Today someone emailed me about using 1/4-inch concrete backer board to make mosaic birdhouses more weather-resistant and durable. I think they had read we recommend thinset mortar for all outdoor and wet mosaics and that thinset mortar was not recommended for wood, which is true, and they were wanting to overcome the problem by laminating the concrete backer board to the outsides of the plywood birdhouse.
That might make the birdhouse more durable, but it would not be practical. For starters, the birdhouse would be so heavy that it wouldn’t be safe to mount it anywhere overhead, at least in the type locations usually used for birdhouses (i.e., on top of a pole).
The thing to remember about mosaic birdhouses is that they are no more durable than the wood from which they are made. That means you should use plywood instead of board to make the birdhouse, and the inside of the birdhouse should be coated with several layers of exterior paint. (I’m not sure how that might affect a bird’s willingness to nest there and am speaking only in terms of durability.) The reason you would paint the inside is to help prevent moisture from the air degrading the plywood from the inside over time. I am speaking of humidity not precipitation.
Generally we recommend white PVA adhesives such as Weldbond only for dry indoor mosaics. However, birdhouses are generally kept on a post or some other location where water cannot pool. With that in mind, a birdhouse could be made with Weldbond provided the inside of the birdhouse was painted and the mosaic was thoroughly sealed with a tile and grout sealer. If those things are done, the mosaic should outlast the plywood on which it is made.
Some people recommend epoxies and silicone based adhesives because they are waterproof. I don’t like the fumes and fast curing times of epoxies or how they can’t be cleaned up with soap and water. As far as silicone adhesives, I’m not sure why I am seeing them recommended so often on the Internet. I’ve not used any in my mosaic work, and maybe they have some better versions now, but the silicone caulks and sealants and adhesives I used for home repairs in the past tended to be too soft and flexible when cured and peeled too easily from glass.
I suspect that I am seeing silicone recommended for mosaic work so often on the Internet is that there is a lot of misinformation that tends to get repeated and that many people aren’t very concerned about long-term durability, at least not as concerned as they are about selling something. I see a lot of craft advice on the Internet that is fundamentally unsound in regards to durability and weather resistance, including advice at the websites of large corporate-style players that have their own television shows. Word to the wise.
Why put all that work into a project if it isn’t going to last? Also, it takes a lot of fossil fuels and mined materials to manufacture products like tile and grout and adhesive. If you choose to use those materials, there is a practical and moral responsibility to make sure that whatever you make is made well enough that it doesn’t end up in the landfill within a mere 2 to 3 years.