Lean-to rafter spacing?

[EddieWalker]

What type of roof is going on top of the lean to? If it's metal, then spacing the 2x6's to four feet is plenty strong enough. Then put your 2x4 purlins on the flat every 2 feet and attach your metal to the purlins.

What is the pitch of your lean to going to be?

Do you have snow?

You can easily go 12 feet with 2x6's on a metal roof with a 4:12 pitch. Even 2:12, but then you start to push the limits of what it can shed during a heavy rain without leaking.

If you are going with shingles, then that additional weight means you need to stay at 24 inch centers and you have to be 4:12 or steeper.

You measure the span from inside edge to inside edge of what you are resting the lumber on. That is where the load begins.

Eddie

 

[cockeyedMFer]

Your rafters span starts and stop at bearing points...top of walls, joist hangers, beams. Will you nailing the rafter help, yes, would i count it,no. When i say bearing points i mean the rafter is supported by a min. of 1 1/2". I am not a fan of using lumber to its max span. Just buy another 16 more for about 150 bucks and be happy you did.

Good point, that's what I was after. The one thing I'd add is that according to span tables, the minimum bearing surface is .4". Which seems surprisingly small. Another option might be to add a ledger board below the eave to provide additional support.

And silly me, I did overlook the fact that every other rafter wouldn't have a truss to nail to, so you're correct that I'd need some bracing in between.

Have you addressed column spacing based on dimensions and length?

I haven't checked my math yet, but we were planning 8' OC 4x6 columns in the middle and 6x6 columns at the corners. 2x8 inner and outer top plates, 2x6 girts all around.

 

[BHD]

IMO it will work fine,
I have buildings with 2x4 on 24 spanning 12 foot, slight sag after 90 years,

another building 2x4 24 on center spanning 10', no seeable sag, 70 years,

barn lean to sheds, 2x6 24 oc, 15 foot, span slight sag it is 105 years old, (all lumber is Douglas fir)

all have approximately the same slope as your drawing, all of my buildings are 1x sheeted and have been shingled, with asphalt shingles, the barn is sheeted but tinned now,

not saying your not pushing the modern limits of construction, but if you have enough 2x6's one could double up a few one on each side of the existing rafter, or as far as shear, if your concerned, could consider a split ring type of attachment,
(basically take a hole saw, saw in about 1/2 inch, and put about a 7/8 inch long section of pipe/like in that hole saw cut).
TECO Split Rings (Timber Rings) On Cleveland Steel

Split Ring Timber Connectors | Portland Bolt

sample rafter using split rings (just posting this for conformation of there use)
http://freeplans.domerama.com/plans/construction/6005.pdf

 

[cockeyedMFer]

What type of roof is going on top of the lean to? If it's metal, then spacing the 2x6's to four feet is plenty strong enough. Then put your 2x4 purlins on the flat every 2 feet and attach your metal to the purlins.

What is the pitch of your lean to going to be?

Do you have snow?

You can easily go 12 feet with 2x6's on a metal roof with a 4:12 pitch. Even 2:12, but then you start to push the limits of what it can shed during a heavy rain without leaking.

If you are going with shingles, then that additional weight means you need to stay at 24 inch centers and you have to be 4:12 or steeper.

You measure the span from inside edge to inside edge of what you are resting the lumber on. That is where the load begins.

Eddie

We're matching the existing barn which is 4:12, typical 9" rib metal roof with purlins on edge at 24". No ceiling or anything. Southwestern Ohio, so there is about a 20-25 psf snow load.

 

[BuilderML]

If you really don't want to buy or deal with 16"o.c. Atleast do a couple rows of blocking the whole width along with doubling up every other one and that should lock things together nicely. Lets do some math just for the heck of it. Snow load 20psf Dead load 10 psf. on 12' your looking at putting a possible load of 720 lbs on each 2x6 rafter. Thats a heck of alot of weight. So do NOT go to 4' o.c. Also a row of blocking where your attaching to the existing roof truss would be nice.
Have fun with your project.:thumbsup:

 

[KennyG]

A little more thinking on this - 24" centers would PROBABLY be OK, but the tables are intended to insure that it will always be OK. It all depends on how much margin you want. Blocking and tieing to the existing trusses will have some value. However, I've seen roof with one cracked rafter after snow loading, apparently because one rafter had a weak point. I've seen the old 2x4 roofs also but you have to remember those are larger dimensions than todays lumber and may be oak or old slow growth softwood that might have twice the strength you can depend on from todays lumber.

The way this project looks, it's not going to completely fail at once if a rafter cracks and it's not like someone is going to live there, so use your own judgement.

 

[JPinnell]

I just added a 12'x 40' enclosed leanto type addition with a 2:12 pitch to my pole barn. We then did an add'l 12'x 40' open leanto hung off of that. 2x10 rafters on 4' centers with joist hangers attached to a 2x10 header sitting on 10' pole spacing. Purlins were 2x4 laid flat on 2' centers spanning the 4' between rafters. The additional lento's have less bounce in them than the existing pole barn roof did that was trusses on 10' centers.

If you have the height could you stack your 2x6 and make a 2x12? I would not recommend this unless you really have a ton of extra 2x6 material. Then you could bump up to 8' centers to match your current truss spacing. Then run your 2x4 purlin on edge, not flat.

 

[cockeyedMFer]

That sounds like a great addition to your barn. Do you get a lot of snow in MO? That sounds a bit light for Ohio winters, but then again my neighbors building is built with 8' OC light trusses and its held up well.

I measured to our property line just to be sure, and 12' is the max I can go. There's an old broken down fence/hedgerow that is just on my side of the line, we need to leave 10' open area between the building and neighbor. From my notes I see we spec'd the barn to be 20' from that fence, but fortunately guy that laid out the our barn was off in my favor by almost 3'. So that leaves us 12' to work with.