Common Rafters, Gable Roof Framing & Cutting Rafters
from several types and sizes of material. Common rafters are measured and cut the same way gable roof framing is figured. Horizontal span is the determining factor in this process and regulated by strict building codes. Each state and city will have its own code enforcement rules to be followed.
Common Rafters are figured by taking the total depth of the structure and dividing
it by two. This will give you what is called the run. From this length you must
deduct 1/2 the ridge board thickness.
By knowing the allowable spans or horizontal allowable distance for the type of material you will be using you can easily determine the size of your common rafters and prepare to calculate the diagonal length they will need to be to span from the framing wall plates to the side of the ridge board selected.
Now you would determine the roof pitch angle to be used and proceed to calculate the required length for the common rafters and also the extension or overhang desired.
Your now ready to start cutting your common rafters. By using our example drawings its easy to see how you would use your framing square set on 4x12 to mark the top plumb cut or 18.4349 degrees and cut with your saw set at 0 degrees. See Figure 1b below.
The front wall plate seat cut can now be designed. 2006 IRC code R802.7.1 will allow 25% of the rafter body to be notched at the ends. This notch is commonly called the seat cut or birds mouth. The plumb line to the top would be called the "HAP" and must be exactly the same on all common rafters as well as jack rafters. It is also used on the hip rafters and then lowered for hip drop. This birds mouth can be cut using a skill saw and finished up with your hand saw so as not to weaken the overhang portion of the rafter.
The plumb line in Figure 1c above must be followed on all common rafters, jack rafters and hip rafters to complete a quality job, gable roof framing would follow the same rule. The only exception is the hip. It must be backed off on top or the plumb line in the seat cut can be reduced by 3/16" in this particular application to lower the outside edges to the same elevation as the common rafters.
The overhang part of the rafters would be the same on all except for the hip rafter. Most modern framers would allow for a 2x rough fascia to be included in the total overhang desired and wrapped with aluminum later.
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