- Today, we're looking at how
to frame a common rafter roof by only using the numbers
found on a framing square. There are many different ways to calculate the length of rafters, and I've shown them all
here on this channel. However, I've never shown in detail how to use the rafter
tables on a framing square. If you look on the blade
of a framing square, you will see the rafter tables, which are basically pre-calculated numbers used to find the lengths
of common rafters, hip and valley rafters,
and even jack rafters. As with every roof, you first need to determine
the total run of the building, which is half the distance of the span minus half the width of
the ridge board material. For our mock-up, the total span or total width
of the building is four feet. And the ridge thickness
is an inch and a half. So half the thickness of
the ridge is three quarters. And half the span is two feet. Therefore, two feet minus three quarters gives us a total run of 23.25 inches. So now that we have the
total run, which is here, the next number to figure out is the length of the rafter itself, which is this diagonal measurement here. In order to solve for that, we need to determine the
ratio of rise to run, which is also sometimes
referred to as the pitch. For example, if you want a roof with a steeper slope or pitch, then going with a higher
number on the rise, like 10, for an example, would produce a steeper roof. On the other hand, if you
want a shallower roof, you may want to go with
something more like a four-inch rise, like this. The rise of the roof
is normally determined by an architectural design or codes or personal preference from the builder. And in this case you might be the builder. So for our example, let's
just go with a six-inch rise. Back at the framing square,
the first line of numbers are labeled as "length, common
rafters, per foot of run." And if we slide over to
find our six-inch rise, which is here, directly below that is our unit length for a six-inch-rise roof. And that number is 13.42. What that means is that for every 12 inches of unit
run on a six-inch-rise roof, the diagonal length will be 13.42. And you can see that if I pull a tape between those two numbers. Back at the drawing, we need to translate
the unit length of 13.42 into our theoretical rafter length here. To do that, we first need to convert our run, that is in inches, to feet. And that's easily done by dividing our total run of 23.25 by 12, which of course is how many
inches are in one foot. And that number is 1.94, if I round up. Again, this number is
now in units of feet. What that means is that there's 1.94, or almost two full units of
the 13.42 in our rafter length. And I know that sounds confusing,
but the math is simple. Multiply the 1.94 times the 13.42 for a theoretical rafter
length of 26 inches. With that number figured out, we now can see the length of our rafter, which is from the side of the ridge board down to the outside wall. This measurement does not
include the overhang amount, and that's why it's called
the theoretical rafter length. Like the slope of a roof, the overhang amount, including
the fascia and soffit cuts, are determined by
architectural design codes or personal preference. All right, let's lay out
and cut this rafter to size. With the framing square held to the six-inch mark on the tongue and the 12-inch mark on the blade, mark out the ridge plumb cut line and cut it out with a circular saw. Hook a tape measure over the end and measure down the
diagonal length of 26 inches, and make a mark. With the square still
holding a six-twelve, slide it up or down until
it lines up with your mark and draw the exterior
building line, or heel cut. For this roof, the overhang
amount wants to be five inches. So measure over five and
draw the fascia cut line. From there, measure down four
inches on the fascia line and draw the soffit cut. The last thing to do is mark
out the seat cut length, which makes up the birdsmouth cut here. The length of the seat cut is
normally the same thickness as the exterior wall and plywood. So that makes ours four inches. To lay out the seat cut of the birdsmouth, flip the square over and line up the square
with the heel line here. Then slide the square up or down until the length of the seat
cut measures four inches, and draw your seat cut,
completing the birdsmouth. The birdsmouth then gets cut out, and of course the overhang amount as well. And when the ridge is installed, these common rafters are looking great. Before we end for the
day, I do want to show you one more set of numbers
found on the framing square, and that is the length
of hip and valley rafters per foot of run. In a previous video, we built a hip roof, which actually had the same
run as our example today. And in that video, we used
a calculator to work out the length of the hip rafter, which goes from the bottom corner here all the way up to the ridge. The length of that hip rafter
was 34 and seven eighths. If we look back at our framing square, we can see that just under the number for our common rafter length is the length for the
valley or hip rafter, which for our six-inch rise is 18 inches. Therefore, if we use the
same total run of 1.94 feet and multiply that by 18, which again is the unit
length for a hip rafter, we get 34.92. To convert the 0.92 into a fraction, so it makes more sense
on your tape measure, all you have to do is
multiply the 0.92 times 16. The 16 represents sixteenths, which is the smallest unit of measurement we normally use here
in the US for building. So 0.92 times 16 is 14.72. To get the top number of our fraction, we simply use the first number here, and then put that over 16. So that would be 14 sixteenths,
or seven eighths of an inch, which means that 34.92 is the
same as 34 and seven eighths. So as you can see, the
framing square works perfectly for figuring out all
of your rafter lengths. I hope this video has been helpful. If you still have questions, be sure to watch my rafters playlist, where I go over all of the
other methods available. I hope everybody is doing well. And I pray God's blessing
upon you and your family during this holiday season. (upbeat music)
