Lean-To Design

[jaw22w]

I had a 30x40x12 pole structure built and am just about to finish up the interior. I can see it all ready. It ain't big enough! Now I want to add a lean-to on a gable end. 28x10. I had a 2x12 installed at 12 feet elevation continuous along the 30-foot wall under the siding just under the truss for a header beam for the lean-to. I don't want to mess with removing the corners of the building, so I am going to inset the length of the LT about a foot off the corners to make the lean to 28 feet long. I will attach rafter hangers to that 2x12 at approximately 4-foot centers.
So now the other end of the rafters needs something to land on, hence the question. With a 28-foot length and 4 posts that makes for 9'-4" spacing. That doesn't fit with the 4-foot spacing of the rafters. My pole barn has poles @8' centers with a post at the truss then one in the center on 2x8's inside and outside. This lean to would be different with 2 rafters landing between the poles but only 2/3 of the rafter span. So, the question is what size should the beams be in my scenario?
Well, I have probably confused the heck out of you. Here is the rough sketch I made. I figure a drawing is worth a thousand words.



I would just go with 2x10 or 2x12 and be done with it, but I am trying to maintain maximum head height at the open 28' side. I have some equipment I want to store in the LT. I need as much headroom as I can get. With the beam height plus the 2x8 rafter and the purlin and trying to maintain a 3:12 slope, it is going to be pretty tough.
I can't raise the rafter attachment point any higher on the end wall. Possible ways to gain headroom? Maybe a 2x6 rafter would be sufficient for a 10' span? Maybe cheat 3" or 4" on the 3:12 slope?
Any suggestions on lumber sizing or how to gain some headroom at the beam would be appreciated!

It should be an official Murphy's Law. No matter how big you build your building, it is never big enough!
P.S. If someone knows how to rotate that pic, it would help

 

[billconner]

What is the design snow load for your area? That determines load for header size.

 

[jaw22w]

I live in Northcentral Indiana. A little research shows my snow loading to be 20psf.
Are there charts to show beam sizes somewhere?

 

[Firebrick43]

I live in Northcentral Indiana. A little research shows my snow loading to be 20psf.
Are there charts to show beam sizes somewhere?

https://www.southernpine.com/span-tables/headers-beams-size-selection-tables/

 

[ycgoat]

I am not a carpenter but I would expect a beam across the the posts at the front, or did I just not notice it in the diagram

 

[dougf]

I was going to post the same chart that Firebrick43 posted, but he beat me to it. Know your snow load and live load, check the span table and move one size up to give yourself some leeway in heavy snow.

You could also size things up and move down to a 2/12 pitch to give yourself some more headroom. Remember the rafters will sit on top of your end wall, so you could just build your open end to your desired minimum height and let the pitch be waht it is. That's what i'm doing with my shop extension and i end up with a 1.5/12 pitch.

 

[Copymutt]

Amazed at the snow load for your area. Thought it would be closer to 50psf.

 

[jaw22w]

https://www.southernpine.com/span-tables/headers-beams-size-selection-tables/

Thanks! That will be a good reference. Unfortunately, the table only shows sizing for spans of 16' or greater. I am only spanning 10'. Unless I'm reading the table wrong.

After a night's sleep, I have a new idea on post to post spans for my 28' foot LT. I'm thinking if I went with a 8'-12'-8' spacing that would line the posts back up with the trusses. Use 2- 2x8s in the 8' bays and 2 -2x10s in the 12 foot bay. for the beam header.

To use for comparison, in my pole barn, the 10 wide doors have 2- 30' span trusses landing in the opening on 2-2x12s. The doors have a pole on each side to the ground. The max snow load on this area is: 10' (door opening) x 16.3' (rafter length) x 20psf = 3260#. The snow load on an 8' span in the barn on 2 2x8s is 2613#. The 12'x12' area of roof in the lean to comes to 2880#.
My homespun math is probably an oversimplified way of doing this, but from this reasoning, I know that the 8' bays will be fine with 2-2x8s. I think the 12 foot bay's span would probably be good with 2-2 x10s, but I would feel better with 2-2x12s. That will leave me with 2- 8' wide bays with an extra 4" of headroom. I can live with that.
Thanks for getting me thinking in the right direction.

 

[dcg9381]

After a night's sleep, I have a new idea on post to post spans for my 28' foot LT. I'm thinking if I went with a 8'-12'-8' spacing that would line the posts back up with the trusses. Use 2- 2x8s in the 8' bays and 2 -2x10s in the 12 foot bay. for the beam header.

That makes sense to me. Depends on what you're doing out there, posts can be a pain in the ***. You could easily span 28' with an LVL, no need to post it in the middle. Course an LVL is heavy, hard to handle by yourself, blah blah blah.

 

[krapie]

What dimension lumber are you using for purlins? Will likely need to be larger for the 12’ span.

 

[jaw22w]

What dimension lumber are you using for purlins? Will likely need to be larger for the 12’ span.

The 4' on center rafters will continue right across that 12' span. I will be using 2x4 x12' and 16', laid flat.

 

[billconner]

I think your conclusion is fine but to maximize clearance you might keep original post spacing and see if a 5 1/2" deep lvl would work. Also consider a 2:12 pitch.

 

[jaw22w]

I think your conclusion is fine but to maximize clearance you might keep original post spacing and see if a 5 1/2" deep lvl would work. Also consider a 2:12 pitch.

Yeah, I'm not married to that 3:12 pitch. That would be the easiest way to gain headroom. Would a 2:12 pitch be sufficient? Actually, I think I could get by with a 2.5:12 pitch, if that is enough. I was just kinda stuck on 3:12 because that's what the barn is.