# Adding a lean to



## FarmerDavid (Jul 16, 2012)

our barn has an enclosed lean to on one side. I’d like to add a roof to the other side that matches the existing roof on the lean to. I don’t plan on enclosing it and would like to know how big of a header beam I need and how many support posts I need to use. I plan on putting support posts on the corners but I’d like to get away with one or none on the side. It will be 30’ foot long and 15’ wide. How do you figure what size beams and supports you need. In SWMO don’t get much snow anymore.


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## TnAndy (Sep 15, 2005)

30' is a pretty good span. You'll need two or three glue-lam beams, and they'll have to be fairly tall.....like 16" or so. Drill and bolt together. 

You could also use 3--2x12 with two 1/4" x12" steel plates between them, drilled and well bolted....though I like the glu-lam above, or the truss deal below, better.

You could also go with a floor type truss. I built such a truss when I built my house to have clear span in the garage. Used 2x6's, 20" tall, glued 3/4" plywood to each side, it spans 24', and from the attic it has absolutely no bounce. No engineering data to back it up, but I wouldn't be scared to span 30' with one and run a shed roof on it.

IF you decide to build a truss, put a slight arch in it as you build it....stretch a string from one end to the other along which ever side you decide is the bottom, and bow the center away from the string couple inches....after the glue on the plywood has dried and it's well nailed/screwed to the inside web, that arch will add a lot of strength to the whole thing.


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## FarmerDavid (Jul 16, 2012)

Hadn’t thought about a truss style. I can run bracing from the new roof structure across to the existing barn wall for support. I just know that wherever the supports are they will be in the way, so minimizing them minimizes those issues.


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## Fishindude (May 19, 2015)

A sketch or some photos would be real helpful envisioning what you are talking about.


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## JohnP (Sep 1, 2010)

You're going to have to break up that 30 feet.

I would do three posts @ 10 feet apart. Notch those for a 2x8 or 2x10 to sit on each side. Then you need some rafters for purlins to sit on. With shed roofs and pole barns, the common thing is rafters on 4 foot centers with 2x4s laid flat for purlins. With thinner stuff, some people like myself do 2 foot spacing on the purlins. I used 1x6s for purlins because I happened to have a bunch of it. Spacing on those and rafters kind of depends on the thickness of your metal. I'm assuming metal. If it's shingle, just use official construction rafter spacings. 
https://www.mycarpentry.com/rafter-span-tables.html

Think about wind lift when putting posts in. Very deep with something attached to the bottom of the pole to prevent pullout or concrete weight to prevent pullout.


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## mreynolds (Jan 1, 2015)

A 10" C purlin is exactly 30' long. I have used them for joists on a commercial attic. They spanned 25' with a 5' cantilever. Then they loaded it down with gas pumps that weighed about a ton each. Maybe 6 of them. Never sagged a bit. That should hold a few rafters easy.


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## ridgerunner1965 (Apr 13, 2013)

I have often done 30 ft long lean to with a 2x10 header on the outside of the posts. and a post in the middle. so a 15 ft span with a 2x10 header. then what I do is shove another 2x8 header up on the inside of the posts and spike it in. then what ill do is take maybe 5 ft long braces nailed between the two headers down to notches in the posts. and then spike them in with pole barn nails. a bit of overkill im sure. 16ft rafters on 24" centers.

im in MO too, we occasionally get a lot of snow. a few years ago we had a snow that collapsed many poultry houses. all my buildings were fine.


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