Mosaic Street-Number Sign by artist Monika Walter damaged

Warning: Marine Plywood Not For Mosaic

Marine plywood cannot be used as a mosaic backer for outdoor and wet mosaic.

Yes, marine plywood can withstand the outdoors and wet days for many years, but it is completely unacceptable as a mosaic backer because it swells and contracts with changes in humidity in the outside air. That amount of swelling and contracting is tiny and might not be significant in construction projects, but it is fatal for mosaic. Absolutely fatal. It’s only a matter of time, and it’s usually not long.

People recommending the use of marine plywood as a backer for outdoor mosaics are not considering one critical detail:

The swelling and contracting of wood due to humidity isn’t trivial where adhesives are concerned, and the displacement (movement) can be measured. Imagine rainy days versus dry days. The displacement is more than enough to work glass free from adhesive because the glass isn’t swelling or contracting at all.

This is not speculation. I am an engineer and have worked in a materials testing lab.

Another piece of evidence I could bring to any argument about the use of marine plywood in mosaic is that I have received photos of tragically-damaged mosaics for 17 years, and marine plywood wins hands down as far as being the worst cause of grief, and the reason is simple:

Marine plywood SEEMS like a solid safe option because contractors will talk about the life they have gotten from it on certain jobs, and so the people who make the mistake of choosing it tend to be people who are making a design with a lot of work and care for the details. They took the time to choose a “good” backer because they knew they were going to put a lot of effort into their mosaic.

Seeing these mosaics damaged is much more painful than seeing some hasty work falling apart because the technical details were just outright neglected.

That brings me to an email I received from Monika Walter.

Artist Monika Walter

Monika Walter says she doesn’t consider herself to be an “artist,” but she has some solid work at her mosaic website, and she makes tables and mirrors and clocks for craft shows. They all look well-executed to me, and a couple of her mosaics make me jealous. More about that later.

The first thing to know about Monika’s work is one of the reasons why she takes so much care with her andamento is that her husband is blind, and he gets to appreciate her work by feel.

The worst part about it is that Monika’s husband was the one who recommended using the marine plywood.

In his defense, they do live in a desert climate. And the thinset she used wasn’t Versabond, and it might not have been sufficiently hydrated.

And contractors in the forums do talk about using marine plywood and just putting Versabond right on top or regular plywood for quick and dirty backsplashes and stuff like that.

But you have to remember that a backsplash isn’t getting exposed to wet outdoor conditions from the backside like an outdoor plaque would, at least not until the house settles or the face of the mosaic gets damaged or a pipe leaks.

The benefits of using stone or tile backer board instead of plywood are obvious if you consider the mosaic below:

A Tale of Two Mosaics

Dream in Blue Mosaic by artist Monika Walter
Dream in Blue Mosaic by artist Monika Walter

Monika tells us that the Dream in Blue mosaic above was made on concrete backer board and has been exposed to the elements of southern New Mexico (heat and relentless sun, torrential rainfalls during monsoon season, and occasional sub-freezing temperatures during winter) for years without even the slightest crack in the grout.

For the address sign that started failing, she used marine plywood, and the pieces of glass began popping off after only 8 weeks.

Note that the damage started at the bottom of the mosaic, a clear indication of water absorption being the root cause of the failure.

Color Family Harmony

I am jealous of the Dream in Blue mosaic above.

I am jealous of how Monika used a harmony of related hues and how they go from violet to blue to to cyan to turquoise green to the yellow highlights of the iridescence. I like how the iridescence expands the range of hues at both ends.

I also like how the stained glass shapes are incorporated with concentric swirls of smaller tile and how there is some figure-ground reversal going on. I’m a little jealous of that and how the stained glass composite shapes don’t all use the same scheme size/shape for the pieces in it. Each is different. That makes the mosaic more interesting.

Monika says that all the materials in this mosaic came from us, except for the glass gems.

TIP: Many glass gems for crafting aren’t meant for traditional mosaic because that are colored plastic on clear glass, and the grouting process is usually enough to scratch the plastic. You should test gems purchased at craft stores by cutting one up or scratching it on a rough concrete surface. Use gloves.

Mosaic Mirror Joyful Reflection by artist Monika Walter
Mosaic Mirror Joyful Reflection by artist Monika Walter

Joyful Reflection

If you wanted to make an abstract mosaic of archetypical shapes and borrow from ancient Egyptian hieroglyphs and their color scheme of gold, carnelian, turquoise blue, and green, and you wanted to make sure you kept it abstract enough, I don’t think most mosaic artists could have done a better job than Monika’s Joyful Reflection mosaic mirror.

The motifs in this mosaic aren’t copies of hieroglyphs. Instead they are abstracted interpretations of the colors and shapes used in the hieroglyphs. Or at least that is what it looks like to me.

If so, this really is a good example of using traditional archaic art to inform and inspire contemporary art. It reminds me of how the art of Walter Inglis Anderson was heavily influenced by Adolpho Best Maugard’s book Drawing Method: Tradition, Resurgence, and Evolution of Mexican Art, which explains the seven elements in drawing and discusses how archetypical shapes in nature (spirals, wavy lines, etc.) became the archetypical shapes in primitive art.

I like the flat colors of the shapes and how that contrasts with the mottling of the gold background. The happy bright colors that catch the eye, and the mosaic is aptly named.

My Recommendation

My recommendation for salvaging the street number mosaic was to dry it out thoroughly and repair it. I would then paint the sides and back with multiple coats of an outdoor paint (oil based). Then I would relocate the mosaic underneath a porch or better still inside.

Alternatives to Marine Plywood

For an outdoor mosaics, there are better alternatives ranging from light-weight foam-core boards to heavy concrete:

The foam-core board would need something to cover the edge, but so would marine plywood.



24 responses to “Warning: Marine Plywood Not For Mosaic”

  1. Paj Tognetti Avatar
    Paj Tognetti

    Thank you for your informative article. I am concerned about all my various ‘glass’ gems. I don’t quite understand how you would know by scratching the gems if they were made of glass or plastic?? I would imagine that both materials would scratch? Could you please explain the process and perhaps include some pictures to see the difference?

    Thank you!
    Paj Tognetti

    1. Beverly Roberts Avatar
      Beverly Roberts

      I called laticrete and they told me I can’t use hydroban board for exterior use. Did I speak with the wrong guy?

      1. Joe Moorman Avatar
        Joe Moorman

        Hi Beverly,

        You might be right. It might only be for indoor wet mosaics. I am researching and will edit the article.

        Thanks for the heads up!

        1. Val’s Blank Canvas Avatar
          Val’s Blank Canvas

          I use cement sheet for exterior mosaics, same stuff used to line wet areas inside houses. . .showers, bathrooms, laundries, etc. after cutting the shape requires. I cut it with an angle grinder, and always do that outside wearing goggles, mask, and ear protection. Dust is not as lethal as asbestos fibres but still nasty stuff.

          Cement sheet can be nailed with galvanished cleats. After fixing to whatever, coat with waterproofing compound . . . again same stuff used to seal sheets before tiling interior wet areas. Leave to dry before tiling in the usual way.

          I used the above process to create mosaic ‘cushions’ atop a couple of log seats four years ago. Been outside in all weather, -4 to 48 degrees centigrade, scorching sun, pelting hailstones. Tiled seats as good as new. Logs not looking as flash.

  2. Waldron Jennifer Avatar
    Waldron Jennifer

    Thank you. Very good article

  3. Carolyn Adler Avatar
    Carolyn Adler

    Would several coats of varnish/polyurethane on all sides of the marine plywood make a difference? I am trying that As an experiment for a mosaics that will be under a 3 foots overhang, on the outside of a house. So far it seems that the tiles are sticking to it with the thin set. Have you ever tried that?

    1. Joe Moorman Avatar
      Joe Moorman

      I’m sure it would work for a while, but eventually ambient humidity would take its toll.

  4. Roberta Avatar

    Great article, thank you.
    Why does the foam core of backer board need to be meshed and thinnest? If the piece is going inside, or in a frame, or tiled along the edge, doesn’t glue just hold on the tiles? Is it for outdoors when it has to be covered?

  5. Marilyn Keating Avatar

    I’ve successfully used any exterior sheathing plywood including osb for years in the northeast. If it’s prepped like a stucco house it’s fine. This means a water barrier like tyvek or tarpaper followed by stucco lath and a stucco rough coat. I live in a hundred year old stucco house and I expect my work to last just as long. At this point in time I have 20 year old pieces that are as stable as when I made them.

    Marilyn Keating

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Marilyn,

      I am sure that you are correct technically, but I suspect that would be problematic advice for the general public making small mosaics on plaques, especially those living in damper climates where stucco construction isn’t common. If they have one flawed place in the mosaic, then that “Achilles heal” it would be fatal because many of those mosaics would be sitting with their bottoms or edge on the ground and subjected to standing water during a rainstorm. Experience has taught me that whatever recommendations we make online have to be robust enough to work in different climates and to work when they aren’t well executed. Thanks for the feedback and tip. I am sure there are people and Mexico and California who are in complete agreement with you!

  6. mariann mawcinitt Avatar
    mariann mawcinitt

    Your advice is so welcome and I thank you for saving me time and misery.
    I look forward to every one of your emails. Thank you so very much.
    M. Mawcinitt

    1. yvonne m moses Avatar
      yvonne m moses

      Good information… I have a 4′ marine grade plywood that I am using for a mosaic outdoor table… Do you recommend not using it at all or will it be ok to put the concrete board on top? If I end up not using the plywood and just go with the concrete board, what thickness do you recommend I use since it’ll be a 4′ piece? I will be attaching it to the top of a wine barrel, I want to make sure that it will be strong enough.

      1. Joe Moorman Avatar
        Joe Moorman

        I think I would use a piece of 1/4″ Hardie backer on 3/4″ plywood. Thanks

  7. Louise Roberts Avatar
    Louise Roberts

    Thank you for all the information and advice that you provide. I have learned so much by reading through the website and your posts. I am making an indoor mosaic using the glass tiles you sell on this website along with the large sanded backer board plywood. I will be pre-sealing the board with diluted Weldbond to make sure that the wood doesn’t suck water out of the grout while it is curing as you suggest. My question has to do with the fact that the entire board will not be covered with mosaics. I will be painting the top left portion and stencling in some words there. The mosiac will flow in and around the painted and stenciled section. I would like to know if I have to be very careful not to get the paint on any part of where I will be placing the mosaic? In other words will the paint interfer with the glass mosaics adhering to the board? Any further suggestions or advice you might have for this type of mosaic would be greatly appreciated? Thank you.

    1. Natalija Moss Avatar
      Natalija Moss

      Hi Louise, sorry for our late reply, but in case anyone else was wondering the same thing, one should avoid mosaicing on top of paint. The paint might come loose over time and tiles could pop off.

      1. Marilyn Keating Avatar

        I also love Mosaic Supply!!!! You can learn a lot from tradespeople too. I love custom building products flexbond on wood and versabond on other things. Flexbond is like it’s name flexible and sticks to anything and can move a bit. Great on wood underlayment so can translate to art. Trades people use it to set tile on wood underlayment. Anything a trades person uses to set flooring will be great for art. A newer product for tile in the trades is wedi-board or other similar brands. It’s very light and you can apply mosaic to it with thinsets and apply it surfaces before grouting with construction adhesive. I’ve also had success using tacky glue or weldbond on mesh then thinset on drywall and then grouting with a polymer additive grout for kid projects.

        1. Joe Moorman Avatar
          Joe Moorman

          Hi Marilyn,

          Yes Flexbond can be used directly on plywood, and you can make indoor mosaic art that way. Thanks!

  8. Jodie M Snyder Avatar
    Jodie M Snyder

    Thank you for such an informative article! I am a newby mosaic maker but am thinking about creating an outdoor coffee table later this year. I live in relatively arid Arizona but am taking your advice about avoiding marine plywood to heart. What could I use as a backer for the outdoor table? Thank you for any advice you could provide.

  9. Frances Avatar

    What do you recommend for mosaic letters used for an outdoor sign. I was going to use ply as I can cut each individual letter out.

    1. Joe Moorman Avatar
      Joe Moorman

      I would make the shapes from folded wire mesh and then apply multiple layers of Versabond brand thinset mortar. Sheep/goat fencing works well for making shapes if you offset each layer by roughly 25% percent so that cells are filled in.

      You should integrate your mounting hardware (such as bolts or loops of heavy wire) into the wire frame before the thinset mortar is applied. The part of the hardware that should be kept bare of thinset can be wrapped in tape.

      The first “layer” of thinset mortar is packing the wire matrix full of thinset so that the frame isn’t hollow. There should be at least one additional layer of thinset on the entire external surface. Additional coats can be made on the face only if weight is an issue.

      You should wear a denim shirt and gloves and eye protection when working with fencing wire, even if you normally do some shop tasks without gloves or safety glasses. Wire is springy and sharp. Fencing wire is springy and sharp in multiple directions all at once. It can surprise you.

      Use bricks to weight down the sheet as you cut it from the roll. Sheep/goat wire is easier to work with than hog wire because the hog wire is a heavier gauge and stiffer and more springy.

  10. Jen Avatar

    Thank you for all the information.
    What are do you recommend for indoor tables? Can marine, birch, or pine plywood be used? Should the back be sealed with PVA glue, oil or water-based sealer, or paint?

    1. Joe Moorman Avatar
      Joe Moorman

      Indoor tables can be made on sanded plywood such as birch. We prefer cabinet grade plywood instead of sheathing because it warps less, but ordinary pine can be used if it is anchored to the frame of the table. PVA adhesives are fine for dry indoor mosaic.

  11. Andy Avatar

    Hi from Gold Coast – Australia.
    I’m building an outdoor laundry with concrete block partition walls and marine ply top. Each of the 3 openings (washer/dryer/sink) will be 700mm wide so I need the 19mm structural ply to span that distance and support to 50mm x 50mm tiles. After reading this excellent article and comments, I believe a cement sheet now placed on top of the marine ply could be the right process?? Assuming I would nail the cement sheet onto the ply with some small evenly placed “seal n flex” silicone blobs would assist in absorbing movement of the ply underneath??

    Apologies for metric measurements..


    1. Joe Moorman Avatar
      Joe Moorman

      Hi Andy,

      I think thinset mortar is used to attach concrete backer to plywood because thinset is flex tolerant over large surfaces, but I have seen construction adhesives used as well.

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