It’s a great cut list, but then you find out the board you purchased doesn’t match what you ordered. Your joinery won’t fit and the board are narrower, and yes, even thinner. It’s something that happens to nearly all beginner that buy softwood lumber for the first time.
Why? It is not because they are poor carpenters, but because of the confusing difference between nominal and actual sizes. Knowing how to tell the difference is most important skill in buying lumber efficienty.
Nominal vs. Actual Sizes
But as the chart above illustrates, there’s a lot of material that gets lost in processing. A “nominal” two-by-four is never really two inches by four inches. By the time it’s been planed smooth on all four faces and dried, the wood have shrunk to one and a half inches by three and a half inches.
The loss isn’t arbitrary. It’s systematic. To account for shrinkage when the wood dries, plus some space for milling, rough green lumber begins life oversize. Next, the mill plane off each face, removing about a quarter of an inch per side. This results in what you’ll find in the store: the dressed-up standard sizes.
Use the nominal numbers in your designs and your joints will be sloppy. Your cabinet sides won’t fit together. You’ll underestimate depth of your shelves and they’ll droop.
To make things even more confusing, there’s the pricing system. Wood is sold at lumberyards by board foot, which is based off the nominal size, rather than the actual one. That means you’re paying for the two-by-four, when what you actualy recieve is the one-and-a-half-by-three-and-a-half stick.
And this applies to each of the other softwood species in the guide. Hem-fir, southern pine, Douglas fir and mixes of SPF all adhere to same dimensional standard. They differ in grain and strength properties, but not dimensions from the mill.
In your mind, maintain these two systems as distinct. Plan orders and budgets in nominal sizes. Cut and fit using actual sizes. Get them mixed up, and it’s frustration and waste.
Look at the thickness classes. The one-by family (which is typically shelving or trim) goes down to three quarters of an inch, twenty five percent less thickness. So if you’re figuring a rabbet or a dado on a piece with one-inch thickness, it won’t work. Remember to factor in lost quarter inch on both faces.
Joists and studs make up the two-by family, but they settle in at one and a half inches thick; half an inch less per face. The beauty of this sameness is that connectors and other hardware can be made to fit certain thicknesses; without it there would be no way to frame.
WASTE: Waste also depends on length. The majority of yards carry 8, 10, 12 and 16-foot lengths. Wall framing will be mostly 8 feet long. Rafters and joists can sometimes benefit from longer lengths. However, transporting them becomes difficult and the cost per linear foot increases. Fewer off-cuts result from purchasing the appropriate length.
For example, if you require a 4-foot wide shelf, purchase a 12-foot board, no waste, three shelves. Purchase an 8-foot board, 2 shelves, 4 feet of waste. Thinking before you buy saves you money.
Read the grade stamps The structural grade, as well as moisture content and species, is all listed in the ink stamp on the side of the board. If you’re working indoors, look for “dry” lumber only; anything over nineteen percent moisture content will warp down the line. Outdoor projects call for pressure- treated wood (the same nominal sizing rules apply, but it won’t rot).
Before leaving yard with your boards, check them for twists, bows or general warping. Straight is right, and it’s worth a few bucks more.
This is dressed lumber. Now we need to adjust our minds a bit to go from rough sawn to dressed lumber, because that’s what you can use. When you shop, try to switch gears and think in terms of actual sizes.
Before going to the store, measure out what you need. Next, tack on an additional amount for the milling loss (this is how much they cut off). It’s such a simple mindset change that makes a chore far more manageable.
Your cuts will fit, your joints will line up and you’ll no longer be able to blame the lumber yard for your screw ups. The wood isn’t trying to lie to you. You simply need to learn to speak its language. If you get inside your head about the fact that what you see on the tag doesn’t equal what you feel in your hands, then building gets a lot easier.
