Sandbox Design Advice

Thanks Slaw.

Bright Star used a space of about 1/2" between the floating platform and the outer box of their Big Rock. In that space was placed a length of foam, to cover the sand. The Big Rock was made of MDF, but I'd use something stiffer for the floating platform, like a layer of Baltic Birch and another of granite, with constrained layer damping between the two. Check out ASC Wall Damp. 

MDF might work but it’s not the most durable material. You’ll for sure have to depend only on the glue as it doesn’t have much strength without a lot of bracing. Another good choice would be poplar as it paints very nicely, much more strength than MDF and your carpenter can joint the corners instead of a butt joint or a miter that may open up. Another option would be to use Baltic Birch Plywood if you want something a little nicer. All depends on what you want to spend, but you’ll have more in labor by far over the material regardless. As far as the lid or base goes, that would be fine as MDF as I think the sand is doing all the dampening and not the granite. Do a multilayer sandwich base of MDF/Cork/MDF. You could also line the inside of the box with Dynamat before the sand. I would think a 1/8” gap (ea.side) all around would be adequate. With a hardboard material, instead of MDF, to make the box it will be better for attaching the bottom plate as well as your carpenter can do a rabbet joint so you don’t see the edge of the bottom plate from the sides. If you’re r going to hire a carpenter, a trim carpenter/cabinet maker and not a framer... do it right. Good luck.

Thank you bdp and adg. I really appreciate your expertise on this.

Any thoughts on sand depth?