20ft rafters for flat roof?

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Old 02-01-19, 11:57 AM
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20ft rafters for flat roof?

hey all
i posted a previous thread about a roof I am trying to put on a cabin. My plan has changed a little. What do you all think of this idea? (See pic)
build up the eastern wall slightly (1:12 pitch)
use 20ft rafters flush with the eastern wall with an overhang on the western wall. (Porch will be added later)
what kind of rafters should I use 2x4, 2x6, 2x8, 2x10?
any advice on type of wood?
do you think they will sag??
thanks you all!

[img]blob:https://www.doityourself.com/a890e3c9-ea9c-4978-bf82-43658e95ad6f[/img]
 
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Old 02-01-19, 12:01 PM
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Like I said before, you need I joists to span that far. At least 12"ers.
 
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Old 02-01-19, 02:47 PM
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There is no dimensional lumber that will span 20'. You can go to timbers which are more expensive and difficult to transport and set. You need some type of engineered product whether a I joist, scissor truss, LVL or glue lam or go to steel. Of all those choices a I joist would be the cheapest and easiest to work with.

Especially with an I joist you will need to protect it from the weather. They do not like being wet. So, even though you are not going to do the porch right away you will need to complete the roof all the way and enclose enough to protect the joists.
 
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Old 02-01-19, 06:46 PM
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An engineered truss will give you more room for insulation and HVAC ductwork if needed.
 
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Old 02-02-19, 06:30 AM
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The folks may be able to supply your needs:

https://www.americanpoleandtimber.com/
 
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Old 02-02-19, 09:11 AM
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Thanks everyone for the great suggestions.
So I should get 20' I-Joists to span the 18'2" building right?
 
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Old 02-02-19, 10:04 AM
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Include your cantilevered overhang in the length. The amount of cantilever needs to be considered as you size the joust. The more you cantilever, the taller the joist must be.
 
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Old 02-03-19, 12:05 PM
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Thanks XSleeper
how do you suggest I build up the side that needs to be higher?
 
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Old 02-03-19, 12:25 PM
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Personally, if your rafter layout, for example, is going to be on 19.2" (or 24") centers, and your I joists are going to be 12" tall, I would just frame up a 30" tall wall with double top plate and double studs everywhere that are also on 19.2 (or 24") centers.

That way your joist hangers will have something solid to nail to everywhere. The joist hangers will need to be centered on the double studs so the the joist hanger flange catches both sides. And you technically should have fire blocking at the bottom edge of the joists.... 12-13" down from the top plate if you use 12" I joists. Or instead of blocking you could sheet the wall with plywood before you put the joist hangers up.
 
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Old 02-05-19, 10:01 AM
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Awesome advice XSleeper
By double top plate, you just mean one on top of another instead of a standard top plate?
 
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Old 02-05-19, 10:07 AM
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One more silly question; if I build up a 30" stud wall on top of double top plate (1.5" each), does that make my rise 33"? Or should I subtract the lower top plate making it 31.5"?
 
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Old 02-05-19, 10:08 AM
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2 horizontal boards make a double top plate. And joints should be staggered. Such as 8+12 and then 12+8. The double top plate will be at the TOP of your 30" wall. The rise is 30" total.
 
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Old 02-05-19, 10:21 AM
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You are amazing!
So to clarify; I am replacing the top plate on all four walls of the structure. I am then building up one side with a 30" stud wall. On top of that, I am installing a double top plate. The top and bottom double plate should not both have joints over the same stud.
Sound right to you?
 
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Old 02-05-19, 10:29 AM
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The wall plate you are replacing and bolting down is the bottom plate of your 30" wall. 30" is measured from the top of that wall plate because the I joists sit on that in front. Your studs would be 27" tall.... nailed into the wall plate (bottom plate). On top of the 27" studs you put a top plate. Then you put another top plate on (double top plate). Total height =30". Hope thats clear.
 
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Old 02-05-19, 03:40 PM
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The bottom plate of your 30" wall only needs to be a single because it's continuously supported along the bottom. The top plate needs to be doubled-up because your joists may not end up over the studs in your 30" wall and the double top plate provides extra strength to span the distance between the vertical wall joists.
 
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Old 02-05-19, 04:47 PM
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Not that this is the only way to do it, but in this case, I'm recommending he make his studs line up "exactly" (double studs that fall to either side of the layout) since the i joist hangers will need to nail into the inside of this end wall.
 
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Old 02-06-19, 12:56 AM
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I understand, thank you so much guys. I am now deep into I-Joist depth research...
 
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Old 02-06-19, 10:30 AM
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Hey all,
Does the picture attached seem like the correct diagram for what I am doing?
 
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Old 02-06-19, 10:40 AM
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That's one way to do it...
 
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Old 02-06-19, 11:12 AM
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...and you can do it however you want. I'm suggesting you do similar to the way it is illustrated in J4 on page 28 of this brochure. Except instead of you having a header, you would have a wall with double studs that is sheathed with plywood or osb. The hangers would be attached to the side of the wall. And you wouldn't need any blocking at the top of the rafters... only where they sit on the front wall and cantilever. Your front wall would be as depicted in H2.

Scroll through these pics. It depicts something similar, but they use a single header on top of a wall plate.
 
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Old 02-06-19, 11:31 AM
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GOT IT! That's the picture I was looking for. You are the man!!
 
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Old 02-06-19, 02:21 PM
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If I am building up a 30" wall and the span is 18'2" and I want a 2' overhang off the low end and the high end is flush with the wall, what size rafters do I need? I can't figure out the measurement with the rise...I'm sorry guys..
 
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Old 02-06-19, 02:31 PM
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Like I mentioned earlier... the place where you buy them will help you size them. We don't know what brand they sell.

An LP brand "LPI 18" on 19.2" centers would work.
 
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Old 02-07-19, 04:45 PM
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Hey XSleeper, with regards to the 30" wall;
Use regular 2x4 for the double studs and insulate the gap left between the studs?
 
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Old 02-07-19, 05:13 PM
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Yes, although I can't tell how wide your wall plate is.
 
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Old 02-07-19, 05:20 PM
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The plate is 1.5 x 5.5 (or 2 x 6)
 
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Old 02-07-19, 06:24 PM
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Do you think 2x4 is ideal XSleeper?
 
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Old 02-07-19, 06:47 PM
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Then 2x6 might be better.....
 
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Old 02-07-19, 07:55 PM
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Hahaaa thanks for bearing with me. Can you explain 'double studs'?
Two directly next to each other (double thickness) for each stud?
 
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Old 02-07-19, 07:57 PM
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yes. You could also frame it normally, and do it like in the JLC photos, with one continuous rim joist.
 
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Old 02-07-19, 08:09 PM
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Continuous Rim Joist seems simpler for a newbie like me. Would you still build up the high side the same amount if you were doing a continuous rim joist?
 
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Old 02-07-19, 08:38 PM
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If you use a rim and the joists are 11 7/8 the wall would be about 18" to give you a 1:12 pitch.
 
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Old 02-07-19, 09:12 PM
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Double top plate still though right?
 
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Old 02-07-19, 09:28 PM
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Yes, definitely. Single studs.
 
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Old 02-07-19, 09:34 PM
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You're the man, so helpful. Can't wait to share once it gets done
Single studs where?
 
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