Hi and thanks for using JustAnswer.com.
If the cabin is 13 ft wide, the ridge is centered at 6.5 feet. A 2X8 runs from this for 12 feet, which leaves 5.5 feet past the cabin outer wall. From this point another 2X8 runs for 18 more feet, or a total of 23.5 feet beyond the cabin outer wall- so the addition is 23.5 feet not 13 feet OR your cabin is not 13 feet wide. Please correct your math so that I can assist you. Also please give not only the width of the cabin, but also the lenght so that there can be no further confusion as to where the ridge is located.Also the run/rise of the new shed roof. Aside from this ,18 feet at a shallow pitch, is no match for snow loads!
here is a link to a span chart for southern yellow pine.
http://www.southernpine.com/pdf/table38.pdf
notice that with #1 max at 16 OC is 14ft2in and at 12OC is 16ft4in.
This is for 50lb snow load plus 15lb dead load, medium roofing. If your 30 foot run was supported midspan you are still over the limit. The splices you describe, for practical purposes, have no strength whatsoever. You can support at the outside wall, truss at the splice, and support midspan from the outside wall to the new outside wall. Gusset the splice/truss support with 3/4 plywood-both sides- glued and nailed with 8d nails in a 1.5 inch square grid- same on the bottom chord of your truss.
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Thank you. I need clarification on some points as i don't know much about construction.
What do you mean by "truss at the splice"? In your comments about gusset, I think you are saying to put the plywood along the rafter over the splice. But I do not understand its relationship to the truss or what is meant by "bottom chord of the truss"
The contractor will be putting in 2x6 cieling rafters for the new addition which will span between the old exterior wall and new exterior wall. Can these be used to support the midspan of the rafters as you suggested?
Think of 2 X 4s in the shape of the capital letter A. Now sit that A on top of your 2X6 ceiling joist, and your rafter splices on top of the point of the A. Put plywood on both sides of the rafter and top 6 inches of the A. Put plywood on both sides of the 2 X 6 and the bottom 6 inches of the A. If you use the shape of a capital W and do the same thing you have created a Web Truss. The 2X6 is the bottom chord. In a truss there is a vertical 2X4 under the top splice centered in an upsidedown V. The letter A was just an example. Here is a link to some examples. Bear in mind your are using 1/2 of a gable truss.
http://www.woodworkersworkshop.com/roof_trusses.php
I don't think this will work. the problem is that the splice in the new rafter is located in the space above the old roof. The old roof still has plywood on so there are no ceiling rafters in this area--just the old roof. So, how do I brace/support the splice?
One idea I had was to put a vertical 2x6 under the splice to support that area. I would need to mount the bottom of the vertical support on a 2x6 mounted flush with the roof and running parallel to the long axis of the roof. The mounted 2x6 would help spread the force over the roof. One problem is that the new rafters are 16 oc while old rafters are 24 oc so only every third rafter lines up. On those rafters I could run the ceiling mounted 2x6 on the old roof top of the old rafter to help diffuse the pressure of vertical support along the entire old rafter. Does any of this make sense? I have photos.
You could do that, but double up the 2X6 laying across the old roof rafters and use Southern Yellow Pine. Also relieve as much load off the old roof as possible( strip all shingles). In addition make the 18 foot side of the span scissor trusses. See that web link and look at scissor trusses. And include opposing webs that meet the new 2X6 directly over the existing cabin outside wall.
General C&ontractor
35 years of troubleshooting construction, remodel, component & material failures. What to do next