Lean-to rafter spacing?

[cockeyedMFer]

We're planning a quick lean-to addition in a few weeks when I have a couple vacation days due. Our existing barn is a typical pole barn with 12" eaves and 2x6 top chords on the trusses.

To keep things simple, I'm planning on sliding the new rafters alongside the chords and resting them on the top row of banding. The new roof will slide under the existing metal. Length will be 54' long and as wide as I can make it in the space we have. It will eventually be enclosed but will not have ceiling or insulation.

One goal is to make use of a lot of extra lumber I have, which includes 2x6's for the rafters, so I'd like to push it out to 12', on 24" centers.

According to the span charts, the limit of a 24" OC 2x6 is 10'9". Now looking at my drawing, can I consider the span to begin where the new rafter is fastened to the existing top chord, rather than beginning at the wall? In other words, does nailing the rafter to the chord count as support? If so I can go out to an overall width of 12' for the lean to while not overloading the rafter.

(for some reason the attachment comes out upside down so I posted it 2 ways...they're both the same image.

 

[kencunningham]

If you can answer no to both of these questions, then go for it. Do you plan on selling your place in the futere? Do you ever get enough snow to overload the roof. The type of lumber you use should also have an affect on the strenght of the rafter.

 

[cockeyedMFer]

We have #2 douglas fir available locally, which is what I'm using for the rafter table, and our snow load here is 20psf unless I'm mistaken.

Just trying to understand what constitutes an attachment point for a rafter. Either its the top of the wall, or the end of the eave.

The other answer is to just buy another 16 rafters and put everything 16" OC.

 

[Gary Fowler]

I think I would go with the standard 16" O.C. spacing and you could likely increase your span a bit at the same time.

 

[BuilderML]

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.

 

[ace10]

The outer truss carrier (assuming there are two) is going to receive a significant additional load. Beyond what the engineer who designed the structure intended. You should address that.


$0.02

 

[KennyG]

Go with 16" on center. I think you have two problems here. The first is that if you don't consider the full width, you are assuming you can take the roof load on the nails in shear. You don't want to do that. The second problem, as you have drawn it, is that the upper edge of the rafter is supported on a point load. You need to notch the upper end so it can bear on at least the thickness of a 2x. This may mean you need to scab 2x sections between your trusses.

 

[Underdog57]

I would definitely go 16" oc. And I would scab short 2 x 6 (14.5") in between the rafters at at least the middle and maybe even put 2 rows of them in. If your rafters are sitting on both plates it looks like you will need at least 14' lumber? I am not sure but shouldn't the span be calculated by the length of the board or is it the horizontal measurement? I am not a pro, but I have built a few buildings in my time, just another opinion to think about. :2cents:

 

[KennyG]

I am not sure but shouldn't the span be calculated by the length of the board or is it the horizontal measurement?

My rafter tables are based on horizontal span, not length of rafter.

 

[HillStreet]

Have you addressed column spacing based on dimensions and length?

 

[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.