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Author Topic: Rafter span chart/calculator for larger timbers  (Read 5244 times)

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Offline Winter86

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Rafter span chart/calculator for larger timbers
« on: February 10, 2016, 10:08:31 AM »
First post for me.  I've been a long time lurker and am going to jump into my first timber frame this summer (28x30 shed/barn).  I'm getting the plans together for the county currently.  I've been calculating my loads and using the beam span calculators on this forum - what a great resource.

What I'm trying to find is a rafter span chart for sizes bigger than 2x6, 2x8, 2x10 etc.  I can't find any information for a 4x6 for example or a 4x8.  I'm milling all my own lumber so I can make the rafters any size. 

The roof I'm working on is a 12 foot span with a 65 lb snow load and 10 lb dead load.  Our county here in Colorado doesn't have any adjustment factors for roof pitch or snow load duration - they plan all rafters as a flat roof. 

I calculated that a 2x12 every 24" OC would work, but I'm wondering what other possibilities would work that were larger than 2x?

Thanks!

Offline Jim_Rogers

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Re: Rafter span chart/calculator for larger timbers
« Reply #1 on: February 10, 2016, 12:06:32 PM »
First of all, welcome.
Next, DonP's calculators in the red tool box are not designed for figuring 2by lumber. They are for 5x5 and larger timbers.
I would not use them for figuring 2by stock.
You could use them for figuring 4x6's as one dimension is over 5".

Jim Rogers
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Offline Winter86

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Re: Rafter span chart/calculator for larger timbers
« Reply #2 on: February 10, 2016, 03:08:10 PM »
Thank you Jim.  At first I was looking for a rafter span chart, but now I realize I can use the beam calculator and divide the load by how many rafters I have to get the individual load on each rafter.  That makes sense. 

Now I can get back to drafting the plans for the county.  This project is a big undertaking for me, but the worst part I think is getting the county to approve my plans!  lol

Thanks again 

Offline Jim_Rogers

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Re: Rafter span chart/calculator for larger timbers
« Reply #3 on: February 10, 2016, 08:31:55 PM »
Calculate the load for one rafter. If the spacing is 16" oc then use that as 1.33 ft times the run (in ft) and that will give you the square foot of area that one rafter will support. Then times the pounds per sqft of live load.
To figure the dead load you need to use the rafter length for the weight of one rafter. Adding to that the roof decking dead load. I usually use something like 10 or 15 lbs per sqft dead load for figuring conservatively.
Then use the calculator with these adjusted load numbers.

Good luck and if you want me to review it post the numbers you come up with and I'll double check it for you.

Jim Rogers
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Offline Winter86

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Re: Rafter span chart/calculator for larger timbers
« Reply #4 on: February 10, 2016, 10:28:13 PM »
Jim, I'm not quite following but I will post up some numbers showing how I was figuring.

This is a shed roof.  The rafter span is 12 ft and the run is 30ft.

I was figuring it as 12ftx30ft = 360sqft of roof
360 sq. ft x 75lbs (snow and dead load combined) = 27,000lbs total on the roof.
If I was to run rafters every 2ft OC there would be 16 rafters in that span. 
27,000lbs/16 = 1687.5lbs on each rafter.  That's what I plugged into the program as my "total load on beam".

Please correct me if I'm not thinking about this correctly.




Offline Jim_Rogers

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Re: Rafter span chart/calculator for larger timbers
« Reply #5 on: February 11, 2016, 08:54:50 PM »
You're saying you have a 30' wide span?
How are you going to do that? two piece rafters?
You'll need some center support some where.
I don't get how it is going to be.
You need to do a screen capture picture of your drawing so I can see the 30' span.

Jim Rogers
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Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Offline grweldon

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Re: Rafter span chart/calculator for larger timbers
« Reply #6 on: February 18, 2016, 02:07:50 PM »
I may be confused as well, but I think he was using the term "run" as the length of the building...  That would make his figures correct...

It's a shed roof so if rafter length was 12' and the length of the building was 30', that would be 360 sq. ft. which is what he says is the roof size...

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