If you’ve ever wondered why a two by four doesn’t actually measure two inches by four inches or what the grading stamp B&BTR means, you’re probably not alone. Unless you learn to speak the language, a trip to the lumberyard can seem like visiting a foreign country. The best way to begin is by understanding how lumber is made in the first place.
When logs are sawed into boards, they’re cut to rough dimensions equal to their full thickness and width. While most lumber is sawed into either one inch or two inch thick planks, other dimensions are available as well. For this reason the thickness is often expressed in quarter inch increments with one inch referred to as 4/4 and two inch as 8/4.
Softwood lumber is usually cut in two inch wide intervals (4”, 6”, 8”, 10”, and 12”), and even lengths (8’, 10’, 12’, etc.). Hardwoods, on the other hand, are cut to whatever width and length the log allows.
Since freshly sawn green lumber has a high moisture content, it is stacked with spacer strips between the layers and dried before being sold. To speed up this process, it is often placed in a heated kiln which can have the added benefit of killing any insects present in the wood.
As wood dries it shrinks in width and thickness until it reaches equilibrium with the air around it. Wood continues to shrink and expand to some degree even after it has dried. This seasonal movement must be taken into account when building cabinets, furniture, and millwork.
The moisture level inside a climate controlled house can vary from 5% to 13% in the United States, depending on where you live and the time of year. Softwood lumber is stamped at the mill to indicate how it has been dried, though the actual moisture content may differ if it has become wet or has been treated after it was marked.
Some of the common drying designations stamped on boards are:
- S-GRN: “Surfaced Green” Not dried, moisture content over 19%.
- S-DRY: “Surfaced Dry” Air dried to a moisture content less than 19%.
- KD: “Kiln Dry” Dried in a heated kiln to a moisture content less than 19%.
- MC 15: “Moisture Content 15%” Dried to a moisture content of 15% or less.
- HT: “Heat Treated” Heated to at least 133° Fahrenheit for 30 minutes at the board’s core to kill any insects present in the wood.
After it has been dried, most softwood lumber is run through a planer where it is smoothed and cut to uniform width and thickness. Planed lumber is designated as S4S if it has been surfaced on all four sides or S2S if the edges are left rough. Since thicker wood shrinks more, one inch boards are planed to ¾” while two inch stock is reduced to 1½”. This is true of width as well, with ½” being taken off boards 4” to 6” wide, and ¾” removed from boards over 6” wide.
While it’s possible to buy unplaned lumber straight from the sawmill, it is called “rough” for a reason, since it can vary in size from one board to the next. This might not be a problem if you’re building an unfinished shed, but in most cases it is important that the dimensions are all the same.
Hardwood lumber, however, is often sold rough. This allows cabinet and furniture makers to plane and straighten boards to their own specifications.
Excellent resource. I had always been daunted by the selection of lumber even at home depot, but now it all makes sense and I can choose wisely. Appreciate the help!!
Nice article, thanks.
A friend of mine has told me that only a few lumber-worthy boards come from timber – his words: “Many logs have very little high grade material per log. The heartwood is seldom used for anything except cants for pallets or ties. This leaves a few good boards along the edges for grade. Walnut is an exception to this, the walnut heart is used (preferred) for grade. ”
Can you back that up and maybe throw out some approximate percentages? The trees I’m growing here in Pennsylvania include red oak, black cherry, red maple, walnut, Norway maple, and baldcypress.
will the mill NUMBER be same on different runs?
Looking at decking materials for a house in the suburbs of Boston. I like the idea of cedar and using hidden fasteners. Local lumber shops don’t seem to be familiar with best product for decking. Can you let me know what grade of cedar “heartwood common” is??
Above it says: HEM-FIR: Hemlock or fir
Probably not. It is a specific species, I think. Other sources agree.
‘Hem-fir is a species combination of Western Hemlock and five of the True Firs’
HemFir is NOT Hemlock OR Fir.
My name is Robert Cook .I will like to make a inquiry on Redwood
Lumber, below is the specification of the Redwood Lumber.
Size:4×4 s4s 8ft
I want you to go ahead and quote me the
total pick up prices plus tax and also advise me the method of of payment
you do accept. Awaiting your response. Best Regards
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Good luck, and thanks for your question!
if you take a 18′ 3 X 4-DF #2 and cut it so it now becomes two 3 X 4-9′ will it change it to a DF #1
What would I expect to see on a 2″x4″ with a. Grade stamp indicating premium , #2 or better, etc?