Roof Rafter and Purlin Spacing for Steel Roof

Started by db4570, August 15, 2016, 10:41:40 PM

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Can anyone give me a rule of thumb for roof rafter and purlin spacing? I want to build a couple real simple structures with steel roofing: an extension roof off my shed to park my popup camper under, and perhaps another small shed on our recreational property. I realize that if this were a residence I would need to calculate snow loads, etc., but is there a basic starting point I can use for these simple applications?

I am in Upstate New York (can get a couple feet of snow).
I'm thinking shallow pitch, 3:12 or less.
How about 2x6 rafters 16" OC, with 2x3 purlins 24" OC?
No sheathing.
Cheap home center lumber.

The resources I see on-line all talk about determining live load, etc. Do I need to make it so complicated?

Thanks for any information.



A lot of NYS was 50psf snow load. I was going to say it's easy to just look at the map.. it used to be. There was a zoomed in map of NYS in the 2010 codes and you could easily see what snow load to use. Now it is a zoomed out map with most of NYS being blank, and written in the north east corner of Ohio, and in PA it says "CS" which means case studies. Assuming that applies to NYS too.

There is no way they want everyone in upstate NY to hire some snow load case study specialist at $500 a job to do a case study before putting on a dang roof.. I swear.. these people that write these codes have probably never swung a hammer let alone built a house. They think they are going to get people building better houses but they end up getting the average guy to circumvent the code because it's so intimidating and potentially so expensive.

A 50psf snow load 2x6 @ 16" oc can span around 10 feet. I used ABC panels to do my roof (Curtis Lumber carries them), and their installation guide would give you the spacing and sizing of the skip sheathing (purlins). a 3-12 roof is the lowest slope you can go with metal roof panels.


I'm considering the same question for a sawmill shed I plan on building.  My location doesn't have a permit or inspector to worry about, so I'm just concerned about a roof that will withstand the elements, primarily snow.

I'm in a 70 lb per s.f. area for ground snow load, but applying wind and importance factors knocks it down to 50 psf.  At least I'm comfortable with that.  The next part is pretty easy, and just requires some basic math.  I'm using big rafters widely spaced, shooting for 36" on center.  The math part is figuring out the area and load each rafter will be supporting.  That is the rafter spacing times the horizontal distance between the beams.  In my case it's 10' 5/16".  Call it 10 for even easier math.  So each rafter is supporting an area of 3' * 10' or 30 s.f.  The load includes the design weight for snow plus the weight of the roof itself.  You can calculate out what that is, but a convenient number to use is 10 lbs per s.f.  So my rafter will be supporting a load of 30 s.f. * (50 psf + 10 psf) = 1800 lbs.

There are some really useful calculators at Forestry Forum at  If you scroll down there you will find Don P's Calculators and you want to select the Beam and Column Calculators link.  Then click on the second link, which is for a Uniformly Loaded Simple Beam (the URL is at  This tool will tell us if the desired rafter spacing is adequate for the rafter and load.  The field you need to input are:

  • Total load: I input 1800
  • Dead load: the weight of the roof borne by the rafter, 300 lbs
  • Span of beam in inches: 120
  • Width of beam in inches: 4 because I'm using rough-sawn.  A 2x6 would be 1.5 inches
  • Depth of beam in inches: 6
  • Species: I selected SPF #2 B+S, which covers the lowest structural grade of spruce, pine, and fir.  You would pick what is available from your lumber supplier

Then click on Show Result to have the calculator determine if the beam passes or fails.  If you are following along with this example using the calculator you find that it fails.  The next thing I tried is going deeper, using a 4x8 rafter.  Keeping all the other numbers the same, I changed Depth to 8 inches and clicked on ShowResult.  It fails again, but just barely.  So if I lessen the rafter spacing to 32" it might pass.  32" equals 2.67 feet, so my load will be 26.67 * 60 = 1600 lbs.  Dead load is 267 lbs.  Using these loads for a 4x8 spanning 120 inches, my beam passes.  So for my location I can put 4x8 rafters on 32" center.

For purlin spacing, I'm assuming 24" o.c. is adequate, but that really depends on the strength properties of the roof panel.  A panel of 24 gauge steel will accommodate a much greater purlijn spacing than a panel of 29 gauge steel.  I'll have to figure that one out when I buy my roofing panels.
My cabin build thread: Alaskan remote 16x28 1.5 story

Dave Sparks

" really depends on the strength properties of the roof panel.  A panel of 24 gauge steel will accommodate a much greater purlijn spacing than a panel of 29 gauge steel.  I'll have to figure that one out when I buy my roofing panels."

Could you use the cheaper steel and gain overall strenth buy using a 5/8 " OSB underneath? Or maybe 1/2".

We used 5/8 OSB with Fiberglass Class A Fire paper under the standing seam. I think we used H clips on the OSB but can't remember why after 12 years.
"we go where the power lines don't"


Regarding purlin spacing, OSB under the roofing would certainly do the trick.  For standing seam, for example, the installation guide should spell out exactly what spacing to put screws.  Without sheathing, that pretty much dictates the purlin spacing.  So I'd say to select your roofing and see what it requires.

My local Home Depot carries a steel panel called Pro-Panel II.  A link to the installation guide is here:
It doesn't actually give much useful information, but I'd infer from the load table that for 50 psf I should have my purlins at no more than 30" spacing.  24" would be much better, and 18" in heavy snow areas.

Edit: I think MtnDon posted this link in another thread:
My cabin build thread: Alaskan remote 16x28 1.5 story


Thanks for the replies. It seems like it shouldn't be so involved for something so simple. I looked at the roof of my shed, which has stood for probably 30 years just fine, and it is 2x4 rafters on, I think, 24" centers. So I'm probably overthinking this.

I, too, thought the steel roofing manufacturers would have useful recommendations, but I have been to several of their web sites and found nothing as far as purlin spacing. I'll dig a little deeper.

I'll probably use one of the load spec calculators recommended, and get within reasonable fudging range.


Dave Sparks

I would also make sure that you screw the metal roof down with a proper screw/ rubber washer.

Do not use a roofing nail on a standing seam roof! They tried to on my 12/12. Do it right the first time as Don says....
"we go where the power lines don't"


This is one company's basic detail manual, go to purlins, girts and roof decks
Most manufacturers have detailed instructions and a helpline, more critical when the panel is the diaphragm, the rack bracing, such as a purlined roof.

avoid oak purlins, high tannic acid content.