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Framing Square Know-How

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Some of my favorite tools are those that conceal many levels of usefulness under the guise of utter simplicity. The best example I know of is the standard steel carpenter's, or framing, square. It looks like nothing more than a simple reference tool, one you might casually grab to use as a ruler if you'd misplaced your tape measure. What secrets does this plain piece of steel hold? I'm going to show you a few.

All Kinds of Angles

The framing square has a body (the wider blade) and a tongue (the narrower one), set conveniently at 90� to one another, but it's much more than just a 90� reference guide. When you're building a roof, a flight of stairs or any other angled structure, it can be used to accurately lay out any angle from 0� to 90�. But to use it, you've got to learn to speak a different kind of angle language than they taught you in high-school geometry class.

The way angles are expressed for use with the framing square is based not on the division of a circle into 360� but, instead, on a particular kind of fraction. A carpenter will say that a building has, say, a 5/12 roof. That's framing square talk-it means that for every 12 inches of horizontal run, the roof rises 5 inches. The reason the angles of roofs and other building features are sometimes expressed in fractions is that this mode of expression makes angles so easy to lay out with the framing square. (Although any combination of numbers can be used to express an angle, it's a matter of standard building convention that angles are expressed in relation to a 12 inch horizontal run, which forms the denominator-the lower half-of the fraction.)

The drawing here shows how the gradations on the outside edges of a framing square are used to determine the critical angles necessary to lay out a typical roof rafter. For any given pitch of roof, this simple process automatically calculates the correct angle at the peak, the correct rafter length and an accurate angled pocket (called a bird's mouth) at the bottom where the rafter will mesh with the top of the wall.

Another standard feature of the steel carpenter's square is the labeled, six-line rafter table etched on its side. This amazingly informative table gives (among other things) the correct lengths for ordinary rafters for every pitch from 2/12 to 18/12, and also the lengths for hip and valley rafters for all these pitches (assuming that they'll intersect with the peak of the roof at a 45 � angle). All the information is based on a 12 inch unit of horizontal run.


One carpenter's square accessory that I'm particularly fond of is my pair of angle buttons. These are little brass clamps that grip the outside edges of your square, to aid in building rafters, stair stringers or any other angled objects you need to lay out accurately. To use them, clamp the buttons so they line up with the numbers corresponding to the angle fraction you're after, and they act as reference stops that make it easy to hold the square at exactly the same angle during a series of repetitive positions. Absolute precision for less than $7.00!

Tuning Up Your Square

How square is your square? It's surprisingly easy to check and, if it's a little less than precisely accurate, you can adjust it. Here's how: Find a 4' (or longer) straight-edged piece of wood that you don't mind drawing on. A fresh sheet of plywood is ideal. Hang the inside edge of the longer, 24 inch blade (the body) of the square against the edge of the wood and use a sharp utility knife to score a line across the face of the plywood, following the outside edge of the tongue. (Don't use a pencil, which would make too fat a line.)

Now, flip the square over so that it's in the same relative position, but on the other side of the scored line. (The inside edge of the 24 inch blade should still be against the edge of the plywood.) If your square is perfectly true, and the edge of the test board straight, the tongue of your square will line up exactly with the scored line. If it doesn't line up, you have a little work to do.

First, lightly incise a 45� line across the point of the square and, depending on whether your square needs to be opened or closed, use a center punch to make a series of dimples in the metal, either at one end or another, using a hammer and center punch. Do this work on a solid-steel surface, such as an anvil or bench vise.

This kind of careful checking and adjusting isn't really necessary if all you intend to use your square for is framing. But if you use it for laying out marks on large furniture parts, you'd be wise to invest the time.

Seeing Clearly

Over time, the numbers etched into the surface of a steel square get harder and harder to see. I solve this problem -for a few years, anyway-by wiping a light-colored coat of paint onto the surface of the square, then wiping it off again, so the paint remains only in the etched markings and makes them easier to read.

I'm a long way from being able to use a carpenter's square to the limit of its capabilities. (Mine has at least one table of numbers on it that I've never used.) I've heard of old-timers who could use a square to effortlessly draw circles of all sizes. Who knows how much more I don't know? Part of the carpenter's square's attraction for me are the mysteries I haven't yet plumbed.