Common rafters are, well, the most common rafter when it comes to residential roof framing. Gable roofs are built with all common rafters, as are shed roofs. A hip roof, depending on the length of the building, has at least four common rafters. The rest of the rafters in a hip roof are called hip and jack rafters.
The first step in cutting a common rafter is finding its length. To find the length one must know the distance the rafters must span to make up the roof. The span is found by measuring the width of the building. This measurement is taken from the outside of the walls including the wall sheathing. If the width of the building is 30 feet, and the roof pitch is a 6/12, these are the factors that will be used to find the rafter length.
There are several methods that are used to get the rafter length. Two of the most popular means of finding that rafter length are the construction calculator and rafter table books ("The Full Length Roof Framer" by A. F. J. Riechers). Both ways will give a precise number to cut your rafter to. The book "The Full Length Roof Framer", is also full of roof cutting secrets.
For our example we will use the 30 foot building width and a pitch of 6/12. This means our rafters will rise 6 inches for every 12 inches of run. Using the book "The Full Length Roof Framer", and opening it to the rafter tables for a 6/12 pitch, you will find all kinds of information about that roof pitch. Under the common rafter table and the span column in feet, read down till you find 30 feet. Next to it you will find the length of the rafter which is 16'9 1/4". This is the length of the rafter but there are a few more calculations to be factored in.
To lay out the rafter cuts on a 2x, I like to use a framing square. I feel its more precise than a speed square, which is a great layout tool in its own right.
To start, pick out a straight 2x to make your rafter cut layout on. This will be your pattern to cut all your common rafters from. If it has any crown at all, that will be the top of your rafter. Lay the 2x on a pair of horses with the top away from you. Since our roof pitch is a 6/12, these will be the numbers we will use on the framing square.
Starting on the left end of our 2x, lay the framing square on the 2x with 6" on the outside edge of the tongue on the bottom edge of the 2x. Put 12"on the outside edge of the body of the square on the bottom edge of the 2x. Move the top of the tongue till it hits the upper left hand corner of the 2x. Scribe a line along the outside edge of the framing squares tongue. This is your plumb cut. If there will be a ridge board, the rafter will have to be shortened half the distance of the ridge. We'll get to that a little later. There are brass stair gauges that can be bought that clamp onto the square at the pitch you are using. Instead of lining up 6" and 12" every time, all you need to do is bump the gauges to the bottom of the 2x.
To get the length of the rafter to the birdsmouth, hook your tape measure to the upper left hand corner of the 2x. Pull the tape and mark 16' 9 1/4" on the top edge of the 2x. Put the framing square at 6" and 12" on the bottom of the 2x and line up the tongue of the square with the mark on the top edge of the 2x. This line represents the outside of the wall and the back of the birdsmouth.
The level cut for the birdsmouth is usually the thickness of the wall. If it is a 2x4 wall with 1/2" sheathing, the level cut for the birdsmouth will be 4" long. To mark the birdsmouth, put the tongue of the square against the line. On the outside edge of the body, put 4" on the bottom edge of the 2x and draw a line on the outside edge of the square. The birdsmouth should end up being 2" deep.
Say we want to add an overhang to our rafters. In this instance lets say we want to add 12". Since we are adding 12" to the rafters on both sides of the house, we need to add two feet to our 30' span. Again we go to our rafter table book ("The Full Length Roof Framer"). Open the book to the 6/12 page and look up the span for 32 feet. The overall length of our rafter will increase to 17' 10 5/8".
To mark the plumb cut on the rafter tail, pull the tape from the upper left hand corner of our 2x (long point of our plumb cut). Mark 17' 10 5/8" on the top edge of the 2x. Again put 6" and 12" of the framing square on the bottom edge of the 2x and line it up with the mark on top of the 2x. Draw a line along the squares edge and this will be your tail cut.
Two more things must be considered to complete the layout of our rafter. As mentioned earlier, our rafter must be shortened to allow for the thickness of the ridge. In most cases the ridge board is a 2x or 1 1/2" thick. This means we must deduct one half the thickness of the ridge from our rafters or 3/4". On our 2x mark another line 3/4" to the right of our original plumb cut line. Do not measure along the top edge of the 2x but rather off of our original line. Either make one mark and draw a 6/12 pitch or make two marks and draw a line through them. Either way will work. This is your new cut line. Mark out our erase the original line so as not to get confused.
The other consideration is shortening the rafter tail to allow for the thickness of the fascia board. If the fascia is to be a 1x, shorten the tail 3/4". If its a 2x, shorten it 1 1/2". Shorten the tail cut in the same manner as the plumb cut. Make a line the thickness of the fascia parallel to the original rafter tail cut line. Again cross out the original line to lessen the confusion. The bottom of the tail cut may have to be clipped so it doesn't hang below the fascia board.
You now have the layout for a common rafter. You can now cut the lines that are marked on the 2x. When making these cuts, make them straight and with precision as this will be the pattern for marking and cutting the rest of the common rafters. When cutting out the birdsmouth, its okay to overcut the lines to completely remove the material.
I like to nail stops to the top of my rafter pattern. I use scraps of plywood about 4" long and about 3" wide. I nail one about 3 to 4 inches from the plumb cut, letting it hang over 3/4" on each side of the rafter. The other stop is nailed just above the birdsmouth, hanging over 3/4" each side of the rafter. Now all one has to do is put the stops against the crowned side of the 2x and trace the cuts to be made. If you laid out all your rafters individually, it would probably take 4 times longer or more.
It might sound like a complicated process, but after you have done this a couple times you can see how easy it can be.
Mike Merisko (c) 2007