Remember from Lesson 1 that wood moves. Because of its cellular structure, wood is constantly ‘out of balance’ with its environment. When the air is humid, anything made of wood is absorbing moisture and swelling as it does. Conversely, when the air is dry, wood products are losing moisture and shrinking as they do.
This is a fairly simple law of nature. The difficulty arises when we push the material to do more than what’s natural; like staying straight, flat and not fall apart which are basic requirements for useful furniture and cabinets. Or when we try to prevent wood from moving at all. It’s common knowledge that we make wood products from trees, like lumber, cut the parts to the sizes that we need and glue them together. Is there anything else we really need to know than that?
For most people, no. For anyone who designs a product with wood as the material, though, knowing some basics is essential. In the first place, there are just 2 categories for wood building products: solid wood and plywood.
Some rules of thumb
Generally, plywood is for large panels and solid wood is for making frames. Plywood is ideally suited for economically covering large, flat areas. It’s also much more stable than solid wood due to how it’s made. Unlike solid wood, plywood is a man-made wood product comprised of several layers of veneer that are laid perpendicular to each other. Veneer can be anywhere from about 1/32″ to 1/8″ thick and from 8″ to about 30″ wide depending upon the species and how it’s cut out of the tree. The core part of a plywood sheet is made of alternating rotary-cut sheets which is not very sightly. The faces, which are the visible parts of the board, are made of matching leaves of the best veneer, usually called ‘fancy-face’ veneer. The reason plywood is so suitable for large areas is that it does not expand or contract in width. The downside of plywood is its edge:
There are few designs which call for the edge of plywood to be left exposed. It must either be ‘contained’ by a frame or banded with a veneered edge.
MDF, short for medium density fiberboard, is another type of plywood core. In this case, the ‘ply’ is comprised of many layers of paper laid up or glued on top of each other but having no grain direction like veneer core plywood. MDF is arguably the most stable core available but it’s also much weaker than veneer core plywood without the alternated wood layers. It’s also more vulnerable to water damage as it can absorb more than plywood.
Here are 2 examples of a ‘fancy-face’ table top:
Notice the frame around the perimeter and the highly figured wood in the center of the panel; both are tip-offs to plywood construction. In fact, a frame containing a solid wood top is not good construction as it would not move with the whole top. It would show gaps in the joints after one year.
Another type of veneered top:
Marquetry is fancy fancy-face veneer. It could be called the veneer-joiner’s art as it is usually made up of many small and intricate pieces of veneer carefully held together until they can be pressed permanently onto the table top. Then a solid wood frame is built around the outside to protect and conceal the edge of the core.
Here are 2 examples of a ‘plank’ style table top
Notice the ends of the planks are exposed which is a clear indication of the solid wood construction (in some cases, ‘breadboard’ ends are attached as a way of covering up the end grain if it’s objectionable). All of the boards are running in the same direction, usually lengthwise in this kind of a table. Plank tops are more straight-forward, tending to lend themselves to ‘quieter’ or robust designs. There is a certain predictability about them.
Solid wood tops are best for this kind of expansion table which has all exposed edges and is intended for less delicate use.
Another type of solid wood table:
This kind of top is extraordinary because of the width of the plank. In this case, 1 board is wide enough for a full top. The edges are left unworked and if the bark still adheres to them, are called ‘live edge’. The wood is so beautiful by itself – even the ‘flaws’ have a certain wild feel – that very little additional detailing is required. There are not many species of trees anymore that lend themselves to a 1-piece top. Those that are available tend to be relatively pricey.
Which one is better, plywood or solid wood?
It’s a common misconception that one kind of material is intrinsically more valuable and desirable than the other. This is partially due to the unsavory reputation plywood has as ugly and common. As we learned above though, neither one is inherently ‘better’ than the other. Each approach has advantages and drawbacks. Solid wood seems to be more durable because its thickness protects it from superficial damage; but, it’s heavy. Veneer can show off the choicest cuts of the rarest woods in intricate and unique patterns; but, its beauty is literally only skin deep.
The best choice for any design is made from knowing what the strengths and limitations are of the materials and from having a sense of how a product might be used in any given environment. Style and personal preferences also play a part in the equation. Plank or solid wood table tops must be carefully held down or restrained both to restrict a tendency to warp and to allow a substantial amount of expansion and contraction across their width. While plywood tops must be structural, they don’t have quite the same movement issues requiring restraint like their solid wood counterparts.
solid wood: simple, robust design. Thickness, length and width of lumber expressed.
plywood: fancy face. Emphasis upon face; available for matching or patterned veneers. Primarily 2 dimensional.