Squares and Measuring Archives

May 10, 2006

How to use the Swanson Speed Square as a Framing Square


Since I published the first review of the Swanson Speed Square, many people have been wondering how to use it. There are actually a large number of things you can do with it, but the most common is probably to use it the same way a framing square is used - to determine the correct angle to cut rafters.I'll show you how to determine the angle you need to use.

But before we get there, a couple words on how the pitch or slope of rafters is stated. In the US, rafter slope is typically given in terms like "4 in 12" or "12 in 12". The first would be close to the minimum slope for a shingled roof and refers to a vertical rise of 4 inches for every horizontal run of 12 inches. The reason that 12 is used as the denominator is that by doing so it is easy to do the math. If you have a 10 foot run, and a pitch of 4 in 12, you simply multiply the 10 by 4 and arrive at a vertical rise of 40 inches.

Now, calculating the correct angle for a common rafter is simple with a little trigonometry, but the speed square takes away the need to drag out a calculator. Just set the pivot at the edge of the board and then rotate the square around that point until the edge of the board aligns with the correct mark on the "Common" scale - marked in red in the picture. Then draw a line along the ruled side of the square (the one parallel with the yellow mark). You can see that at "12 in 12" the edge will make a 45% line.

For hips and valley rafters, the angle is slightly different because the ridge itself is at an angle. This is a bit more difficult to do the math for, but once again, the speed square takes away the need to do math. Just do the same procedure as for common rafters, but instead use the "Hip-Val" scale. You will see that because the boards meet at an angle the cut is not as steep.

One thing the speed square won't do for you is determine rafter lengths. However, simple geometry will tell you that the square root of (rise-squared + run-squared) will give you the length of the other side of the triangle. For this you may need a calculator.

One other feature which I've found useful at times is the scribe feature. In the area marked in yellow in the picture you see a number of notches a quarter of an inch apart. You can set a pencil in the notch and slide the square along the board and have a mark parallel to the edge.

As you can see, the speedsquare is rather coarse in its markings. That isn't to say it is not accurate, but rather, the fineness of the markings is better suited to wood framing rather than fine woodworking. So buy one of these and throw it in your toolbags. It should last a long time.

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November 8, 2006

How to Check and Adjust a Framing Square

Framing or carpenters squares last a long time and are big enough that they can get stepped on or dropped or have something put them out of square sometime over their life. Fortunately it is easy to check and adjust them. To do this simply find a flat surface that you can draw lines on. This may rule out the dining room table.

First draw a straight line twice as long as the leg of the square. With the heel of the framing square in the middle of the line, draw a line up along the edge of the framing square (perpendicular to your base line). Then flip the square over with the heel in the same place and draw a similar line.

Framing Square
The two lines should match. If they don't then the square is not square.

To correct it is simple, but go slowly. If the square is at an angle of less than 90 degrees (bent towards itself) then take a punch and a hammer and make some small dimples near the inside corner of the square. This will expand the metal and make the legs of the square bend further apart.

If the square is wider than it should be, make the dimples near the point of the square. This will bring the legs closer together. Keep checking while you are doing this. If you don't have a punch you can get away with placing the square on a hard surface and whacking it with the corner of the hammer face. Repeat until you get a square square.

December 15, 2006

How to Use a Framing Square to Draw an Ellipse


Most people use a square to make things straight or square, but it doesn't have to be that way. It can be used to make graceful curves as well. Here is how to draw an ellipse with a framing square. Use it to lay out a gentle arch over a window or doorway.

Take a straight piece of wood and put two brads into it. The first goes at a distance equal to the height of the ellipse (shown as distance "A" in red). The second is at half the width of the ellipse (shown as distance "B" in Blue. Place the square with the inside corner at the center of where you want the ellipse. Tape or hotglue a pencil or piece of pencil lead at the end of you stick. Run the brads along the inside edges of the square. Flip the square and then do the other side.

January 6, 2007

Japanese Framing Square (Sashigane) vs. American Steel Square

The American steel square dates from the industrial revolution. It served as a form of calculator in the pre-electronic days and was the subject of a number of treatises and innovation. In contrast, the Japanese square - called a "sashigane" is much simpler, but it has a few features that the American square doesn't which make it better suited for Japanese style woodworking and carpentry.

The first obvious difference is in the size. There are a few explanations for this. First the US square is typically 24 inches by 16 inches. These dimensions match typical framing spacing. Most studs are on 16 inch or 24 inch centers. Making the square 36 inches would make it unwieldy. Japanese carpenters work with a measurement called the "Shaku" which is about 30.3 cm or a bit less than a foot. The shaku is a sixth of the length of a tatami mat which forms the traditional module for Japanese architecture. It is my supposition that the American steel square is bigger because it could be bigger. The availability of large quantities of relatively inexpensive, mass-produced rolled-steel made large squares economical - something that wasn't as possible when hand-forging steel. American manufacturers took advantage of this and stamped rafter tables, inch-decimal conversions, octagon scales and all manner of other arcane and now abandoned reference material on the broad sides of their squares.


Of course size has its disadvantages too. The wide American square is inflexible, while the Japanese square can bend. This is useful when drawing a line square to an already planed edge. The following picture shows the use on a dimensioned piece of lumber, but on a log it becomes more useful.


Coming from a tradition of starting with lumber that is in round form (logs basically) the markings on the Japanese sashigane are intended to help with this. For example on the back side of the sashigane is a scale with measurement in "kaku-me". This unit is equal to the normal measurement (may be centimeters, shaku or even inches) multiplied by the square root of 2 (1.41427). This is used to determine what size square lumber can be cut out of a round log. First place the kaku-me scale across the end of the log and take a reading. In this example it is about 11.


I've drawn a square inside the circle showing the size that can be obtained and also how the square root of two was obtained (you can use the pythagorean theorem to prove it to yourself.)


Finally, the profile of the sashigane is not flat like the American steel square. because Japanese carpenters use ink for marking rather than pencils, the edges are relieve to avold sucking the ink under the square and causing smearing. Sorry I don't have a diagram showing this, but both edges have a slight bevel on both top and bottom and there is a hollowing out in the center.

There are other differences which I don't have time to write about here, but it is interesting to note that the utility of both of the different squares is well tuned for the type of use they are put to. Even if you have no use for metric or classical Japanese units of measure, the light weight and flexibility of the Japanese square is very useful for a lot of things. It is definitely worth having both in the toolbox.

For more about Japanese carpentry, "Complete Japanese Joinery" offers a good instruction in tools, techniques and typical joints/details.

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