– In today's video we're looking at the simplest and easiest way to lay out a common rafter. For those of you that don't know, there are four main methods for laying out common rafters. And they are the Pythagorean-theorem, unit length, a calculator and the step-off method, which is what we're covering today. Out of all of them, the step-off method might be the easiest one to understand. And that's why a lot of trade schools actually begin with teaching this method. To understand this method, and really all the other methods, it's important to know the difference between the theoretical rafter length and the actual rafter length. Theoretical simply means the measurement or the rafter size before you add or subtract anything else to it. And the actual rafter length is what the rafter will actually be cut to in order to get it to fit. To better explain this, let me make a simple drawing. Here you can see a drawing of a common rafter. A ridge, an exterior wall with a double top plate and an overhang. The distance from the outside wall here to the center line of the ridge board is the theoretical rafter length. The theoretical rafter length is technically found along the measuring line, which is here, which runs from the corner of the birdsmouth, all the way up the rafter. And this is the method that the step-off method and other methods calculate. The actual rafter length on the other hand is the theoretical length plus the overhang amount, whatever you choose that to be, minus half the thickness of the ridge board. Again, whatever that might be. Bottom line, the theoretical length is a fixed measurement because the overhang amount and the ridge thickness can vary from roof to roof. So now that you know that, let's look at the step-off method in more detail. The first number you need to know is the total run, which is simply half the width of the building's total span. And for our mock-up, the span is four feet or 122 centimeters, which means that half that distance would be the total run, two feet or 61 centimeters. The second number we need to know is the unit run, not to be confused with the total run. The unit run is really easy to calculate because it's always the same for common rafters, and that is 12 inches or 30.5 centimeters. The third number to determine is the unit rise. Unit rise is the amount that the rafter rises per 12 inches of horizontal run. Now, unit rise is normally determined by a set of plans or drawings, or you can simply go with what you think looks the best for your project. So for our little project here, I'm gonna go with a seven inch or 18 centimeter unit rise. All right, so now that we have a unit rise of seven inches and a unit run of 12, it's time to take those measurements to the rafter using a framing square. For this example, we'll be using the inside numbers of the framing square and marking on the outside. However, depending on the width of your framing material or just personal preference, you can totally use the outside numbers of the framing square as well. With the square position with the tongue side on the right, line up the seven inch mark with the top of the lumber here. And on the blade side of the square, line up the 12 inch mark. I like to use these framing jigs by SquiJig to hold my numbers so that I can easily slide the framing square up or down, or to move it to another board without losing my settings. The first line to draw is the ridge cut, which is this line here. Then measure over 12 on the blade and make a mark for your first 12 inch unit of run. Now, slide the square down and line up your tongue with the 12 inch mark and draw another line. Remember that our total run of the building was two feet or 61 centimeters. So we're going to need to lay out two full units of run to equal our total run. So with our first unit here, measure over 12 inches again, make a mark and slide down the square to step off the second unit run. Now we have a total of two units each at 12 inches for a total of two feet, which is the total run of the building. Our theoretical rafter length then is the measurement from the ridge line here down to the outside building line, or heel cut of the birdsmouth. More on that in a moment. Because our mock-up has a total run of two feet, it was really easy to lay out because the total run worked out perfectly with our two 12 inch units of run. But sometimes buildings don't end up like this and can be a little bit more, say, two feet, one inch or 63.5 centimeters. How do you lay out for that? It's actually really easy. Once you draw your ridge line, instead of moving right into marking out the whole unit of run, which is 12 inches, you would first measure over one inch or 25 millimeters. Then from that line, you measure over 12 inches on the square, make a mark and draw your first full unit of run. If you do it this way, it takes care of any smaller or odd numbers first, before stepping off the full 12 inch units of run. All right, with the theoretical lines marked out, let's go ahead and mark out for the birdsmouth and the overhang. Now the birdsmouth is made up of two cuts, a heel cut and a seat cut. The seat cut is what sits on the top plate of the exterior wall. And the heel cut is what fits tight to the exterior wall sheathing. If you look back at our rafter, we can see that the last line that we drew was both the exterior building line as well as the heel cut. Therefore, all we have to do is mark out the seat cut to have our completed birdsmouth. The seat cut is often the same length as the exterior stud and the sheathing combined. In our example, the stud is 3.5 inches or 89 millimeters, and the wall sheathing is a half inch or 13 millimeters, which means that the total length of our seat cut is four inches or 102 millimeters. To lay that out, simply use the scale on the bottom of the square to measure over from the heel line four inches and draw a line. With that done, now you can see the birdsmouth, which will get cut out later on. Before we move on, I do wanna quickly bring up the one-third rule. The idea is that you never want the corner of the birdsmouth to be more than one-third the width of the rafter. So if I break our rafter up into thirds, you can see that the birdsmouth is within the first third of the rafter. Codes vary from place to place, so be sure to double check all your local codes before building your roof. Moving on, the next thing to work on is the overhang, which gets added to the theoretical line here, making the rafter longer. Overhang amounts like unit rise can change from building to building, from project to project, because of design or personal preference. For this project, we're gonna go with an eight inch or 203 millimeter overhang. With the square still set up as before, slide it down and measure over from the outside building line, or theoretical line, eight inches and draw another line. This line represents the fascia or sub-fascia line. The fascia line is where
