Basics 2: Lumber Dimensions, Wood Grades and Engineered Wood

I was born into a family of carpenters and had sawdust introduced to my environment at age 6 when my father bought an old house. It was built around 1900 and was definitely considered a fixer-upper. Dad wasted no time in starting on his project and continued it until he sold the place about forty years later. I grew up in the construction world experiencing my father performing miracles almost every weekend in the remodeling to our old house.

I had an interesting email from Marc in Manhattan who suggested our website should have an article on lumber dimensions and plywood to inform the novice. Marc wrote:

"Actually, I'm talking about all pertinent information that a woodworker takes for granted but a novice needs to know in order to make the smartest decisions. The sizing issues with wood products drives me insane. Its history has been explained to me by professionals but, in my mind, that doesn't justify why a 2x4 shouldn't measure exactly 2 inches by 4 inches. This logic should apply across the board, so to speak. Then there's the issue of wood species, soft vs. hard and which application is best for each. I just discovered that 4x8 sheets of 3/4" oak are available for purchase. I never knew that. It just seems reasonable that your very charming website should feature an education on how to decipher and select wood products sold in a typical lumber store, such as Home Depot, Lowes, etc."

Thanks Marc. I thought everybody would know lumber dimensions, but realized through my emails that many people out there were not as fortunate as myself in growing up in a lifetime of remodeling. A subject as broad as this should start at the beginning. "In the beginning God created the heaven and the earth." Oops, maybe not that far back!

From Logs to Lumber

Living on the West Coast of Canada, since a boy, I'm familiar with logging trucks loaded with logs heading to the mill. At the mill the logs are removed from the trucks and are graded by dimension and quality on the ground by workers called scalers who record the volume of each log as well as the species of tree the log is from. Most lumber we buy at the lumber yards are limited in lengths up to 22 feet. The mill is given a cut order. The scalers are aware of the lengths they need and mark the logs accordingly. The sawyer in the mill also has a copy of the order and cuts the logs according to the lengths, widths and thicknesses needed. At this stage the wood is quite wet, called green, and is commonly cut to lumber dimensions of 1", 2", 4", and 6" thick and 2", 4", 6", 8", 10" and 12" wide.

Editor's Note: This article was written by me about 20 years ago, so mill practices has changed dramatically. Now a days, a computer operator runs a computer system that scans the log and decides the best size board to cut out of it. This information is fed to a saw which does the cutting.

The wood species in my coastal area are mainly softwoods such as fir, hemlock, spruce and cedar. Softwoods are evergreen trees opposed to hardwoods, which come from deciduous trees (trees that lose their leaves in the Fall). Softwood lumber is used primarily for structural strength in framing and hardwood is generally used for finishing purposes.

After sawing the logs into sizes of dimension lumber the piles of lumber called lifts are moved to an area suitable for air drying or into a kiln. Kilns are large ovens, where entire lifts are brought in to be heated and dried. Most softwood lumber is air dried to a moisture content of approximately 19% by weight and hardwoods used for furniture, cabinets, flooring, paneling and molding require a final moisture content of 6% to 8%. Drying wood is critical to its overall performance and value. I've read that the kiln drying process is the most time consuming and expensive of the conversion of logs to lumber. As the wood dries its dimensions reduce.

Years ago our houses were constructed using lumber that was left rough with full dimensions. Our walls were plastered (wet plaster trowelled over rough slats) which allowed fairing out any unevenness in the studs on the inside and wood shingles were usually used for siding, which did the same on the outside of the studs. With the market changing to drywall (dried gypsum between sheets of paper) and paneling for walls, our studs had to be straighter and more uniform. Planing lumber to consistent sizes became the norm. The rough sizes remained at increments of 2" and 4", but the finished products were planed down to 1 5/8" by 3 5/8", which eventually became 1 1/2" by 3 1/2", the common 2 by 4 (2x4) of today. The rough 1" boards became 3/4" thick. One can still purchase rough lumber in sizes such as 4"x4" and 6"x6" as well as special orders of rough lumber of almost any dimension.

Lumber Sizes

1x23/4 x 1 1/2
1x33/4 x 2 1/2
1x43/4 x 3 1/2
1x53/4 x 4 1/2
1x63/4 x 5 1/2
1x83/4 x 7 1/4
1x103/4 x 9 1/4
1x123/4 x 11 1/4
2x21 1/2 x 1 1/2
2x31 1/2 x 2 1/2
2x41 1/2 x 3 1/2
2x61 1/2 x 5 1/2
2x81 1/2 x 7 1/4
2x101 1/2 x 9 1/4
2x121 1/2 x 11 1/4
4x43 1/2 x 3 1/2
4x63 1/2 x 5 1/2

Grades of Lumber

Cedar fence boards still come rough on one side and smooth on the two edges and one side. We now have designations for these boards: S4S is surfaced four sides; S1S2E is surfaced one side and two edges, etc. T&G denotes tongue and groove. Along with the smooth lumber came lumber stamps which designated where the lumber was milled, its grade, moisture content when surfaced and species. All framing lumber used in residential and commercial applications must have the grade stamps visible for building inspectors to view. For those who cut their own lumber, check with your local jurisdiction for rules on lumber grading; an engineer may have to be hired to approve the grade. A 2x4's grade is: Construction, Standard, Stud, Utility or Economy. In larger sizes the grades are referred to as Numbers 1, 2, 3 and # 4 is Economy. In house construction grades such as #2 and better are used for studs, joists and rafters and #3 is used for plates and joists and rafters for short spans. Economy is only allowed for strapping walls, etc. where no structural integrity is needed.

While on the subject of dimension lumber, today a common product is pressure treated wood (PTW) that is treated with a toxic chemical (forced deep into the wood through pressure) to protect it from pests in the soil, which can protect it for up to 40 years. In some dry areas PTW foundations are also approved for houses. PTW does not protect the wood from the elements (air, water, etc). This is best done with the use of exterior finishes such as stains, paints, and clear finishes containing ultra violet (UV) inhibitors, such as Thompson's Water Seal and Cetol.

Cedar is a good choice of wood species as a finish material on an exterior surface, such as siding, shingles and fences. Cedar has natural oils, which stand up to a certain extent to the weather. Cedar has a tendency to grey as it weathers if unprotected. This is a natural protection to a point. Cedar is best stained rather than painted. A painter once told me that on my house we should stain the cedar trim and in two years or so when it weathers, cover it with an exterior acrylic latex paint, which I've done. Cedar comes in most of the same sizes as dimension lumber. It is quite common in fence board sizes of 1x6, 8 and 10 inches, actual sizes if purchasing the rough sizes or trimmed down to 3/4"x 5 1/2", 7 1/2" and 9 1/4". Cedar is not a structural material; in other words it doesn't have the strength to support floor and roof loads as does hemlock, Douglas fir, some of the pines and spruces.


Plywood is an engineered wood: this is a wood product that is manufactured to precise national and international standards by binding together wood strands, fibers, or veneers with adhesives. Plywood is made from veneers of real wood. The wood from the log is peeled off in layers on a huge lathe rather than cut to dimensions for lumber. The plywoods are considered hardwoods if the top and bottom veneers are from the popular hardwood species such as: oak, pine, poplar, maple, alder and cherry, among others. The inner layers are veneers of the cheaper softwood or cores of particle boards which will be discussed later. As with dimension lumber the plywoods come in assorted sizes and grades. Four feet by 8 feet has been designated as the common size, however 4'x4', 2'x4' and 2'x8' are usually available in building supply stores, as well as larger sizes according to the availability in your area. The thickness of plywood starts at about 1/8" and goes up to 1". Paneling is 1/8", 1/4" and 1/2"; sheathing is 5/16" to 5/8" and finishing plywoods are 1/4" to 3/4"; 1" plywoods are used for stair treads. These are the common sizes.

The grain of plywood veneers alternate; one veneer is laid length ways and the next width ways giving the panel great strength. The grain of the plywood, when it's being machined, is considered to go with the grain of the top and bottom veneers, which is lengthwise in the original panel.

The common grades for plywoods are as follows:

  • G2S for good two sides.
  • G1S for good one side.
  • (The G2S and G1S grades are sanded and used for furniture, cabinet doors, partitions, shelving and concrete forms.)
  • SEL TF for select tight face.
  • SEL for select.
  • (Both of these are unsanded and are used for underlayment, combined subfloor and underlayment, sheathing, hoarding and packaging.)
  • SHG for sheathing grade. (Used for sheathing roofs, walls, and floors.)
  • MDO for medium density overlay. (For siding, soffits, paneling, built-ins, signs, or any use requiring a superior paint surface.)
These plywood grades are further broken down to A, B, C, and D, so you can have G2S-B or SEL-D, etc.

Plywood Grades for Cabinetry

Let me offer some tips concerning building cabinets and grades of plywood to use. Another grade to consider, other than G1S when building cabinets and furniture, is shop grade or D grade, especially if the cabinets are for a shop or utility room for storage. Pick the sheets carefully, D grade sheets may have started to de-laminate, they are also not exactly 3/4" if sanded D grade, but are less expensive for some projects.

When building kitchen cabinets I try to use cabinet grade plywood which is G1S which is exactly 3/4" for dado purposes. Melamine, an engineered wood panel made up of particle board faced with a very hard painted surface on each side, is also a good choice for the gables and shelves of cabinets. MDF is ideal for cabinet doors which will be painted. One thing to watch in buying plywood for finishing, other than the grade, is that the sheets are not warped. They should be laying flat on the pile without a corner lifting up. Discard sheets like that. These were probably removed by someone else knowing better and returned to the pile by the clerks.

Fir plywood is our cheapest plywood where I live. It comes in G1S and G2S. G2S is almost as expensive as a shop grade birch plywood which is graded as a G2S D grade. A G2S sheet of plywood always has one side better than the other, although some sheets are harder to tell the difference than others. I use only fir plywood for shop cabinets which will be painted. I've seen fir kitchen cabinets stained and it just goes against my grain. Fir has a very definite grain to it and no matter what stain color you use it still looks like fir plywood. The cheapest hardwood plywood in my area is birch. Ash is priced not too bad and is a good choice for staining to look like black walnut, which is one of the most expensive hardwoods. Oak is cheaper than pine, mainly because it is so popular the price is kept low. The plywoods that are unique in themselves are the open grain woods such as mahogany (Philippine and African) and oak. Japanese mahogany is used mainly on doors and is ugly for a finish, it has a paint grade surface as does fir. These are my personal opinions on the species of plywood in my area.

With your cabinets, you may want to go with 5/8" rather than 3/4". Get a G1S grade. If using European hinges and frameless cabinets the two gables are back to back which hides the bad side of the plywood. Always use carpenter's yellow glue with nails or screws for cabinet joinery.

Other Engineered Woods

Over the years engineered wood has evolved from just plywood to panels and components made of chips, sawdust and fiber. The most popular engineered product in recent years used for sheathing walls, floors and roofs is oriented strand board (OSB). An engineered wood made of thin strips of wood (about 1"x6" lengthwise grain) cross-oriented, glued and pressed together. Also used for the webs in I-Joists.

Another popular panel used for finishing is MDF, medium density fiberboard, an engineered wood made from softwood fibers that are mechanically separated, then randomly combined, glued and pressed so the material has no grain, but is consistent throughout. It swells if wet unless treated, so not good for mist areas such as a bathroom. This includes moldings for baseboard and crowns.

Before MDF was the K-3 particle board, an engineered wood made of particles (wood chips, shavings and saw dust), glued and pressed tightly together so it is dense, heavy and flat. It is cheaper, weaker and less durable than wood or plywood and harder on your saw blades. It swells if wet. This was used extensively in mobile home floors but did not stand up to moisture. It is still used for counter top manufacturing under formica.

Engineered wood applies not only to panels but lumber, as well. LSL for laminated strand lumber is an engineered wood used for studs, plates, headers or rim boards for I-Joists. Also known as TimberStrand.

LVL for laminated veneer lumber, an engineered wood used for beams or headers. These usually come in widths of 1 3/4" up to lengths of 60'. Also known as Gang-Lam or Micro-Lam.

PSL for parallel strand lumber, an engineered wood used for columns, headers and beams. These come in thicknesses from 2 11/16" to 7". Also known as Parallam.

Glulams have been around for years. Glued laminated timber is an engineered wood made of several layers of small-sized dimensional lumber glued together into single, large, strong, structural members that can be used as columns, arches or beams.

And finally you should know about the finger jointed pine moldings made up of small pieces of solid wood held together with strong adhesives and joints made of multiple fingers which increase the surface area for the glue. Finger joint lumber is also found commonly in dimensional boards from 1"X2" to 1"X10" including 2X4 pre-cut studs. The use of small pieces glued together has kept their costs down, resulting in little waste. Finger jointed lumber is not designed for plates, just for studs on the vertical.

I hope this discussion has answered some questions and concerns you may have had. I appreciate people like Marc who are straightforward enough to ask questions, intelligent enough to research a project before just diving right in and are willing to learn new skills for their enjoyment as well as to save a bundle of cash on the labor for a renovation or construction project.


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Hi, I'm Dave Osborne. With over 50 years experience as a journeyman carpenter, foreman and contractor in heavy construction I enjoyed working with apprentices and sharing the tricks of the trade that others shared with me. Now I get emails from Members all over the world and we include many of my answers in our Free Monthly Newsletters. Some of my answers include drawings and instructions specific to a project, but may also answer your questions. I use correct construction terminology, so you can confidently inform your building supply dealers or contractors exactly what you need.


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