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?