A-Frame roofs. A-frames are tall, symmetrical frames, requiring only one type of rafter to be constructed. Gabled roofs. A gable is an extended section of roof that extends from one wall, perpendicular to the roof. Mansard roofs. By including an extra joint in each rafter, a mansard roof slopes in such a way as to allow for living space in the area of the roof. Hipped roofs. These roofs slope on all sides, creating a dynamic effect for the home, but requiring a large number of rafters be constructed of variable sizes to account for the slope.
Fink trusses are the most basic, consisting of a basic rafter, joists and jack supports. You can get these manufactured off-site or you can construct them yourself. You can also have them constructed with “room in attic” cut-aways to allow for livable space among the rafters. Scissored trusses are recessed slightly in the center of the rafter, allowing for a vaulted ceiling indoors. Clerestory trusses are a-symmetrical, with one joist extending beyond the rest of the rafter to allow for a windowed-wall along the roof. [3] X Research source
Run of each rafter (in feet). This measurement refers to the total length of each segment of the rafter. Basically, each truss will be made of two rafters, making the width of the house the length of the run, times two. Rise (in feet). The rise refers to the height of each truss, measured from the bottom of the roof segments to the top-most point or peak of the roof. Think of this as the total height of the roof itself. Pitch (in inches). The pitch of the roof refers to the amount the roof slopes for every 12 inches it extends horizontally, and it usually given as a fraction. A pitch of 7/12, for example, would mean that the roof rises 7 inches every foot that it extends. Length of each rafter segment (in feet). After determining the previous measurements, you’ll need to calculate the length of each individual segment of the trusses–the lumber for the run, for the diagonal, and for the sloping sections of each truss. This will depend on the design of each truss and the geometry of the previous measurements.
You should already have the run and the pitch measured out, but you’ll need to calculate the “adjusted” run by subtracting the width of the ridge beam, the center beam in the roof segment that attaches both rafters and makes the truss segment. [8] X Research source Divide the adjusted run by two to get the actual length of each run (the longer of the two sides that make up the right angle of the triangle made by each rafter). On the construction calculator, you can press the “Run” button to program that number in for further calculations. Next, input the pitch of the roof, which you should have already calculated for your design. Upon inputting this information, the calculator will spit out all the info you need: the length of the diagonal segments, the interior measurement of the rise cuts, etc.
If you’re hiring out the construction of your house, ordering pre-fab trusses also helps to keep the cost down, and material waste to a minimum. You won’t have to pay for the time of the laborers constructing the trusses, as well as the materials that go into it. Nowadays, buying the trusses pre-made is nearly universal.
To keep the weight light, 2X4 inch (5cm x 10cm) nominal boards are suitable, as long as the bracing and the member connections you’ve calculated are accurate. For larger roofs or more complicated designs, denser lumber may be required. Though it will cost more, it’s absolutely essential to get high-quality straight-grained boards that are very dense, well-seasoned, and straight as arrows. If you’re building a roof for a home, the lumber must be top-notch. Avoid splitting, knots, and barked edges when selecting lumber.
When cutting the ridge beam (the center board that conjoins both sides of the truss) make sure to measure the height-above-plate (HAP) and mark it on the ridge. When you’re measuring the ridge joint, it’s important that you account for the width of the wood from the diagonal chords of the rafter. If the lumber you purchased is already more or less cut to size, you don’t need to worry about trimming it. Start by making the seat cuts to fit them together and trim them as necessary.
Input the pitch of the roof that you calculated above (we’ll use 7/12 for reference, throughout) on the framing square. On the body of the square (the longer side) input the 12, and input the 7 on the “tongue” (the shorter side of the square) by lining up the stair gauges at the corresponding point on the square. Line up the square with the pitch-end of the chord and mark the line with a pencil. Some carpenters like to make this cut before moving on to making the seat cuts, because it gives the board a nice edge to grip the measuring tape onto. Other carpenters prefer to make all the measurements at the same time and all the cuts at the same time. It’s up to your personal preference.
Start by marking the plumb line, the horizontal line that marks where the wall will meet the rafter. You should be able to calculate this quickly on the carpenter’s calculator. Line up the square with the plumb line, then rotate it 180 degrees, realigning the square on the other side of the board, leaving at least 1. 5 or 2 inches of bracing for the top edge above the seat cut, and at least 4 inches of width for the support on the top cut. While you’re at it, some carpenters like to measure the rafter overhang, since you’ve got the square on that end of the board. Depending on the design, that usually will involve making two cuts to square up the end of the board, leaving usually about 6 inches or so of overhang beyond the seat cut.
Very simple trusses work on a principle of thirds. You can divide the length of the entire bottom run by three, then measure out that distance across the run to determine where the braces need to go. You can mark the center point, then scribe your cuts along each bracing board, depending on the length. Again, this is a very basic method of bracing a simple truss. more complex trusses will require more complex bracing calculations.
Again, some carpenters prefer to make individual chords at a time, or work from rafter-to-rafter, making complete ones and moving on to the next. It’s up to your personal preference and work flow.
It’s a good idea to stack rafters on top of one another on a tarp to keep them level and out of the way. This process will likely take a couple of days, at least, making it important that you care for your rafters in the meantime, especially in wet conditions.
With a carpenter’s level, draw a level line to the bottom of the first rafter tail, adjusting to the tail hits the end of the overhang. Put a mark there, make the same measurement on the last rafter tail, and draw a straight line connecting the two points, marking the bottom of all the rafter tails, trimming them with a circular saw, if you didn’t do this when you were cutting the rafters originally. Cut and nail the sub-fascias plywood to the rafters, extending as is necessary at the ends to make up for the overhang.
Start the sheathing along the bottom of the trusses, setting the first pieces on either end, then moving to another side so the seams will be staggered and the roof will be strong.
