What size rafters do I need...


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Old 08-03-18, 07:07 PM
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What size rafters do I need...

I just put on a 6X8 addition and need to put a roof on it. It's going to have a 5/12 pitch, what size ridge beam and rafters do I need, 6, 8's?
 
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Old 08-03-18, 07:18 PM
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Depends... how much insulation do you need to get in the ceiling? If you are using 2x12 ceiling joists to get the deepest amount of insulation possible, you would probably also use 2x12 rafters.
 
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Old 08-03-18, 08:03 PM
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My main addition has 2x4 trusses with 8" insulation. The main house runs north and south, the little addition runs off the side of the house east to west. The addition roof will butt against the 5/12 pitch, would I really need 12" rafters over a 6x8 room?
 
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Old 08-03-18, 08:52 PM
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If your insulation is going between the rafters, you need 1" - 2" of air space in addition to the thickness of your insulation. So if you have 7" of insulation, you'll need 2x10 rafters.

If your insulation is between the ceiling joists, you can get by with 2x6" rafters on 16" centers. Keep in mind, if you get a lot of snowfall and accumulation on your roof, you may want to increase the size of your rafters, to accommodate the extra weight.
 
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Old 08-04-18, 04:41 AM
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This little addition roof is only going to have insulation in the ceiling joist and not the rafters. We do get a lot of snow here at times so I think I'll go with 2x8 rafters. Should I also run a 2x8 ridge beam or would I have to go bigger?
 
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Old 08-04-18, 04:49 AM
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would I really need 12" rafters over a 6x8 room?
You may not have heard of energy standards. (See page 14) You need either r-38 or r-49 in your ceiling, depending on your zone. Is this bring inspect​​​​ed? Course maybe they don't care. 2x4 trusses are probably way taller at your top plate because it's a truss. With a stick built roof, you have to build that height with the dimension of the lumber you use... and still maintain height for ventilation. The original part of the house does not need to be brought up to code... they can't force you to do that... but the part being built does. Unless your inspector doesn't care on such a little room.

If you want the rafters to fully bear on the ridge you would use a 2x10 ridge. You will need to clip the ends of your 2x8 ceiling joists at your 5:12 pitch. Don't forget to birds mouth the rafters.
 
 

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