Hardware store lumber aisles are confusing. The wood all appears to be identical, yet it’s priced different with varying grades. You notice Stud grade, Select Structural, Number Two…why? Why does all this lumber has different grades and pricing when they look alike?
That confusion arises because everyone thinks all lumber is created equal. In fact, the industry categorize wood into appearance and strength. Knowing what those distinctions mean will save you money. You won’t pay for features you don’t want. And you’ll ensure that you’re putting the right stuff in jobs where it need to be strong.
Understanding Lumber Grades and Sizes
How do grades and types of wood stack up? That’s what this design value chart compares. If you were building something with unsupported spans over great distances, Southern Pine might be better suited with higher bending strengths than Hem-Fir or Douglas Fir. But again, it all depends on the grade. Select Structural lumber are used on posts and beams where failure is not an option. Number Two lumber is the default for your typical framing. Most people simply grab whichever one is nearest.
Knowing that Number Two boards can only have small knots helps you realize that a Number Two board is OK for floor joists but not when carrying heavy loads. It’s the difference between lasting and needing repairs.
On the edge of the wood there’s a little stamp that tells you about its moisture content and seasoning status. That’s important because unseasoned green lumber shrink as it dries up. You could end up with stuck doors or loose nails if your lumber shrank too much. The S-DRY stamp (below) indicates the moisture content is nineteen percent or less. Perfect for framing, where the majority of water weight have been reduced from the wood.
KD-15 is what you look for if you’re installing hardware or doing finish work that requires tight tolerances. Kiln-dried to fifteen percent moisture content, this wood is pretty darn dry, reducing the shrinkage that occurs after installation to keep connections snug and trim nice and straight. It’s not something you want to ignore. It’ll save you headaches later on if you do.
The grades were created by grading agencies such as NELMA, NLGA and WWPA who establish common standards (the stamps) and spell out exactly how much of this defect and that one is allowed. The rule is: A knot is just the circular cross-section of a branch, but if it lands on the tension edge of a beam, it weakens the board considerable; grading agencies also set the rules for what constitutes a check or a split.
You’re not simply judging whether a board is straight. When you scan a board, you are viewing any weakness the grade stamp has previous noted. You can rely on the stamp, but make sure you inspect the board, too. If there’s anything obvious you see. Such as a twist or a bow, it will present issues when you try to build with it.
The other issue that confuses people is what they call nominal size. Two-by-fours aren’t really two inches by four inches at all. They’re dried and planed down to one and a half by three and a half inches. Always order and design with the actual size; don’t use the nominal size. Roofs and floors are calculated for their actual size. If you make a mistake and put a nominal size into your calculation, you’ll get an under-designed floor/roof. It is a small thing but it makes a big difference if load is part of the equation.
Matching the right material to the job is all part of buying lumber. A simple wall stud doesn’t require premium Select Structural wood, but a load-bearing beam isn’t made from Stud grade lumber. The information is in the chart, but you have to use it. Examine the wood for visible defects; look at the grade stamp; and verify the moisture content. Once you know what you’re buying, you quit guessing and build confidently knowing the wood is up to the task. Until then, it’s merely material.
