Lean-to roof beam size calculation

[bfishman]

Hello folks,

I'm designing a lean-to roof for a chicken run that's attached to the side of a shed, and would appreciate a double-check on some of my load calculations.

There's a 62 psf ground snow load in my location.

The roof will host corrugated PVC greenhouse panels and thus will have very little additional loads other than the potential snow.

The roof is designed to cover a 12' x 12' area. The slope of the roof will be 2' drop over 12'.

The columns are PT 4x4s each set 30" into the ground. They are at a span of 12' apart.

I am going to use a ledger board lag-screwed to the studs of the shed wall.

The joists will be 24" apart on center. Calculations show that I will need to use 2x10" (12' long) joists.

Now for the real question - tables say I need to use either 3x 2x10" for the beam, or a 3-1/2" by 9-1/4” engineered piece.

The 3-1/2x9-1/4 I can source are quite pricey, but given that my columns are 4x4" (nominal), I don't see how I can properly secure the 4.5" (actual) width of the beam to them, were I to glue/screw 3x 2x10" together.

So I guess the questions are:

-Am I calculating these load requirements correctly (particularly for the beam)?

-If so, is there a proper way to affix the 3x 2x10 beam to 4x4 columns? Or am I stuck using the pricey one-piece beam?

-if so, is there a 'best' way to 'laminate' these 3x 2x10s together?

-Minor question, but does it seem important to remove a section of the 3/4" plywood siding from the shed in order for the ledger board to affix directly to the studs? Or am I safe going directly through this 'siding' with some 4" lag screws? (These should be long enough to set halfway into the 2x4 studs).

And to head off any suggestions of increasing the size of the columns, they are already in-place - the run was not originally built with a roof in consideration.

Thank you in advance for any assistance or suggestions you can provide!

 

[bfishman]

Follow-up: I found a different calculator suggesting that 2x ply of 2x12 should be able to span the full length. That would certainly be more cost effective AND easier to fasten with a post cap tie. Could anyone help confirm that finding?

Also, I'm not concerned about deflection.. probably L/120 is fine given there's nothing on top but PVC.

 

[carlaisle]

With the details provided 2x10 rafters look marginal. Will probably be just fine in practice, but if I had to sign off on it, 2x12s would be specified. You will not get a full 12' depth covered with only a 12' long rafter. Since you are not willing/able to upsize the columns, how about adding an additional column mid-span? 5" lag screws cost very little more than the 4" variety.

 

[Hank11]

I’d go for more pitch so the snow can slide off. You and the chickens won’t get much benefit from transparent roof panels with snow on them.

 

[bfishman]

Thanks all. First time using the sizing tables and wanted to make sure I wasn't overbuilding this. Looks like not

 

[bfishman]

With the details provided 2x10 rafters look marginal. Will probably be just fine in practice, but if I had to sign off on it, 2x12s would be specified. You will not get a full 12' depth covered with only a 12' long rafter. Since you are not willing/able to upsize the columns, how about adding an additional column mid-span? 5" lag screws cost very little more than the 4" variety.

I assumed 4" because I was worried about splitting studs with the longer screw. Perhaps I'm thinking of it wrong

 

[njk4o5]

i have a 20' lean to in Maine with 90 psf snow load, 2' drop over the span of 18'. Beam is (3) 2x10 sitting on 6x6 PT. Rafters are 2x10x20' spaced at 16" with metal roofing. Had 3' of wet heavy snow on it for a couple days last winter no problems.