5' Roof Overhang?

[builderpat]

I am designing the roof for a cabin in the pacific northwest. Here are the current plans:

None of the walls are load bearing, instead the roof is 21' 2x6 24" o.c. and the rafters sit across 3 4x6 beams. The rafters run parallel to a 1:16 pitch while the beams run perpendicular to the pitch. Each beam sits on two 6x6 posts. The beams span 12' across the posts and the rafters span 8' between the beams.

The main question is the remaining 5' on the rafters will overhang past the last beam. The idea is for the roof to extend long enough to offer protection for the 4' deck below. Is there sufficient support for such a big overhang? Do I need to adjust the rafter dimension or spacing?

 

[StrEng007]

There are several unknowns that you need to determine before you can properly analyze this. My recommendation would be:

Determine your loads:
You need to consider your roof wind loads, roof live loads, snow loads and dead loads analyzed with the proper load cases.

Trace your load paths:
Since you have no load bearing walls, I'm assuming you're working with some sort of braced frame, moment frame or cantilever system to support the overall lateral and vertical systems. You'll need to check each of the components for loading on the individual members, as not to exceed the unity or necessary design checks on your materials. For rafters, I typically check them with components and cladding loads for principle directions (not concurrently). For anything combining more than one directions I'll use MWFRS.

Trace the loads from your rafters, to support beams, to posts, to foundation. Your beams and posts may be acting as "beam columns" depending on how you setup your connections and boundary conditions. How you design your connections is going to determine whether or not this configuration will work.

Know your materials:
If you're using wood, be familiar with the NDS and know what type of wood you're using at the project site (Keep in mind that you're most likely not using a moment frame with wood members).

With your site conditions and materials not specified, it's hard to tell whether or not this will "work". That's the question of the hour and I hope this get's you started in the right direction.

 

[XR250]

I think BuilderPat might be a builder, not an engineer.

 

[RPMG]

The answer requires structural analysis, or posts.

 

[BAretired]

The rafters seem a bit light for a 5'-0" cantilever, but I don't know the snow load in the pacific northwest.

Also, the posts need bracing. You can't just erect six posts without some form of bracing and expect them to stand up. I agree with RPMG that a structural analysis is required.

BA