2x6 roof span question

With regards to the chicken coop I am building in the Wood's open air style.  The one roof is approximately 12 feet from one end support to the other.  The coop is 8 feet wide, so my plan was to use 2x6s on edge, 24" on center.  Pitch is very close to 6:12.  The 2x6's will be 14' to allow 1' overhangs on either end.  The plans in the original book call for 2x4s for the roof, but those would be true 2x4, so my plan is to use dimensional 2x6s instead.  So, if I want my coop to last 100 years, are 2x6s strong enough to maintain the roof with no sagging on a 12' span, 24" centers?

Normally they would be good.
BUT, do you have snow loads?

John C Daley wrote:Normally they would be good.
BUT, do you have snow loads?

Yep, sure do.

What you need is a "span table" which will give you span ratings (distance between supports) for various lumber dimensions (and species), at various spacings on center (12", 16", 24"). These will usually specify a load(s) (per square foot) for the system (floor, roof). The decking material you use also has a span rating (between joists) to consider as well.
For a roof, the weight of the roofing is usually considered, as well as snow loads.

If this is subject to inspection/building code, you may find that even though a span table says a certain size of lumber will span, local code might require a minimum size be used (only 2x8 or greater as a floor joist, for example).

As an outbuilding, not a residence, you may not have these issues, but always best to know in advance.

I would go 16 oc at a minimum.

Spanning 12' at a 6:12 pitch, you will get about 3" overhang on each end with 14' lumber. I would use 16' so you can have actual overhangs.

You would also be going from say 4' high at one end to 10' high at the other end. Is there a reason for this much slope and rise?

Snow load is absolutely relevant.  If you're not sure what your snow load is, your local county building office knows!

After that you need to know what species of wood you're dealing with.  The variation across woods isn't great, but there are marginal differences.

Note - the span is the horizontal span (wall to wall), not the length of the rafter itself.  There are many tables, and of course calculators too (e.g  https://www.mycarpentry.com/rafter-span-tables.html & https://www.awc.org/codes-standards/calculators-software/spancalc)

I would go 16 oc at a minimum.

what does this mean please?

John - That's shorthand for "16 inch on center", or about 14.5" gap between rafters.

And then "minimum" is funny b/c he's really suggesting 16" as the maximum, but its the minimum suggested strength.  Ain't English grand?

Hi Eliot,

Interesting point.

I think I'm confused about this building.  Is the coop 8x12'?  Are the 14' rafters oriented with the slope or perpendicular the slope?  And the 6:12 slope means the 8' long walls are higher on one side than the other?  Or is it the 12' walls that are higher on one side than the other?

Trace's description of the project could be more detailed, a photo or a sketch would help greatly.
To me, the 12' span and the 8' width, suggests that the shed roof slopes "the long way". (the shorter 8' span would be more conventional)
A quick search on "Wood's open air style" shows a lot of coops about 8' x 12' with two shed rooves that are offset in elevation, with a clerestory window between the lower and upper roof. These rooves do slope in the long axis of the building, but it's two shorter spans, not one long span... (and therefore 2x4 rafters would make more sense).

My apologies for assuming people knew what coop I was talking about.  Here is a picture of one, not mine.  Mine are different dimensions than this, but this shows the basic shape.  I'll update this more and answer the questions.

To add to my terrible earlier descriptions, the coop is roughly 8'x16' and has two roof sections with a clerestory. The roof section that starts at the front of the building starts at five feet and goes up to six feet. The wall under that roof section is 6'3 1/2" long. The roof section on the back of the coop is ten feet tall at one end and goes down to five feet at the back of the coop. The wall under that section is 9'8 1/2" long. In the original plans from the book, I believe all boards were true 2x4, in Minnesota, with snow loads approximately like ours in Wisconsin. Since I'm using dimensional lumber, I was going to use 2x6s. The plans also show 24" between rafters, if that is the correct word, so 5 rafters for the 8'wide coop. Every span table I look at makes it seem like that isn't enough. I only want to build this coop once, but I also don't want to use double the lumber that I need.  The rear section of the roof is steep enough that I expect it to shed snow easily. The front section is not steep at all, but it's a shorter span. My lumber for the roof is whatever they sell at all American do it center. They just call it pine.

I hope that helps clear things up. Sorry for the confusing first post, and I much appreciate you guys trying to help in spite of my lousy description.

Anything I left out, please let me know.

What kind of roofing will you put on it?  

Where the short roof and the clerestory meet, what holds up those ends of each roof?  Strong cross beams from side to side?  The walls at each end of the building (16 feet apart from each other) should hold that end of the roofs up just fine, I'm wondering how well the other ends of those roofs will be supported.

My house roof was built in the 60 and it's a 14 foot span (peak to outside wall) with 2x6s on 16" centers.  They do sag but I've had 3' of snow on it.  They probably used better wood back then...

Mike Haasl wrote:What kind of roofing will you put on it?  

Where the short roof and the clerestory meet, what holds up those ends of each roof?  Strong cross beams from side to side?  The walls at each end of the building (16 feet apart from each other) should hold that end of the roofs up just fine, I'm wondering how well the other ends of those roofs will be supported.

My house roof was built in the 60 and it's a 14 foot span (peak to outside wall) with 2x6s on 16" centers.  They do sag but I've had 3' of snow on it.  They probably used better wood back then...

The current plan is to use 3/4" osb with corrugated steel roofing, but I'm open to changing that.

Yes, where the roof meets the clerestory, I have some rough cut true 2x6s that will go across the coop for the roof boards to rest on. They will also butt up to a cross beam that I will nail through and into them.

If necessary to keep them from sagging, I can easily put the roof boards at 12" OC, or use 2x8s at 16" OC. I'm not sure which of the two is stronger.

If it's a slippery (non rusty) metal roof, you won't have much snow at all sitting on the steep side.  So 2x6s on 24" centers is plenty (in my opinion).  For the shallower side, the rafters are pretty short so again, I wouldn't be worried even if it has 2-3' of snow on it.

You can save some money by skipping the OSB and using boards running perpendicular to the joists to attach the roofing to.  Unless the OSB also helps make the coop air tight or something.  

On my sugar shack I have 2x4s running across the face of the roof every 24-30" going up the roof to support the metal.  They're oriented flat to the rafters so they add 1.5" to the height of the roof.  But they're plenty strong enough to hold the metal up.  If you had scrap lumber of a consistent thickness you could use that as well.  Old barn boards, 1" thick boards, etc.  The thinner they are, the closer they'd need to be.

Here's a link showing the roof of my sugar shack.  The trusses on the inside of the building are the equivalent of 2x4 rafters.  The building is 14' wide so each half of the roof covers 7'.  The pitch is 4:12 so much flatter than yours.  The trusses are 24" on center.  Where you're thinking of using OSB, I have 2x4s running across them that you can see in there.  This roof sheds snow about half the time and when we got 140" of snow two years ago I didn't even worry about it since it slid off before it got too heavy.

https://permies.com/wiki/141158/pep-homesteading/Homesteading-Badge-Oddball-thread#1107363

Mike Haasl wrote:
You can save some money by skipping the OSB and using boards running perpendicular to the joists to attach the roofing to.  Unless the OSB also helps make the coop air tight or something.  

The OSB - or plywood - can add a lot of strength to the whole structure.  Of course, galvanized panels 24" wide have a similar effect (especially if you are generous with the fasteners and affix the long edges well).  I find roof sheathing is necessary if you are putting down tar paper and then composite shingles - metal roofing can indeed be just fine with the boards as Mike suggests.

I'd say 3/4" OSB is far too thick ... most roofers on residential structures only go with 1/2" (although that's generally on 16" centers).  I'd suggest its better/cheaper/easier to have 16" centers on rafters and 1/2" OSB than 24" OC and 3/4 sheathing.

Mike, that's a beautiful building.  I wouldn't be ready to tackle something like that :)  Great job on it.  

There are three reasons I planned on using OSB under the metal.  As you mentioned, it would really help keep out drafts.  A large part of why that coop design works so well is the area in back with no drafts.  As Eliot mentioned, I assumed it would add support to the structure.  And finally, I figured it would be much quieter for the birds in case of rain, hale, high winds.  Hopefully less stress for the birds.

Eliot, thanks for the input.  It's easy enough for me to use 1/2" on 16" centers.

Thanks, you can do it too after fiddling with building stuff for a few years...

That all makes sense.  In that case I'd also go with 1/2".  The osb would help knock the noise down a bunch.  

Make sure the support beams for the clerestory section sit on top of some posts in the side walls.  Having a beefy beam that is just nailed on the ends doesn't confer as much support as you'd think.

I have been told that 5/8 OSB is needed to hold the screws for the metal roofing.

[quote=Tom Berens]I have been told that 5/8 OSB is needed to hold the screws for the metal roofing.[/quote]

I'm going to try to arrange the metal roofing in such a way I can go directly into the roof boards, but I'll lay everything out ahead of time and see if that will work out.  If not, I'll go with thicker OSB.  Thanks for pointing that out.

Mike Haasl wrote:Thanks, you can do it too after fiddling with building stuff for a few years...

That all makes sense.  In that case I'd also go with 1/2".  The osb would help knock the noise down a bunch.  

Make sure the support beams for the clerestory section sit on top of some posts in the side walls.  Having a beefy beam that is just nailed on the ends doesn't confer as much support as you'd think.

Once the weather is nice enough to build the base, I'll move the walls outside and mock them up and take some good pictures.  Hopefully you guys can review and point out anything I may have missed, be it needing more supports, heavier boards, whatever.  Changes are easy before the roof goes on.

A house we lived in 10 or 12 years ago had a 8:12 pitch.  When they put a steel roof over the single roof, the snow started just sliding off - no snow load.  It's something to consider.

Phil Swindler wrote:A house we lived in 10 or 12 years ago had a 8:12 pitch.  When they put a steel roof over the single roof, the snow started just sliding off - no snow load.  It's something to consider.

Hey Phil, that's a large part of the reason I decided to go with the steel roof, I'm hoping it will act exactly like that.  The open end of the coop faces south, and that makes the shorter, flatter roof face south as well, so I'm hoping that when we get sun, it will melt off that side pretty fast. The long, north facing roof is much steeper, so I'm hoping the snow will slide of most of the winter like you said.  Worst case scenario has me dragging the snow off the flatter roof, and as small as it is, that shouldn't be a problem if I need to do it.

Thanks for taking time to read the thread and comment.