How to determine roof slope?

I'm having trouble understanding exactly how to do this.
Our shed will be 10x20 and we'd like an overall 6' wall height and for it to have a simple shed roof like a lean to sloping towards the back of the building.

Would it be enough of a slope to have the front wall be 6' and the back be 5' or no? We live in Alabama. We get a good bit of rain and only occasionally do we get snow. Maybe a few days a year if that.

Is there a formula for determining the slope of a roof based on wall height?

Hi Felicia, I'm guessing that roof slope would be ok for Alabama.  It won't shed any snow so be sure to have strong enough rafters and walls to handle the weight of all the snow that could fall on it in the worst case.  Flatter roofs don't usually work well for shingles since the water isn't running off of them as fast.  So you may want to check which roofing materials you can use for your slope.

Roof slope is generally described as the "rise over the run".  In your shed roof that would be a rise of 1 foot and a run of 10 feet if the walls are the height you proposed.  For construction purposes, roof slopes are usually in units of 12 feet.  Many residential roofs are somewhere in the 3:12 to 4:12 arena.  So for every 12 feet of run, the roof rises 3-4 feet.  Your proposed shed roof would calculate out to a 1.2:12 pitch.

Are people going to spend any time in this shed?  I'm 6'3" and I would be disappointed to have to work in that shed

Mike Jay wrote:Hi Felicia, I'm guessing that roof slope would be ok for Alabama.  It won't shed any snow so be sure to have strong enough rafters and walls to handle the weight of all the snow that could fall on it in the worst case.  Flatter roofs don't usually work well for shingles since the water isn't running off of them as fast.  So you may want to check which roofing materials you can use for your slope.

Roof slope is generally described as the "rise over the run".  In your shed roof that would be a rise of 1 foot and a run of 10 feet if the walls are the height you proposed.  For construction purposes, roof slopes are usually in units of 12 feet.  Many residential roofs are somewhere in the 3:12 to 4:12 arena.  So for every 12 feet of run, the roof rises 3-4 feet.  Your proposed shed roof would calculate out to a 1.2:12 pitch.

Are people going to spend any time in this shed?  I'm 6'3" and I would be disappointed to have to work in that shed

No people in the shed. We're building it as a house for our outside cats. The only time people will be in there is when we go inside to handle their food, water, love on them, etc each day.

Would a front wall height of 8' and a back wall height of 6' work? We get about 1.6" of snowfall each year and about 56" of rainfall. For the rafters, we plan to use 2x6 lumber, 16" on center and then running 2x6 lumber in the opposite direction between the rafters for added support. Will this be strong enough?
For the wall frames (we'll be doing a post beam straw bale shed), should we use 2x6s or will be 2x4s be strong enough?

Also, how much of a slope would you recommend for the roof of the pen we'll be attaching to the shed? Picture a chicken coop with an attached run. Our structures will be set up like this where the shed is in the back and the pen is attached to the front of the shed and runs off of it. The pen will be 20x30.

If we build the front of the shed with an 8' wall, will we be able to attach the pen there and slope down to a 6' wall if the length is 30 feet. Will a 2' drop be sufficient?
Hopefully that makes sense.

If I had to tend your cats, I'd still be grumpy about the low ceiling...  Constantly bending over is fine when you're young but eventually you'll have a sore back and it won't be as fun.  Even if you're well under that height, other people may be tending them for you or you may want to sell the place.  Of course it's up to you so I'll get off my soap box :)

8' at the front and 6' at the back sounds great.

Those rafters sound plenty strong for 1.6" of snow.  That is assuming that the roof slopes in the 10' direction and the rafters are 10' long (plus extra to give an overhang).  Running rafters 20 feet won't work without extra support (they'll sag excessively).  I'd even say that you could skimp on the 2x6's going cross-ways.  I bet you'd be fine with some salvaged 1x6 boards.  The rafters are doing the work, the cross pieces are to give the roofing something to attach to and keep from sagging.  

2x4's is plenty strong for wall studs in a normal house.  If you're doing a 2x4 wall and filling in with straw bales, I'd say that 2x4's would still be strong enough as long as you have one at least every 4'.  Maybe put one every bale?  Just make sure you have a good header going across the tops of the 2x4's to carry the rafter weight efficiently to the studs.  Maybe someone with more straw bale experience can chime in.  

Ok, so the 20'x30' pen is in front of the shed.  The shed has a 8' high front wall, sloping back to 6' high.  The pen would match the 8' high wall and then have a roof on it that slopes forward 30 feet to a final height of 6'?  I'm thinking the slope for that would be fine again.  As long as it sheds rain you'd be ok.  The big issue is the span.  No rafters will carry a load over 30'.  So you'd need to put posts and cross beams at 10' and 20' from the shed with beams on them to support the roof rafters.  The rafters could basically also be 10' long and you'd be creating three roofs that are each 10' long and connected.  Hopefully that makes sense.

Are you sure you need to roof over the whole pen?  Could you get by with a roof over 10-12' of it closest to the shed?

Great, then my thinking is right for everything so far. For the pen, we had already factored in putting support beams at the 10 & 20 foot marks for the roof. We'd love to not have to roof the whole pen but they need 24/7 access to the pen even during bad weather. So, we figure for the shed and pen roofs, we'll create at least a 2 foot overhang on every side to protect the lime plaster and the roof itself will be corrugated panels over cdx.
Hopefully this all works out to be pretty close to the plan.

Yay!  You may be able to skip the CDX if you're using metal roofing, especially over the pen...  That's what the cross pieces are for (1x6, 2x4 or whatever is handy).

It would definitely be cheaper to skip the cdx over the pen but we're wanting to insulate everything.

Are there any special precautions we need to take if we decide to do a straw bale roof?

Hmm, I'm a bit confused.  When I think of a pen, I'm imagining an area with a fence around it to keep good critters in and other critters out.  You're putting a roof on it to keep it dry (most critters would approve).  How and why would you insulate it?  Any insulation you put above will get defeated by hot or cold air passing through the perimeter fence.  Or am I missing some details?

I don't know much about straw bale construction so I'm not sure about precautions for a straw bale roof.  That would add some weight for the rafters to carry and I'm sure you'd need to keep it dry.

Yeah, sorry, the pen roof won't need to be insulated. I was thinking of something else when I was typing. But it will need to be roofed completely for the elements and we hate the idea of adding wire to the roof but the thought of only having panels on the roof makes us nervous. If there's any damage to a panel then there won't be anything between them and a way out. So, we figured adding cdx to the roof before the panels would eliminate a need for wire but provide a barrier between the cats and the panels just in case. Maybe that makes sense.

Gotcha, and for some reason I was imagining pigs or goats or something.  I didn't realize you were farming cats.  Are they meat or dairy cats?

The metal panels I'm imagining are 3' wide wavy sheet metal that come in long/custom lengths kind of like this from Home Depot.  When they're screwed down to the wood cross pieces (purlins) they aren't going anywhere unless you get a hurricane or tornado.  I would imagine they are plenty secure for a herd of cats.

Yeah that's exactly what we're going with. I was hoping it would be strong enough to not need cdx or anything under it. We do get several tornado warnings each year though. That makes me nervous.

If a tornado is coming through there's nothing you can do to keep that roof from coming off.  All you can really do is build it to handle a severe thunderstorm with high winds.  

Go look at what other folks in your area do for farmyard roofs and you'll feel pretty good about putting the Home Depot style roofing on proper rafters for your animals.

That's true. And that's a great suggestion. Thanks for all your help!

My pleasure, good luck on the build!

Thank you!

I'm concerned about a completely roofed 20' x 30' pen in Alabama. I believe it would get unbearably hot in summer sun, even if the walls are fully open fence. I would advise keeping any solid roof to just sheltering the pen walls; overhangs with attention to eliminating paths for climbing out & over the roof parts would be secure against any escapes or predators aside from hawks. If you are concerned about flying predators, I would stretch wire mesh across beams/poles spaced 4-5' on center, or even wires stretched every foot or so both ways to prevent large birds from flying through.

I also think keeping cats in a strict "indoor" space without real weather is bad for them. A yard/pen with grass and bushes would probably give them much more stimulation than a plain floor (nothing much can grow in the shade of a roof that wide.)