Semi Solid wood floors have multiple thin ply layers (see picture below) that are glued together. The center core is generally a softer wood material and is used to make the tongue and groove. A hardwood finish layer is glued on top of the center core and another softer wood ply is attached underneath the core. This top ply is also called the finish layer and comes in a wide varity of species. The thickness of the board is generally 14mm, of which 3-4 mm is of the hardwood itself.
They come is two formats, plank and 3 strip and come in a wide variety of colors and available in domestic and exotic hardwood species. Depending on the manufacture you can also sand and refinish, or recoat most semi solid wood floors up to 3 times if properly done, which adds to their appeal. They should last 20-40 years before needing replacement depending on the amount of wear and tear the floor will get.
Semi Solids are designed for the floating installation, but most can also be glued-down, or stapled down. Because these floors can be installed with the floating installation method they can be installed over a wide variety of sub-floors.
Semi Solids are ideal for under floor heating. One of the benefits of using a semi solid floor is that the floor boards are locked together at the joints of each board and not nailed or adhered to the sub floor. This allows the whole floor to move as a single unit if a dimensional change within the wood floor takes place. Another reason is that a semi solid floor with a stable plywood backing makes for a more suitable floor with less chance of dimensional movement than a solid wood floor.