Rafter span and spacing for 10/12 or 12/12 pitch
I did a quick search on here but coulnd’t find a thread that fit my situation. I know there are a ton of rafter span calculators online that either give you info for under 3/12 pitch or over 3/12 pitch. I can’t help but think that the requirements would be different for a 10/12 or 12/12 pitch roof. I don’t want to under build nor do I want to overbuild. I am wanting to put up a carraige house to match our Victorian home that has a 12/12 pitch roof(s). Our home roof is 2×4 (probably SYP) that is spaced at 24″ or more. They still have the one by furring + decking + asphalt shingles and it doesn’t seem to budge an inch when is snows because it doesn’t hold much snow with that pitch.
Here are some of my details that I hope someone can help me figure out what i need. First I live in south central Kansas so my snow load is 15 – 25 psf. Being in the southern part and based off recent history I would lean more towards the 15. I would like to frame with 2×4’s now that the price of lumber has come back down. So it would be the standard KD WW that the big box stores carry. My building will be 24ft wide with each rafter side being 17′ 6″ long if I calculated it correctly based on a 10/12 pitch with an 18″ overhang.
I plan on putting on a metal roof so just furring strips + 28 ga metal for live load weight. Again I would prefer to use 2×4’s but can go with 2×6’s if I can’t get away with 2×4’s. Just basically need to know if my spacing should be 12, 16 or 24 inches.
Thanks in advance for the help,
Dennis
Replies
Um, you’re wanting to span over 17 feet with a 2x4 rafter—is this some kind of joke?!
2x8s barely make it with 20 live:
http://www.southernpine.com/app/uploads/SPtable17_060113.pdf
And that’s on 12” centers
Not a joist, a rafter. I live in a house that has been standing more than 130 years with 2x4 rafters spanning more than 15 ft on 24" centers and have abloslutely no sag or issues. I'm sure it has to do with the 12/12 pitch which is the pitch I will be doing on the shed. Not that it makes a huge difference but I forgot to mention that almost 2 ft will be overhang. Have a great evening!
The chart I sent was for rafters. You don’t see 2x4 rafters in much construction of the last 50 years or so, except maybe a small back yard shed. Figure out your total span, from ridge to plate, and consult the chart I sent. I get that your house has 2x4 rafters, if you think that’s acceptable, then I guess build it out of that. If you want it to be code compliant it will need to be beefier, I would spend the extra bucks and use the proper lumber. Today’s stock is undoubtedly weaker than the old growth material your homes rafters are made from.
You are correct in that my current house is old growth southern yellow pine which will burn up saw blades if you aren't careful. I am going to beef it up but more for peace of mind than for code. I'm at the edge of very rural Kansas town which is pretty much country. Not much as far as codes goes around here unless insurance gets involved, which they rarely do.
A 2x4 roof rafter span is limited to 10'10" with 12" on-center spacing. for Douglas Fir-Larch #2 or Southern Pine #2 for 10 Lb. dead load, 20 Lb. live load. For 20 Lb. dead load, 30 Lb. snow load, the 2x4 roof rafter span drops to 8'5" and 9'0" for Douglas Fir-Larch and Southern Pine respectively.
For the 10 on 12 pitch you're talking about, the span is more like 15'7" depending on details, such as the ridge board and wall thicknesses, so 2x8's should work at 12" on-center.
I don't know how your roof is constructed to have 2x4's spanning more than 15' on 24" centers, but there would have to be additional framing supporting the roof that effectively reduces the distance that is spanned. If you could post a picture showing how your house is framed, that would be very interesting. You could also be benefiting from "real 2x4's", not today's 1.5"x3.5", and the greater strength of old growth lumber, but I can't imagine 2x4's spanning 15' that are unsupported by other framing structure.
For reference, look at Fine Homebuilding's video on YouTube where a Shaker shed that is only 12' in width is built using 2x6 roof rafters plus they used collar ties every 4', that are not shown in the video. https://www.youtube.com/watch?v=10pAABFXvhE
I STAND CORRECTED, per UncleMike42's input on rafter spans. It's the HORIZONTAL distance spanned that is listed in the tables for rafter size selection. So, for this 24' wide building, 2x4's are not usable as roof rafters, 2x6's would require 12" on-center spacing for a 20lb. dead load/30 PSF snow load, and 2x8's would be fine at both 16" AND 19.2" on-center spacings (assuming #2 Douglas Fir-Larch or #2 Southern Pine).
Thanks, that helps me to decide on which route to go. I will just calculate the cost difference between 2x6 @ 12" centers vs 2x8 @ 16" centers.
If your building walls are 24 feet apart, the span of roof rafters is 12 feet. The slope does not change this, nor does the overhang.
https://www.awc.org/codes-standards/calculators-software/spancalc
The ability of a wood rafter to carry a load depends not only on the size and spacing, but the species and grade of the board.
you can use the calculator I pointed to to understand how these vary, and make value decisions based on the costs of acceptable choices that meet the structural demands.
https://codes.iccsafe.org/content/IRC2018/chapter-8-roof-ceiling-construction
Interesting, I did not know that about it just being the horizontal distance rather than the lenght of the rafter. Thanks for the links UncleMike!
Thanks everyone I really appreciate your feedback. Sorry it has taken me so long to reply, I only got the one notification that there was a reply and never got any more emails, so I assumed no one was commenting. I think I will go with 2x6's on 12" centers. I will definately send you guys a picture or two of the attic in our 1887 house and show you the long spans of 2x4' on 24" centers. Yes it is old growth southern yellow pine but there is no additional support.