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Using the Swanson Speed Square for Plumb and Seat Cuts

The speed square has a couple advantages over a normal steel framing square for marking and cutting rafters. Because of the pivot point at the corner of the square, you only need to line up one number rather than two. For example, to make a plumb cut on a rafter for a 6 in 12 roof, you just put the pivot at the corner of the rafter and line up the 6. Here is how it looks for a common rafter:


A second advantage is that the diamond hole in the square can be used to easily mark off seat cuts. The seat cut is where the rafter sits on the top plate. To do this, just line up the diamond with the plumb line you have just drawn and move the square towards the board until the pivot rests against the edge of the rafter. Becasue the distance from the diamond to the pivot is 3 1/2 inches (the same as the width of the top plate) the cut line will be 3 1/2 inches long and will be at a right angle to the plumb line. If this doesn't make sense in words, take a look at a picture or two. Line it up like this:


Then remove the area here:


Of course, the speed square is an old design. Where I am, the building code increasingly calls for the use of 2x6 framing instead of 2x4. This means that the diamond does not work anymore. But with a file and a black sharpie, you can modify your speed square to make seat cuts for 2x6 framing as well. To make aligning with a plumb line easy, I scribed a line entirely across the square. It looks like this now:


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  • Japanese Framing Square (Sashigane) vs. American Steel Square
  • How to Use a Framing Square to Draw an Ellipse
  • How to Check and Adjust a Framing Square
  • Using the Rafter Table on a Framing Square
  • How to use the Swanson Speed Square as a Framing Square
  • Swanson Speed Square Review
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  • Setting Things Straight - Stabila 187 Level Review
  • Squaring Walls and Foundations using diagonals and the 3 4 5 Right Triangle
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  • Comments (8)


    It should be noted that the diamond method should not be used for very steep roofs. Such as a 12 in 12 roof, or 45 degree slope. If you were to use the diamond method there would be hardly any rafter left once you made the seat cut.

    Instead the HAP or height above plate should be found. Basically you want 2/3 of the rafter to be left above the seat cut, so you find out what 2/3 of the rafter width would be. Then just measure down from the top of the rafter 2/3 of it's width, and where that measurement intersects with the plumb line is where you would line up the edge of the speed square.

    Note: Good point and thanks for the comment. It is dependent on the depth of the rafter as well. For example, using the diamond on a 12 in 12 rafter would require a 2x8 rafter. All carpentry requires a little common sense. - Jack

    this site was very help full to me, answered all the question I had, thanx very much. made the job go very smoothly as I am just a home handy man.

    eric;m oelschlegel:

    i;m trying two find a video on' how two use the swanson speed sqare correctly?and understand what it can do for me;two make me a better carpenter.ps. had some trouble with the trim i was laying out?


    Great site! Thanks for posting!

    Lance Rabago:

    This is the best and the only site i came across that teaches you how to use a speed square for common rafters.


    Thanks for explaining this so clearly with words and decent pictures. I've seen quite a few pages (dozens in the last couple of days) that didn't get the idea across.


    I am a female carpenter thats been out for a couple years and just needed to polish up on some math. Great site for it!

    Captain Slappy:

    Thank you for the speed square info!

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    This is a single article from STUFF dated January 6, 2007.

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