Gable roof framing plans
and Calculating how to cut roof rafters
Gable Roof Framing Plans
Calculating Gable Roof rafters for normal gable roofs are cut the same way as the common rafters when figuring for a regular pitched hip roof. Dividing the span of the building in half and deducting half the ridge thickness will give you the run
measurement to be used in this calculation.
You can use Figure #1 for standard gable roofs.
Once you know the run to be used for your project you will now need to determine what pitch or angle the roof should be and how much overhang is needed.
In Figure #1 above we are using 4x12 pitch for our example. I have selected 2x12 syp (southern yellow pine) material but you can use whatever size and type you want so long as it will meet your building code requirements. Working with your chosen material stacked on top of a good set of saw horses you will need to crown each peace. You can stand at the end with the rafter laying flat and raising your end look down the top edge to determine which way the bow or bend is going. You will not likely find many peace's that will be exactly straight so by doing this you ensure your roof will lay as flat and straight as possible given that the total weight will deflect the lumber's crown back to a straight position.
With all your material crowned away from you, your ready to start laying out the rafter pattern that you will use to mark all the others.
This is the best place to start in laying out your roof rafters so that all parts will be the same. Using your framing square and having crowned your rafter material away from you place your square at the left end of the material with the tongue set on 4 and the body set on 12 at the top edge of the rafter material shown in Figure #1b below and mark along the tongue.
With your framing square still in place you need to determine how much overhang will be required or wanted and mark along the bottom edge of the body of your framing square that length then keeping your 4x12 alignment, then slide the square up to that point and mark your material. This will give you the plumb cut at the seat cut or birds mouth portion of your rafter. You will want to determine the depth of the rafter you are permitted to cut away. It is usually 25% to 30%, in this example I will use the former giving me a total rise for my plumb cut of 7 5/16" and mark this in front of the tongue on your framing square.
Now flip your square over and again align your 2x12 marks and then slide the square over to your seat cut mark you just made and mark along the body of your square. You can figure the end of the overhang and slide square over to that mark and also mark it. That gives you your seat cut and overhang complete on your pattern.
From the point of your birds mouth or the bottom of the plumb cut you can measure the length of the rafter. Or a easier way is to measure from the top edge of your rafter pattern starting at the plumb line and measuring along the top edge to the proper length you need. As shown in Figure #1 above our example would be 15'-5 13/16" to the long point of your new roof rafter pattern.
Now that you have your roof rafters I would link to point out a few very important things for your installation process.
When your marking other rafters from your rafter pattern you want to be sure that you flush the top of the rafter pattern with the rafter beneath it. If you don't do this it will cause problems later. The marking of the birds mouth area can cause humps in the finished roof and the overhang as well.
Secondly and equally important when your installing the roof rafters you "must" pull very hard to keep the heal of the rafter tight against the exterior walls. Do not allow the rafter being installed to even touch the ridge board at this point until your partner has finished nailing the rafter seat against the ceiling joist. If this is not maintained through out your project it will not turn out as well having sags and high spots.
When you start installing your roof rafters start at the end with front and back and just let them rest together, then move inward to the end of your ridge board and set one rafter front and the matching rear rafter letting them just rest against one another as well. Now very carefully slide the ridge board at both ends between the two. The weight of the rafters will hold the ridge almost completely while you nail each side. Once you slide the ridge in place don't lift up on either rafter or the whole thing will come down causing damage.
Calculating the actual length for the roof rafters can be accomplished in several ways by doing the math or by using a modern calculator designed to do this for you.