What Plastic Lumber Do I Need? The premium grade plastic lumber has the nicest finish, and is the usual choice when there is any milling required. It is also the best choice for decking since it is available in tongue and groove and many other stock sizes and shapes.
The molded grade plastic lumber are still made with high density polyethylene just with a different extrusion method, the end result being a denser board, a little rougher grain, and truer dimensions. This method also limits length availability, and you are more likely to find “voids”, though usually small.
The structural grade plastic lumber has the same attributes as described above for the molded and landscape grades, with one major difference. We add chopped fiberglass to the polyethylene that makes the lumber approximately 4 times stiffer than the other grades. These boards are the choice when free spans beyond the capability of the other grades are required. It is most commonly used in understructure in deck construction, though there are many other applications such as pergolas and arbors. This grade, because of the heavier fiberglass additive is also the heaviest grade on a size by size comparison with the other grades.