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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

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    Make sure that extra post isn't in a place where it will be hit by a car door as you attempt to exit the vehicle. You may remember, but your guests/children/etc. may not, leading to dents in doors.
    – FreeMan
    Commented Mar 18, 2021 at 12:44
  • Then again, you in the dark, late at night, when you're tired and in a hurry may well forget that extra post is there. Until it's too late...
    – FreeMan
    Commented Mar 18, 2021 at 17:59
  • the left side post will be next to the fence, so it won't make much of a difference. Nonetheless I can always stick something to the middle post to avoid these incidents. Thanks for the advice :)
    – Juan Cortines
    Commented Mar 18, 2021 at 19:02

2 Answers 2

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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.

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  • Thanks, this is very useful. Since the 5th post is not going to help with the strain on the right beam, I could just remove it. Thanks for explaining that. I also understand the same goes for the roof pitch - even a small pitch should do. I was planning on adding 2 diagonal braces per post connecting with the beams. Should that suffice? I was also planning on using H-shape steel anchors for the posts. They seem quite solid.
    – Juan Cortines
    Commented Mar 18, 2021 at 20:11
  • 1
    The “H shaped” post bases are considered a “pin connection”. The posts will rotate and sway , especially with all that weight above. Two diagonal braces about 3’ long per post at the top should be adequate, but I’d use a diagonal brace for the bottom too. It may have to go post to post each direction.
    – Lee Sam
    Commented Mar 18, 2021 at 20:34
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If your question is:

"Can a lean-to shed be designed so that it can be supported with 3 posts on one side and only two on the other?"

Then the answer is yes, it can be done. Is it too "risky"? Too risky for what? To support the expected snow and wind load? The answer to that is:

"Yes, that too can be done."

I suspect, however, your real question is:

"HOW do I do this?"

The answer to that is:

  1. Seek out some pre-designed plans for such a structure that will be acceptable to your local AHJ to obtain a building permit.
  2. Get a design done for you by an architect or a licensed engineer that has been analyzed to support the expected loading in your area. (The AHJ will want this when applying for the building permit.)
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  • Thanks! In my country there's no need for a permit for this type of structures and I could do some over-dimensioning in order to be on the safe side. But yes, you are right my answer is not clear. I'll edit it.
    – Juan Cortines
    Commented Mar 18, 2021 at 10:22
  • With only two end posts on one side, would need to at least double wood depth compared to the side with three posts. A steel I beam might be better for that side, if you can't put in another post.
    – crip659
    Commented Mar 18, 2021 at 13:44
  • yeah I considered that as well, but I'm afraid then it wouldn't be a DIY project anymore.. Thanks!
    – Juan Cortines
    Commented Mar 18, 2021 at 19:04

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