How To Cut Common
Rafters
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 how to cut
roof rafters is finding its
length. To calculate rafter cuts and 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.
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 and the cuts of the
birds mouth for the rafter, 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.
The process for laying out rafters might
sound complicated, but after you have done this a couple times
you can see how easy it can be.
Mike Merisko (c) 2007
More roof framing
articles.
The Full Length Roof Framer: The
Book
Cutting Rafters Easily and Efficiently
How To
Frame A Gable Roof How To Frame A Hip
Roof
How
To Frame A Gable Overhang
