Author Topic: Rafter/Dormer/Spreading questions  (Read 4948 times)

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Offline AaronB

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Rafter/Dormer/Spreading questions
« on: January 12, 2013, 02:58:46 AM »
I am not a carpenter by trade and I've never worked with rafters. All the projects I've worked on have always used trusses. I've also never worked with dormers or a loft area like I'm thinking of doing.

That said, I've created a  few drawings that I hope illustrate what I've come up with after doing my reading and research.

I want to build a 16'x26' house with 10' high walls and a loft over the back 10' of it which would have a kitchen and bathroom located under the loft area. The roof in the loft would have an 8' wide shed dormer on one.

The roof would be 12:1 pitch. The rafters would be 2x8s on 16” centers. The front 16' would be open to below. I planed on putting a 2x6 ceiling joist every 48” and tripling it up just so it looks like a heavier beam (this is colored in green in one of the pictures). One of my questions is, would that be enough to prevent spreading in the front 16' if these ceiling joists are spaced every 4 feet?

The back 10' would have a loft. I've seen where people use ledgers and hangers to build the loft, but I wanted to use 2x8s across the 16' width with a bearing wall separating the kitchen from the bathroom (that wall isn't depicted in drawings). That means the 2x8s span 10' at maximum.  Also, I wanted to support the ends of the loft floor joists in the walls using jack studs; is that acceptable? Will that prevent spreading in the rafters above the loft?

I wanted to create an 8' wide shed dormer above the loft. Is that too wide? Can I do it the way I've drawn it or is there some major issue I'm not understanding?




Loft Cross Section

Open Ceiling Cross Section

Thank you

Offline flyingvan

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Re: Rafter/Dormer/Spreading questions
« Reply #1 on: January 12, 2013, 06:39:27 AM »
   Seems it would want to spread out since there isn't any rafter ties for a pretty good width.  The roof over your dormer would want to settle straight down which would make your frame under the window buckle out at the plate.   You could beef up that ridge beam your rafters tie to with 6x material, then support it on both ends with a column (then the beam itself keeps the rafters from spreading).
    Also---though you didn't ask for this, I'm gonna mention something else from a style perspective.  To me, where you rafter tails end under the dormer, then start up again, would look funny, sort of pre-fab.  Have you considered building some dummy rafter tails to attach to the wall so that eave is continuous under the window?
    I don't see how you get in the loft either
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Offline PEG688

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Re: Rafter/Dormer/Spreading questions
« Reply #2 on: January 12, 2013, 10:35:52 AM »
I am not a carpenter by trade and I've never worked with rafters. All the projects I've worked on have always used trusses. I've also never worked with dormers or a loft area like I'm thinking of doing.

That said, I've created a  few drawings that I hope illustrate what I've come up with after doing my reading and research.

I want to build a 16'x26' house with 10' high walls and a loft over the back 10' of it which would have a kitchen and bathroom located under the loft area. The roof in the loft would have an 8' wide shed dormer on one.

The roof would be 12:1 pitch.

 The rafters would be 2x8s on 16” centers.


 I don't think 2x8 will allow the R-value you'd need to pass code, this of course depends on where you will be building , IF you need to pass code , where you want to insulate. The open to below area creates some issues with insulation , ceiling finishes, wiring , and other factors. 



The front 16' would be open to below. I planed on putting a 2x6 ceiling joist every 48” and tripling it up just so it looks like a heavier beam (this is colored in green in one of the pictures). One of my questions is, would that be enough to prevent spreading in the front 16' if these ceiling joists are spaced every 4 feet?

Yes. IF you thru bolt the joists , purlins , collar ties , it would be stouter, and  it would look better than  a bunch of nails driven into them. 

The back 10' would have a loft. I've seen where people use ledgers and hangers to build the loft, but I wanted to use 2x8s across the 16' width with a bearing wall separating the kitchen from the bathroom (that wall isn't depicted in drawings). That means the 2x8s span 10' at maximum.

  Also, I wanted to support the ends of the loft floor joists in the walls using jack studs; is that acceptable?

  Yes,  I believe it is , you'll have to provide blocking to prevent rotation. The down side it reduced insulation where the jack stud is located.   


 Will that prevent spreading in the rafters above the loft?

It will prevent the wall s from spreading , but the rafters will need some additional work up above.  It appears you drew in 2x4 ceiling joist , the blue member's, is that right? I'd suggest stepping up to 2x6's as those members will be collar ties / ceiling joists, a solid attachment at the plate / joist / rafter connection point will be critical.   

I wanted to create an 8' wide shed dormer above the loft. Is that too wide?

 No, 8' is fine.  Think about the side walls , you've drawn then as 2x4's laid flat, turn those into 2x6 on edge (normal framing, keeping the inside of the 2x6 in plane with the inside of your doubled up rafters , just move your roof line out accordingly , run the rafters tails longer to make your soffits extend  out properly .   


 Can I do it the way I've drawn it or is there some major issue I'm not understanding?




 You've missed a few , maybe more things , but the general idea is sound. It just needs tweaked a bit.

 


 


 


 


 


 

 

 


 


 

 
 


 


 
When in doubt , build it stout with something you know about .

 

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