I'm planning on building a wooden carport with a lean-to, single slope roof for one car.
The approximate dimensions 3.5 meters or 11.5 feet wide x 5 meter or 16.5 feet long.
The question is, if I could do something like this:
x_____x
| <- |
x <- |
| <- |
x_____x
In words:
3 posts on the left, 2 posts on the right. The arrow indicates the lean-to downwards direction.
I figured this roof configuration would partially relief the right hand structure where there are 2 posts only, relying more on the left hand side where there are 3 posts.
Unfortunately I'm also constrained when it comes to the lean-to roof pitch (around 20 degrees).
I should also mention I live in a region where it snows in the winter but it not really windy.
Is it reasonable to build such a structure taking under consideration the extra strain added by the winter snow?
What would be recommended wooden beam sections to stay on the safe side?
Thanks!
Juan
Best Answer
Structural roof loads are transferred directly downward. Adding vertical supports to one side of a structure (even if it slopes downward to one side) to help support the other side does not work.
In fact, even adding the middle post on the left, does not reduce the “load” on the footing and soil. That is to say, there is just as much load on the soil (1/4 total building load) for the middle post/footing as for the corner posts on the opposite side (which is 1/4 total building loads).
However, if you are transferring horizontal loads, (wind, earthquake, etc.) then more posts will give more resistance. In fact, this structure (as drawn) has a serious rotation issue. Connections at the top and bottom of the posts is critical. Often we use diagonal braces at the top of each post and bury the posts so they can’t sway or use diagonal bracing from top to bottom between each post.
In order to size the beam, we need to know how much snow is in your area. Usually it ranges from 25 psf to 35 psf. Plus the weight of the joists, roofing, etc. is about 10 psf.
So let’s use 35 psf plus 10 psf for a total load of 45 psf. If you calculate the load on the beam, it’s 45 psf times 6’ (half the roof span) for a total load along the beam of 270 plf.
However, to answer your question, Common construction lumber in U.S. is SPF species and a common grade would require a 4x12 spanning about 16’ and a 2x8 or 4x6 for an 8’ span.