is very porous and soft, and so water can penetrate it (through tiny cracks in the glazing). Once this water freezes and expands, it cracks the ceramic tile and often causes the face of the tile to flake off.

Mosaic Bench Detail showing freeze damage

Mosaic Bench Detail showing freeze damage. Note that the empty sockets in the blue tile are NOT where tile has popped off. Instead, it is where the faces of the tiles have flaked off due to water freezing and expanding in tiny cracks and pores.

In the photo above, you can see how some colors were more resistant to freeze damage than others. This difference was not due to the color but to the variety of the tile: some brands of ceramic are harder and less porous than others. Also, some brands have thicker glazes, and that can also affect how permeable the tile is to water.

Preventing Freeze Damage

You can minimize freeze damage by sealing your finished mosaic with multiple applications of a tile and grout sealer from your local building material store. Avoid ordering sealers online during winter months because water-based silicone sealers ruin if they freeze during shipment. You should also clean and reseal the mosaic each fall. Small mosaics such as mosaic stepping stones can be brought inside for the winter.

Mosaic Bench Second Detail showing freeze damage.

Mosaic Bench Second Detail showing freeze damage. Imagine how bright the orange and yellow sun was before Freeze Meister blasted it and flaked off the color!

Mosaic Bird Bath

Artist Lyn Richards sent me some pictures of the mosaic bird bath she made this past summer, and it is worth showing off. The floral design and the colors used in the mosaic were inspired by Van Gogh’s painting “Irises.”


Mosaic bird bath by artist Lyn Richards makes good use of contrasting colors and a Van-Gogh-inspired floral design that help integrate the bird bath with its surroundings. Note the use of an ordinary concrete stepping stone as a footing/base to keep the mosaic work separated from the soil. This is recommended because the acids in decaying organic matter slowly rot stone, concrete and grout.

Material List for a Mosaic Birdbath

Concrete Base

To make a mosaic bird bath such as this, you could use a ready-made concrete bird bath from a lawn and garden center, or you could make an original concrete sculpture if you are looking for something with a less conventional shape. Lyn used a ready-made concrete bird bath that came in two sections: a base column and a bowl.

Thinset Mortar and Tools

Thinset mortar should be used instead of adhesive for attaching the tile. A complete list of tools and materials for mixing thinset and for applying it is given in my instructions for using thinset mortar with glass tile. (Note the instructions were written for doing extremely detailed work, so I might make thinset seem much more difficult to use than it really is.) I have also written a page for how to keep your hands clean when using thinset.

 Glass Mosaic Tile

Glass mosaic tile in non-porous and so water can’t get inside it and freeze and crack it all to pieces. This makes glass superior to stone and ceramic tile when used outdoors. You can use any of the glass we sell. Lyn’s mosaic was made from different brands of 3/4 inch vitreous glass tile, metallic glass tile. iridescent glass, and stained glass.

To find out how much tile you need, you can divide your shape into component surfaces and use our mosaic tile estimator.  What do I mean by component surfaces? For estimation purposes, Lyn’s bath can be thought of as cylinder topped by a disk. The surface area of the cylinder is A = D*Pi*H, where D is the diameter, and Pi is 3.14, and H is the height. The surface area of the disk is A = Pi/4*D*D. Note that this time D is the diameter of the bowl and not the pedestal base. Remember to multiply the surface area of the disk by a factor of 2 because there is a top and a bottom.

As always, remember to inflate your estimate by 5 to 10% to account for cutting scrap and to provide a cushion for error in your estimates. I have always found that it is much better to have tile left over than it is to not have enough. Suppliers sell out from time to time, and the next manufacturer batch might not match what you already have, or at least match exactly. Also, leftover tile serves as inspiration and material for future projects.

Finding and Transferring Patterns

Lyn used Van Gogh’s painting “Irises” for her pattern. She drew the outline of the shapes (blades and petals) onto the concrete using a pencil, and then she went over that drawing with a black Sharpie marker when she was sure she had her lines correct. This is recommended: Don’t use the marker until I work out my lines by trial and error using a pencil. If your pencil drawing gets to be a mess and you need to start over, you can “erase” it by wet sanding it and wiping it off with a rag.


To create a sense of depth and shading, Lyn used multiple shades and hues of green for the leaves of the irises instead of one color.

Choice of Colors and Visual Interest

Note how Lyn used multiple shades and hues of green to make the blades of the iris leaves instead of using just one type of green. This creates shade and depth and visual interest and makes all the difference in the world. Monochromatic color fields are boring, and they take just as long to tile as a more interesting mix of colors. Note the solitary pea green blade of new growth and how it stands out from the other leaves, which are a mix of darker and bluer greens. Similarly, the petals of the irises are made from multiple cyan blues and ultramarine blues instead of one color, and this makes them more lifelike and more visually appealing.

Andamento and Visual Interest

The background of a mosaic is an opportunity to add visual interest to the scene and should never be “bricked” up like the tiled wall of a shower. Andamento is the visual flow of a mosaic created by arranging the tile in rows, and it can make the background of your mosaic as visually interesting as the figures in the foreground.


Lyn’s strong use of andamento creates a sense of motion in the background. When used to its full potential, andamento can make the background as interesting as any figure in the foreground, as demonstrated here.

Observe how Lyn used multiple related hues for the background instead of one color. Also note there is not a single whole tile mixed in with all these cut pieces. Why not? A uniform square would stick out like a sore thumb among all the irregular pieces.

Cutting and Attaching the Tiles

Thinset is concrete that hardens over time. I can usually get three hours out of a batch, and I often work in sections, usually completing about 1 or 2 square feet per session, depending on how detailed my work is. Lyn found it useful to have her tile cut up in advance of a session, but you can always make incidental cuts as needed while you are attaching tile. Again, my thinset instructions explain the details of how to handle and use the mortar to attach tiles. Tip: partially crushed liquor boxes make great cradles for holding heavy pieces of stone and concrete while you tile them.

Removing Sharp Edges

A marble file or a rubbing stone can by used before the tile is mounted to remove any sharp edges. After the tile is mounted, it is a little more difficult to remove sharp edges, but gently rubbing the surface of your mosaic with a rubbing stone can help a lot.


Before grouting, you should clean off any excess thinset from the face of the tiles. Scraping this off will also make any loose tiles fall off so that you can reattach them before grouting. You should wait a day or two before grouting to let the thinset harden. Black grout is best for making your colors stand out. Lyn used a combination of black, gray, and brown grouts, and she blended these to get the exact color she wanted. A few days after your grout hardens, you should buff all the grout haze off and seal the finished mosaic with multiple applications of a silicone-based tile and grout sealer, such as available at any building material store.


Lyn Richard’s mosaic bird bath at home in her garden.

Mosaic Transfer Instructions

Yesterday I wrote up some recommendations and instructions for Outdoor School Mosaics that focused on a project where each child made a mosaic stepping stone on a paver to be arranged in a crazy quilt design. I forgot to clarify that those instructions were written for younger students and beginners needing to play around with tile and get some basic experience forming tile into patterns and shapes. After all, it doesn’t make sense to have young children trying to copy the work of an experienced mosaic artist before they have had the benefit of handling tile long enough to make a simple triangle or smiley face.

More Sophisticated Designs

If you wanted to use make a more sophisticated design with smaller pieces of tile or to render an image, you could adapt that same method and add just a few steps. I have the steps numbered below, but here is a summary of what would be different: Before you covered the cardboard square with contact paper (sticky side out), you would draw your pattern on the cardboard or tape the pattern to the cardboard. Once you position all your tile on the pattern covered in contact paper, you would use some mosaic mounting film to pick it up off the cardboard/contact paper. Then the mosaic could be pressed onto a paver or stepping stone coated with thinset. Once the thinset hardens, the mounting film is peeled off and the mosaic is grouted. This method allows you to lay up very complicated designs in advance of transferring it all at once to the cement.

Pavers vs. Molds

Note that here I am talking about using thinset mortar to attach a mosaic design to an existing stepping stone or paver or flagstone. If you need instructions for how to use a stepping stone mold to press tiles into wet concrete (or pour wet concrete over a mosaic design mounted on contact paper at the bottom of a mold), then read my article on Mosaic Stepping Stone Instructions.

Stepping Stone Transfer Instructions

  1. Cut out a square of cardboard the same size as your stepping stone or paver. If you want to use an irregularly shaped piece of flagstone as your mosaic base, you can cut out a piece of cardboard in the same shape as the flagstone. Just lay the flagstone on the cardboard and trace around it.
  2. Draw your mosaic pattern on the cardboard or on a piece of paper taped to the cardboard.
  3. Wrap the cardboard pattern with clear contact paper with the STICKY SIDE OUT. The sticky contact paper keeps the tiles from sliding around as you position them on the pattern.
  4. Use mosaic mounting tape (or clear packing tape) to pick the mosaic up off the cardboard pattern.
  5. Coat the paver or stepping stone or flagstone with a thin layer of thinset mortar. Smear it around to make sure the surface is wetted thoroughly and then scrape off the excess. You only need a layer about 1/16 inch thick. A little more won’t hurt, and it doesn’t have to be exact, but too much can be a little messy if you press down of the mosaic and squeeze it out the sides.
  6. Press the mosaic into the thinset. It may be easier to lay the mosaic on a table (with the mounting tape side down) and lower the thinset-covered stone onto the sheet of tile.
  7. Allow the thinset to harden for 24+ hours.
  8. Peel off the mosaic mounting tape.
  9. Grout the mosaic with additional thinset if needed. It is better to use more thinset instead of grout because it will match the color of any thinset that pressed up between the tiles when you mounted the mosaic. If you use grout, then the color probably won’t be exactly the same, and your mosaic will look like you grouted it with two different types of concrete. Often no additional grout is needed because enough thinset squeezes up between the tiles during mounting.
  10. Clean any grout residue or haze from the face of the mosaic by buffing with a clean cloth.
  11. Allow the grout to cure for 2 or 3 days and then seal the finished mosaic with a tile and grout sealer purchased from a local building material store.

For more information on using clear contact paper and mosaic mounting tape to lay up and transfer mosaic designs, read my article on Mounting A Mosaic On Clear Adhesive Film. If the cost of mosaic mounting tape is too high, or if you don’t need a whole roll, then you can use clear packing tape as a substitute.

Outdoor School Mosaics

Recently I received an email from an art teacher whose school mosaic project was an outdoor mosaic where each child would create a mosaic on an 8 inch x 8 inch brick paver (paving stone), and then the mosaic pavers would be arranged together in a crazy quilt design similar to what artist Victor Kobayashi created for his mosaic patio in Honolulu.

I really like the crazy quilt approach to school projects because it allows each student to make their own art and have a real art experience instead of copying some teacher’s favorite piece of art, which usually involves more boredom or frustration than it does art. Crazy quilt projects also tend to produce more exuberant and impressive results. Copying something is merely copying something, even if that something is an acknowledged masterpiece.

Normally, school mosaic projects can use 1/2-inch or 3/4-inch sanded plywood as a backer, and the tile can be attached using a white PVA adhesive such as Weldbond, but plywood and glue are for indoors only. For outdoor and wet mosaics, you must use thinset mortar to attach the tiles to the backer, and that backer must be cement, stone or masonry. For large mosaics, a sheet of concrete backer board can be mounted to a metal wall using a frame welded from angle iron, or the mosaic can be created directly on a stone or concrete wall or a brick wall plastered smooth with thinset. In this case, the mosaic can be laid up in advance on fiberglass mesh, mosaic paper or clear mounting tape, and then these sheets can be pressed into thinset spread on the wall using a notched trowel.

This particular teacher decided to use brick pavers for her backers, but the concrete stepping stones/pavers commonly sold at building material stores could have been used in a similar way. The real issue for her project was how the students could use thinset mortar to attach each individual tile without creating a huge mess.

Thinset mortar is a sanded portland cement product with polymers added for strength and adhesive properties, so think of it as sticky concrete because that is essentially what it is. Your students might be mature and competent enough to use a bottle of glue that looks and handles just like Elmer’s glue, but how are they going to fare when they start working with sticky concrete? Now that I have your attention and your hair is standing on end, let me calm you by saying that it can be done, and it can be done fairly easily with a little forethought and planning.

One option would be to avoid setting each tile individually and lay up the designs in advance on clear mounting tape using my instructions for using contact paper and mounting tape. Then thinset could be spread on the pavers and the whole design mounted at once.

But that still involves handling thinset and some point, and sometimes you find situations where the mounting tape method isn’t practical (such as when not all of your tile have the same thickness).

Make A Prototype To Answer Basic Questions

The key too minimizing frustration and mess is to figure out your process BEFORE you involve the children, and the best way of doing that is to make a prototype in advance. In making a small mosaic beforehand, you work out the details of your materials and methods, including how the thinset will be distributed between the different children and how they will apply it to the backer.

Here are some questions you should answer by making your prototype. Please don’t let any of these alarm you because I have a practical recommendation at the end of this article that greatly simplifies everything and even eliminates some of these concerns:

How long does it take to apply tile to a mosaic of this size?

How many classroom sessions will be required?

Would it be more practical to have longer sessions instead of a larger number of short sessions?

How will thinset be applied to the stepping stones? Will the children spread the thinset themselves?

How will the children keep their hands clean while working; buckets of water and piles of rags?

How will we keep the thinset from drying out in the heated winter air or summer AC? Can we use humidifiers if necessary?

How much thinset do you need to mix up at one time? (This is answered by thinking about how many students will be working at once and how much thinset you used in one working session.)

How will we mix up the thinset? Is a parent volunteer available with a mixing paddle, drill motor and 5 gallon bucket? Do we have any parents who work as contractors and have experience with laying tile or mixing up concrete?

All of these things are relatively easy to implement, but they can make things chaotic or difficult if you don’t think about them in advance.

Thinset And Surfaces

There are a few specific concerns related to using thinset and pavers/stepping stones.

Surface Wetting

Sometimes you can drop a clump of thinset onto concrete backer board and it will harden without bonding to the backer board and it will fall right off or come off with minimal scraping. This was because the thinset didn’t really make intimate contact with the board due to surface dust. This can become more of an issue over time as you work and the thinset starts to set up as you are using it. The point is that sometimes you need to smear thinset into a surface to make sure it adequately wets the surface and makes intimate contact. Normally this happens merely by pressing a tile into the thinset, but you might do well to keep an eye out for students who are minimalists in terms of how much thinset they apply and for those who have a butterfly touch and just kind of sit the tile on top of the thinset instead of pressing it in.

Presealed Pavers

One problem you might encounter is pavers or stepping stones that have been sealed with some sort of silicon or polymer that might interfere with thinset bonding well to them. You can test for this simply by dripping some water or spittle on the paver and observing whether or not the water wets the surface. If the water wets the surface and soaks in, then there shouldn’t be any problem. If the water beads up similar to how water beads on a waxed car, or if it fails to soak in, then you know that the pavers have a heavy coat of sealant and should be avoided.

Skin Irritation

Wet concrete is mildly caustic, so it can dry and irritate the skin. A box of disposable medical examination gloves from the drug store can prevent this. You should also have the children wear safety glasses with side shields.

Overly Complex Designs And A Recommendation

Another thing you can learn from making a prototype is how much time is involved and how simple or complex the designs can be in order to be completed in the time allowed. I definitely prefer children be allowed to make original designs so that they get a real art experience, but you still need to give them recommendations about what level of detail is practical and look out for children trying to make overly complex and detailed designs. For this reason, it can be somewhat problematic for children to sketch out their designs in advance. Sometimes the mere act of drawing gets a person thinking in terms of a level of detail that isn’t practical in the medium in which the design will be executed. I have encountered this time and again while sketching out designs for my painting and mosaic.

Instead of sketching out designs, a more practical exercise might be for the students to play around with arranging tile before they decide on a finished design and definitely before they work with concrete.

I recommend making cardboard squares the same size as the mosaic backer and allowing the children to practice laying up their design on the square. If possible, give them one session to play with different arrangements and experiment with rendering different designs in the square, and a second session to finalize their design.

Then the following sessions could be about transferring the tile to the thinset on the paver. Using this approach, it would be possible for a teacher or parent volunteer to spread thinset on the pavers, and then the students merely transfer their tile designs from the cardboard squares/trays to the thinset, which would greatly minimize the amount time the children spent touching concrete.

A Practical Method For Kids And Thinset

  1. Make squares from cardboard that are the same size as the stepping stones/pavers or draw squares the size of the pavers on cardboard or trays. I prefer to cut out the cardboard squares so that they can be wrapped with contact paper with the sticky side out to prevent the tile from moving around.
  2. Have children spend one or two sessions arranging tile into designs on these squares/trays.
  3. Have teachers or parent volunteers mix up and spread thinset on the stepping stones.
  4. The children transfer their designs to the stepping stone one tile at a time. Alternatively, clear mounting tape could be used to pick up and transfer more complex designs made from smaller tile.
  5. After the thinset has hardened for a day, grout the mosaics with more thinset or grout.
  6. After the grout has hardened for at least a day or two, clean off any remaining grout residue by rubbing with a clean cloth and seal the mosaics with a tile and grout sealer.


The above instructions were written for children and beginners who just need to play around with tile to make simple designs. However, you may have more advanced students capable of making more sophisticated images from many small pieces of tile. I have written a second article Mosaic Transfer Instructions which explains how to lay up a more complicated design on a pattern and transfer it all at once to thinset or cement using mosaic mounting tape or clear packing tape.

Terracotta Flower Pots Mosaic Warning

Terracotta flower pots are highly susceptible to freeze damage because the material is extremely porous. Moisture seeps into the tiny pores, freezes and expands, and then the surface flakes off as shown below. However, it is possible to use terracotta as a mosaic base provided the mosaic is mounted correctly and the artist understands the limitations of the material.


Terracotta flower pots are highly vulnerable to freeze damage because they are soft and porous. The damage could have been minimized by sealing the pot inside and out with a tile and grout sealer.

Minimizing Damage To Terracotta Flower Pots

Freeze damage can be minimized by sealing the flower pot inside and out with a tile and grout sealer from a local building material store. Tile and grout sealers are silicone products that plug tiny pores and prevent moisture penetration. They can also interfere with bonding, so we recommend sealing the flower pot AFTER your mosaic is complete. Note that even if you do seal a terracotta pot very well with multiple applications of sealant, it still won’t last as long as a concrete pot would.

Concrete Pots Are Preferred

A terracotta pot might be an acceptable base for an abstract mosaic quickly made from random colorful tile, but detailed mosaic designs take more time and effort than that, and they deserve a more durable base. Concrete flower pots and planters are available at most lawn and garden centers, and whatever extra cost is well worth it. A single winter of hard freezes can totally destroy a terracotta flower pot left outdoors. Also terracotta is also easily broken and cracked during normal use.

Sure you might already have terracotta pots at home that you could use for free, but how much money are you saving if the mosaic doesn’t last six months? Once you take the time to mix up mortar and attach the tile, you’ll be glad you took the time to find a concrete base, even if your design is just random tile.

Use Mortar Not Glue

The tiles should be attached with thinset mortar instead of glue. White PVA adhesives such as the Weldbond we sell are water resistant when fully cured, but there is a difference between water resistant and water proof. Flower pots are containers full of damp soil, and that means the back of the mosaic will be continually subjected to moisture and acids from decaying organic matter. The acidity of the leach water means that flower pots may be a more extreme environment for mosaics than pools and fountains.

Thinset mortar can also be used to grout your mosaic. Both grout and thinset are powdered portland cement products, but the thinset is stronger and more adhesive. If you are going to have to purchase a powdered cement product to make the mosaic, get the better product (thinset), and use it for everything. You can even reinforce the inside of a terracotta flower pot by plastering it with thinset.

How To Make Custom Shapes For Mosaic Backers

Rectangular backers are fine for most mosaic designs, but sometimes you want to make an irregularly-shaped mosaic or a mosaic with a custom shape, such as the silhouette of a common object: tree, automobile, flower, turtle, etc. How you make such a backer and what materials you use depends on whether or not the mosaic will be installed in an outdoor or wet location. Note that not every location in a kitchen or bathroom has to be considered as being “wet.”

First, I will discuss irregularly-shaped backers, and then I will explain how to make custom-shaped backers for both indoors and outdoors. The last section about custom-shaped backers for outdoors should be useful for people making mosaic signs and placards.

Why You Should Not Buy A Shaped Backer

If you buy a shaped backer from a craft supplier, then your mosaic will have exactly the same shape and size as all the other mosaics made from that particular backer. Also, most of that craft crapola is designed in China, and it all looks rather dated. The saddest customer picture we ever received was a picture of a beautiful mosaic design (serious, intense, original) executed on the most boring, cutesy, cliche shape of a ladybug. Oh what might have been…

Irregularly-Shaped Backers

If you want more of a random “found” shape instead of a specific shape, then the solution is to use a piece of scrap plywood or flagstone depending on whether or not the mosaic is outdoors.


For indoor mosaics, you can use a piece of 1/2″ cabinet-grade plywood, and a local carpenter or cabinet maker can give you more than you could ever use. Check with friends and their spouses for a few pieces of scrap, or you can buy a sheet of cabinet-grade plywood at a building material store such as Home Depot or Lowes and have a friend cut out what you need with a jigsaw. You can also buy a decent jigsaw for about $75, but make sure you follow the safety instructions and maybe watch a safety video or two on Youtube if you are a novice with power tools.


For outdoor mosaics, you should not use wood or adhesive. Wood doesn’t even have to get wet for the humidity in the air to swell and warp it. Instead of the glue-then-grout method used for indoor mosaics, you should use thinset mortar to attach the tiles to a stone or masonry surface. Concrete backer board can be used for rectangular and circular mosaics that have some sort of frame (such as the rim of a metal patio table), but the edges of concrete backer board can be crumbly, and that makes it a lot less useful for irregularly-shaped mosaics where the edges are left unfinished, such as you see in the fragments of ancient Roman mosaics displayed in museums.

To make an irregularly-shaped outdoor mosaic in the style of a Roman or Greek fragment, use a piece of flat flagstone such as can be found at stone stores, landscaping stores and some higher-end lawn and garden centers. Avoid slate and sandstone, especially the softer varieties. You can get a general idea of how soft or brittle a type of stone is merely by paying attention to how it has been breaking or scratching or weathering in the big piles or stacks at the stone store. Slate is good and flat and smooth, but it tends to be thin and break too easily for most sane people to care about mosaicing on it.

Custom-Shaped Backers For Indoor Mosaics

For indoor mosaics, 1/2-inch cabinet-grade plywood is my preferred backer, and it is sold at most building material stores. It comes pre-sanded and is more resistant to warping than the cheaper plywood used for construction sheathing. There also fewer if any internal voids in the plies of wood, so the edges are stronger and look neater. The few extra dollars for cabinet-grade plywood are worth the cost.

The shape of your backer can be drawn directly on the plywood with a pencil, or you can first draw the shape on cardboard or paper and then cut it out and use it as a stencil and trace the shape on the plywood.

Cutting the shape out is best done with a jigsaw, which can be bought for about $75, but you can ask friends and their spouses to do it for you if they have one. Most people who work with carpentry or cabinetry will have one, but if you decide to cut it yourself, make sure you watch an online safety video about using jigsaws first. In my opinion, jigsaws aren’t nearly as dangerous as circular saws and table saws, but novices should be extra careful when using power tools.

 Custom-Shaped Backers For Outdoor Mosaics

Metal isn’t recommended as a mosaic backer, but if you are mounting the mosaic outside, then metal will probably be involved in some way, at least in how the mosaic is attached to the building or post. The most obvious solution is to have concrete backer board set in a metal frame made from angle iron, and this frame can have mounting studs (bolts) welded to it prior to painting it and inserting the backer board.

I don’t recommend hanging mosaic signs from chains because mosaic work is heavy, and intense wind from storms can turn the sign into a battering ram. Also, the chains would need to be checked periodically for wear, and the artist cannot guarantee that the owner of the sign will do this over time.

Of course, a frame made from angle iron is really only practical for rectangular shapes.

My approach for making a custom-shaped mosaic for outdoors was to put the steel inside the concrete. Essentially, all I did was cut out my shape in 3/4″ expanded steel using a cardboard pattern as template, then weld mounting studs (bolts) to it, and then I encased it in thinset mortar, which is a type of sticky concrete.


Expanded steel 3/4 inch. The 3/4 inch measurement refers to the size of the internal holes, specifically the minor axis (shorter dimension) instead of the longer side-to-side dimension.

The 3/4″ expanded steel was cut using an angle grinder with a thin cutting wheel because I didn’t have a cutting torch and haven’t yet saved up enough money to buy a plasma cutter. (Are you listening Santa?)

I used my cardboard template to outline two pieces of expanded steel, and I made sure that the direction of the expanded metal was oriented at 90 degrees between the two pieces. That way when I welded them together, I was sure that the holes would not line up perfectly. Instead, I wanted the holes in each piece of metal to be partially covered by the other sheet.

welded expanded metal

This structure was made from scraps of expanded metal I had in the shop instead of two pieces expressly cut out for the job, but notice how I made sure the expanded pattern in the top and bottom layer are still rotated 60 degrees from each other instead of perfectly lined up. A rotation of 90 degrees is optimal for ensuring the holes in the resulting structure aren’t too large.

Once I had my shape welded together, I welded some 3/8-inch bolts to it to that the finished mosaic could be bolted to a wall. Then the frame was scoured with the stiff wire brushes that are used to clean welds.

The thinset mortar I used to cover the frame was applied in multiple coats. The first coat was mixed with about 50% fine pea gravel so that the mortar had some bulk to fill the holes in the frame. Note that most pea gravel you see at lawn and garden centers will need to be sieved through 1/4″ hardware cloth or at least have the larger stones picked out. If that seems tedious, then consider how tedious it will be to pick put the large stones once they are coated in sticky concrete but are too big to be pressed into the frame. (Been there.)


This is the underside of a finished outdoor mosaic backer. Note the three mounting studs. Also note that the top surface (facing down) is a lot smoother than this backside. I made the top surface perfectly smooth by applying a second coating of thinset to the top face and then setting it upside down on a piece of construction plastic.

Thinset mortar contracts or “thins” as it cures, so there isn’t much point in making your surface perfectly smooth with the first application. Of course you want it level, and you don’t want any large pieces sticking up, but there is no need to try to smooth it to a finished surface with a trowel. If you do smooth it perfectly, you will notice dimples that get larger each day for about a week as the thinset contracts internally.

Due to this internal contraction and the resulting dimples, I wait about a week before applying this second coat, which mainly involves spreading the thinset on the face with a putty knife or trowel and then turning the mosaic face down on a piece of construction plastic.


Roll of construction plastic. Grout does not stick to plastics in general, but this stuff is especially good about being stick free.

Construction plastic is sold in large rolls at building material stores. A cheaper alternative is clingy kitchen wrap such as the Saran Wrap brand. Kitchen wraps aren’t as strong, and they don’t tend to stay put even when taped down, but an easy solution to this problem is to find a large piece of cardboard and wrap it around the cardboard about 3+ layers deep. Then you can lay the covered cardboard on your work surface.


Use this improvised method and these instructions at your own risk. Like all the instructions on my websites, these instructions haven’t been rigorously tested in corporate laboratories. Neither can they anticipate all the potential mistakes an individual could make in executing them. As always, if you are installing anything for a client, it is your obligation to evaluate the strength, safety and longevity of your art, especially if it is to be displayed in a public space.

All that being said, there isn’t too much if anything in these methods that uses materials in a way that they aren’t commonly used or at least in a similar way. Unless you weld things in an amateurish way or fail to clean the welds, then the backer should have a very long life, even outdoors. The only mode of failure I am particularly concerned about is the possibility of the bolts rusting through over time, although that would be a concern with any heavy sign mounted by bolts.