Rafter Direction vs Floor Joist Direction

[Micah]

Im building a 12x12/16 cabin with a 7/12 gable roof and at the moment I'm using google sketchup to make a mock up and estimate materials. My question is which direction the roof rafters should go. I vaguely remember ready that the rafter should follow the same direction as your floor joists. I really don't want to do that because my floor joists run east/west and I plan to put solar panels on the roof so I want my rafters to run north/south. Its to late to change the floor joists because my posts are already cemented in place.

[MountainDon]

Need more info.
And I'm confused by   "12x12/16"

What is under the joists, or how are they supported?

If I assume this will have a regular rafter roof where the roof loads travel down the walls the rafters sit on those loads need a strong support path to the ground.

[Micah]

The foundation is a post and pier. The posts are 4x4 and go down about 3+ feet into the ground and I used 8" sona tubes. The pier is a 4x8 build up beam. The floor joists are 2x8's. I'll attach a pic of the mock up so far. This is the 12x12 which I think I am going to stick with.

[Micah]

"Need more info.
And I'm confused by   "12x12/16""

Sorry I didn't address this. I haven't decided wether or not to go 12 or 16 foot. I think I have settled on 12 foot though.

[MountainDon]

It would appear you have a problem.

For clarity let's say the roof ridge is running east-west. Let's call that part of the roof that's drawn in the south side and the wall under the roof eve the south wall.

With the roof as drawn 1/2 of the entire roof materials weight (dead load) plus 1/2 of the roof live load, like snow, rain...) comes down on the south wall. That could change as the wind blows; if the south wall gets the wind on it the north wall load could increase as the south load decreases. That south wall bears on only two points, the beams that run north-south. Even if that rim joist is doubled up that's not enough to support that weight.

In order to position the roof as drawn you would need to have the equivalent of a beam under that wall along with another pier in the center of the span. If you had a full perimeter foundation or a slab this would not be a big deal.

With the roof turned 90 degrees the roof loads then travel down the east and west walls. That load is then distributed by the floor joists along the support beam. Then to the ground.


The illustration doesn't show headers above any windows. That's proably just because the illustration is incomplete. However, the walls that transmit the roof loading to the foundation need headers above doors/windows.

[MountainDon]

If the piers are already in place there may be another issue if you switch to a 12 foot length instead of the 16 foot. Are the piers located with spacing that would permit the use of a 12 foot long beam; 12 feet and no longer. If you need to run a beam the full 16 feet because of spacing you may have a weather related issue.

As illustrated you could place a porch on the south side. However with the beams protruding from under the walls there is the very real possibility of water getting under the wall and back along the beam. This could cause future rot problems. PT wood used for the beams could help aleviate the issue, but then there would still be a potential issue of rot on the floor joist where the joists contacts the beam. PT joists would help with that......   

[Micah]

So I should flip the roof around. Its not a big issue but I figured it would be easier for me to mount solar panels on the roof that it would be to make brackets. Glad I asked before I spent tons of time sketching it up. I will repost after I get everything finished.

I had planed to put a porch like you have said. The floor joists will be PT as well. I had also planed on using a long term sealant where the porch and the floor joist butted together. Believe it or not there are headers, cripple studs, and jack studs there. The white lumber is the framing for windows and doors, the red are wall studs, and the green are upper and lower plates.