About 65 to 70 percent of a tree can be used for solid lumber, but what about the rest? In the past the answer was simple: It was burned or dumped in landfills. Today more than 95 percent of a harvested tree can be put to good use thanks in great part to the increased use of engineered wood products. Among these, medium density fiberboard, or MDF, has enjoyed remarkable success over recent years. As the MDF supplier for This Old House TV's Charlestown house, I'm in a good position to tell you about the material's history. The material got its start in the United States in 1966, at a plant in upstate New York. As it became clear that making MDF offered a far better use of residual wood than disposing of it, production soared. Today there are some 27 plants in the US and Canada, and more than 100 worldwide. Recently the industry has expanded its use of recovered materials to include agricultural by-products such as wheat straw and post-consumer recycled wood. MDF is typically made from sawdust, planer shavings and other waste that remains after a tree is milled into lumber. The wood is then cleaned and mechanically refined in a process that reduces it into fine, uniform fibers. Excess moisture is removed and an adhesive resin is added to hold the fibers together. This mix is then formed into a long, thick, homogeneous mat, which is compressed under intense heat and pressure. The resulting MDF panel is sanded to a fine, even smoothness, and cut to the required width and length. Computer-controlled sensors monitor the entire manufacturing process to measure slight variations that even the human eye cannot detect. Finished panels are tested for uniformity, strength and other structural soundness. MDF is used extensively indoors in furniture, cabinets, doors, mouldings and flooring. Like other engineered wood products, it has a distinctively flat, dense surface that holds paint well. It doesn't move like wood, so its joints stay tight and paint doesn't crack. But the glory of MDF is its uniformity; it can be machined into every conceivable shape to create architectural details such as balusters or mouldings. Unlike real wood, MDF has no knots, grain or warping that can make intricate woodworking difficult. While solid wood is better suited to structural applications such as floor joists, MDF tends to be cheaper than solid wood so it's well suited to interior doors, bookcases and kitchen cabinets. In the Charlestown house, the homeowners chose MDF interior doors that we custom cut to match the house's other large doors. We made and shipped the doors in only a few days, at a significantly lower cost than solid wood doors. Once the doors are treated with a faux-wood grain finish, they will have all of the benefits of MDF with the look of solid wood.