Woodshop Wednesday: Checking Lumber for Straightness

If you’re only interested in the progress on Colin’s house, feel free to skip today’s entry, and every other Woodshop Wednesday. I hope most of you keep reading, though. This stuff is pretty cool.

Wood is a natural building material, and it comes with plenty of imperfections. A freshly cut length of dimensional lumber might dry unevenly, causing the wood to bend or warp. For those who work in construction, dealing with flawed lumber is a fact of life, and a good carpenter will check every piece of wood before using it to see if it’s straight enough.

The thing is, a curved piece of lumber isn’t necessarily bad. Depending on which way it curves, it might work fine for certain applications. Carpenters determine the adequacy by utilizing a concept familiar to any structural engineer: the strong axis and the weak axis. Basically, you can orient any piece of lumber two ways – with the taller dimension up-and-down, or flat. Header beams, floor joists, rafters, and the like are usually oriented with the taller dimension up-and-down, because that forces the lumber to bend about its strong axis and makes it (spoiler alert!) stronger.

A picture is worth a thousand words.

A picture is worth a thousand words.

How much stronger? You can calculate a beam’s moment of inertia (a mouthful for sure, but unfortunately that’s what it’s called) using the formula below, where b is the beam’s horizontal dimension and h is its vertical dimension. A beam with a high moment of inertia I is hard to bend. Obviously, since h gets cubed, you want to make h as large as possible… which explains why the strong axis has the taller dimension up-and-down.

I = b * h3 / 12

What all this means is it’s usually no big deal if a piece of lumber curves about its weak axis. Since you’ll orient your beam to bend about its strong axis, whatever the weak axis is doing will not affect its strength. And since the weak axis bends easily, you can tug it straight if you need it to be straight. On the other hand, any curving about the strong axis compromises the strength of the beam, and is very difficult to force out.

So, let’s sight down a few 2x6s and see how straight they are!

Exhibit 1.

Exhibit 1.

Exhibit 1: This is about as good as it gets. Use this 2×6 as a stud, plate, beam, or anything else. Try not to chop it into small pieces; a straight 2×6 is a terrible thing to waste.

Exhibit 2.

Exhibit 2.

Exhibit 2: This one has some curvature, but the good news is it’s only about the weak axis. Since the strong axis is fine, you can use this piece as a beam, too. I’d avoid using it as a stud, but only because you might miss the mark when you try to nail plywood or drywall to it… not because of strength concerns.

Exhibit 3.

Exhibit 3.

Exhibit 3: U-G-L-Y. This piece curves about both axes. All is not lost, however. You can make it a nailer or temporary brace where warpiness doesn’t matter much, or you can chop it up to use as blocks or cripples.

Today’s Woodshop Wednesday was more technical than usual, but it’s one of the most critical things for a carpenter to know. I hope you both learned something new and enjoyed reading.


5 thoughts on “Woodshop Wednesday: Checking Lumber for Straightness

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