Hi, I'm Dave Osborne. With over 50 years experience as a journeyman carpenter, foreman and contractor in heavy construction I enjoyed working with apprentices and sharing the tricks of the trade that others shared with me. Now I get emails from Members all over the world and we include many of my answers in our Free Monthly Newsletters. Some of my answers include drawings and instructions specific to a project, but may also answer your questions. I use correct construction terminology, so you can confidently inform your building supply dealers or contractors exactly what you need.
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When installing barge rafters to support an overhang on a roof, using 2X6's, is there a standard (or recommended) spacing on how far apart to set the barge rafters? My overhang is 32", it's a 10 in 12 roof with a span of 17' plus the overhang. My rafters are on 16" centers and I'm going in three rafters due to the long overhang.
When framing for a gable end overhang, usually an overhang up to 16" can be done without building "ladders" or "look outs", as we call them, for the overhang. In your situation, with a 32" overhang you should use the ladder technigue as you have chosen. The rafters going out 90 degrees with the gable end usually sit on the gable end truss which is cut down by 3½". The look outs then are 2x4 on edge and usually go inboard over the gable end truss into the next truss laid out on 24" centers. In your case with 2x6 rafters, I would do the same, except as you say remove one rafter from the gable end and attach the lookouts into the second rafter over from the wall line, at least 24" from the wall line.
On the wall line build your gable end rafter lower by 5 1/2" (or 3 1/2" would do, too depending on the choice of barge boards matching the fascia). These look outs should be laid out on 24" centers starting from the eave where your sheet of plywood starts. May as well have the 4' joint of the plywood butt up on one of the lookouts. Also, the ridge board should continue and catch the overhang rafter on the top and the rafter trim catch the bottom. When installing the plywood, try to get most of the sheet on the roof to help support the overhang. Stagger the sheets here by only 16" with the longest sheet in the center of the span. That is to make sure the overhang sheet nails into a rafter at least one inboard from the lookout connection. This way, your sheathing helps support the overhang, too.
Here is a drawing to help explain. This can apply to rafters, as well:
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