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neb
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# Posted: 30 Mar 2011 07:53pm
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I want to know if a 2x4 will be strong enough for a rafter that will be open and no supports. It will have a birds mouth cut and fastened to a main beam 2x8. They will be 2 ft center on the rafters. I will have a 32inch rise on gable end and will only have 5 foot span. The shack will be 10x10. I cut a pattern from a 2x4 and allowed for 7/16 sheathing board so it fits tight.
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BlaineHill
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# Posted: 30 Mar 2011 08:10pm
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Load tolerances are a strange thing depentant on not only span, but also roof pitch and other things like expected snow load. Looks like a 32 inch rise and a 60 inch run. If you are using a 2x8 ridge board, a 2x6 cut on angle (looks like about 28 degrees) will cover about 6.25 inches of the 7.5 inch ridge board. I would bump it up to 2x6 rafter and as a bonus you can put in a little more insulation. Also, after you cut the birds mouth you won't have much width left if your use a 2x4.
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turkeyhunter
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# Posted: 30 Mar 2011 08:37pm
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2 x 6 at least....
on my floor joists at the new camp, i could have went with a 2 x 8....but 2 x 10 were only a few bucks more....
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neb
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# Posted: 30 Mar 2011 08:40pm
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BlaineHill
Actually I just went out and measured at the birds mouth and I have 2 inches of wood after the cut. I did a simulation and went up 32 inches 5 foot span and alowed for the main beam and the sheathing also I will have 1 foot of hang over on the rafter. On the birds mouth that will lay on the top plate I have about 4 inches of cut. I might be full of crap but trying to get everything figured out and cut so I can pack everyhting in. Does this seem right to you and thanks again for the help.
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MtnDon
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# Posted: 30 Mar 2011 09:33pm
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32" rise to top of ridge? That makes it about a 6/12 pitch.
An S-P-F 2x4 grade #2 would be just capable of spanning 5'2" with a snow load of 40 lbs and 24"OC spacing. Note that is grade #2 and not stud grade which which has much less bending resistance. Other species and grades will vary.
You could go up to a three inch maximum birdsmouth foot cut without making the rafter tail too weak. The rule of thumb is to keep the HAP (height above plate, as measured vertically from the top plate to the top sloped edge of the rafter, to no less than 2/3 the distance across the sloped rafter. Not the actual lumber width, the vertical distance.
A 2x6 gives more insulation room if the roof is to be insulated. Don't forget about ventilation if you do.
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neb
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# Posted: 30 Mar 2011 09:46pm
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First I would really like to thank all of you for your time. Now if I use a 2x6 that would be know problem for rafter material. The question is now I have a 2x4 pre cut rafter I did for practice. If I lay that 2x4 on the 2x6 and lay it at the top side of rafter and draw out the pattern from the 2x4 to the 2x6 I will have a very larg birds mouth cut. Will this pattern work to just draw it on the 2x6 and of course the cuts would be longer where the rafter meets the 2x8 ridge and the birds mouth I hope this makes sense
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MtnDon
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# Posted: 30 Mar 2011 09:58pm
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Why not lay the 2x4 pattern on the 2x6 at the lower edge of the rafter? You'd de duplicating everything that matters, just making it a little taller on the outside.
Do you have snow load? You could also space the rafters at 16" OC to provide more strength. That would be good to 6'9" with 40# snow. And, oops I made a small error on the first calc; the 2x4 at 24" would span 5'11" not 5'2"
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neb
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# Posted: 30 Mar 2011 10:25pm
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MtnDon
I know I'm stupid but wouldn't that make my rise from the top plate to the top of the ridge beam taller. If I layed 2x4 from the bottem of ratfer instead of top of the rafter. This would add to the height and it would not be 32 inched from top plate to the ridge plank. Call me dumb I don't blame you.
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BlaineHill
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# Posted: 31 Mar 2011 07:06pm
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If I am following this right... if you match the bottoms and trace, you will have to raise the ridge board. If you match the tops and trace, you will have to lower the side wall. I don't think either option is recommended. I would start over. Expect a slightly lower roof pitch with the 2x6 so don't try to transfer over the angle where the rafter meets the ridge either. Good luck.
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Anonymous
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# Posted: 31 Mar 2011 07:44pm
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BlaineHill
Exactly the way I see it and thanks for the drawing. If I want to copy from the 2x4 to the 2x6 I would have to lay it on the top of the 2x6. The birds mouth would be deeper cut and wouldn't think that would be a problem. I still would have 3 inches of wood above the birds mouth cut., This way I could maintain the 32 inch rise that I want. Do you agree with this.
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BlaineHill
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# Posted: 31 Mar 2011 09:30pm
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I don't know the horizontal dimension of your original birds mouth cut. If you line up the tops, trace the board, and the horizontal cut on the birds mouth does not exceed 4" (stud wall plus exterior sheet), and you are happy with the thin tail rafter thickness, I see no problem. Just promise you won't invite me to call you dumb or stupid anymore.
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MtnDon
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# Posted: 31 Mar 2011 09:51pm - Edited by: MtnDon
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My thought was that the overall height was not so important as to a couple of inches, that's why I suggested what I did. If the overall height must stay the same then that changes what I suggested. My misinterpretation.
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Borrego
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# Posted: 31 Mar 2011 09:57pm
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Just use 2x6's and forget all this difficult math :-)
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neb
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# Posted: 31 Mar 2011 10:03pm
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BlaineHill
Again thanks for the help. I know I'm making this thing harder then it should be. If I do the same thing I did for the 2x4 pattern and this time use the 2x6 I could make it less of a cut for the birds mouth. I will set it at the 32 inch rise and give it a try and should get a smaller bird mouth cut.
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