Rafter triangles

The go is often called the run or the span of the rafter or the roof. It is almost always given on the architectural drawings.

The rise or height may be given on the drawings, in which case it is easy to draw in the longest side, measure it and take the bevels or angles.

More often than not though you will be given the information to get the rise in a different way. At the right is how to get it if only the span and the roof pitch are given.

The base is drawn and a line drawn for the rise. A protractor is used to drawing the angle of the roof and that line is extended to meet the vertical to finish the triangle.

In North America the roof pitch may be given as a ratio, usually with the run as 12 with the rise being the variable.

Draw a triangle at the desired ratio. This will give the pitch or roof slope angles. Then draw the rafter triangle inside or outside that triangle to get the length.

Rarely you may have a desired rafter length. Lay out the span and a vertical construction line and then you scribe an arc the length of the desired rafter and where it cuts the vertical is your third point.

The on site substitute for a trammel or a large wing compass to scribe the arc is a length of timber batten with two nails driven through it the desired distance apart. Fine adjustments being done by bending one of the nails.

Looking again at our original sketch at the top of the page you will see that account has to be taken of the eaves overhang.

As you can see from the sketch at the right we could simply add the small overhang triangle at the end of the main span. This is OK, but not really good practice. This makes the drawing larger than it needs to be. The whole purpose of drawing roof sections to scale is to do them at a large enough scale so that good level of accuracy can be maintained.

If you add extras like this then our may have to go too a smaller scale to fit it all in and that means less precision in your measurements.

So the answer is to draw the overhang triangle inside the main one. Less work and greater accuracy.

In the previous example we placed one triangle inside another in the interests of keeping the drawing at as large a scale as possible and of course this trick can be used more than once.

The sketch shows a house with three different roof spans. As long as they are all at the same pitch or roof angle then they can all be combined into the same simple drawing. Just make sure they are all labeled clearly.