Educate Me On Rafter Span Calculations

eXpatRioteX

I'm playing around with a couple online calculators trying to determine what material would be needed to frame a roof. Its high time i figured out how to do this myself, but there are a few things i dont understand. What does deflection refer to, and how do i determine it? How do i calculate snow load for my geographic region? The roof i'd like to frame is 6:12, and the run for the span is 13'3". I think the bare minimum i can use is 2x10 hemfir on 24's, or 2x10 spf on 16's. The rafter length is around 18' with the 1' overhang. Can someone point me to an easily understood reference to show me how to clculate this myself?

ch0mpie

Deflection is basically how much a beam "saggs" when a load is applied to it. As far a everything else, if the online calculators aren't working for you try this book:

http://www.amazon.com/Design-Wood-St...9433789&sr=1-1

Unfortuanately it doesn't come with an engineering degree, so you should probably hire an engineer to design this type of stuff for you.

Tom M

You need a code book. There is a geographical chart that has ground snow loads for your area. Chapter 8 has charts for rafters by snow load, if your area falls in between a chart go to the heavier load its more restrictive. You have a standard ceiling where the proposed attic floor joist rest on the exterior walls so now you can locate the chart you need (30psf, ceiling not attatched). There is 2 options for dead loads to select from before you go further. The 10psf will cover your basic roof products of moderate weight. The 20psf is heavier roof coverings like slate,tile,etc.. Now find the Hypotnuese or slope you want by building width and ridge hieght. Disreguard the overhang its only cosmetic. This gives you your span. So now you know all your criteria to follow the chart. Take your span length say 15 ft for example and you want 2 x 6 --16" o/c the max you can get for a 2 x 6 ---16 o/c is 14-4 for select doug fir you need to bump it up to a 2x8. Now you can get 18-10 for select doug fir or 15ft for #2. You can also reduce your spacing to 12"o/c and now a 2x6 gets you 15 again (under certain species). If I remember correctly the deflection is in inches say L/^=180 THEN you can deflect no more than 1" in a 180" span. For floors its double say 1" in 360" span. Thats it now you know how.

KentWhitten

It's not the length of the rafter. It's the horizontal span. The bearing is from the plumb cut of the ridge to the short point of full bearing on the wall.

Also like tom said, you need to check the code book, but even then it can be off. Call your local building inspector and ask what the snow load is for your town/city.

Deflection is such a mistaken formula to use for construction. But it's my opinion, it's what is used. Best example is 2x6 floor joists compared to 30' long 16" TJI's. Deflection of 2x6's are almost unmeasureable. It's a stiff, stiff floor. A 30' TJI, while able to take the load limits, bounces like an SOB.

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If you correct your mind, the rest of your life will fall into place ~Lao Tzu

Custom Cabinetry - Portland, Cape Elizabeth, Scarborough, Kennebunkport, Yarmouth, Falmouth, Cumberland, Ogunquit, Maine

Salmon Falls Cabinetry

Tom M

It's not the length of the rafter. It's the horizontal span.
It is the lenght of the rafter. The charts dont specify roof slopes by pitch so you need to find your rafter run.

joasis

A copy of Architectural Design and Engineering is a good book to have handy...for sleepless nights, it is a drug free way to induce sound sleep. Seriously, you will get bored with the definitions and calculation, but you will know what terms like "L/180" and L320 mean and how they apply in construction. Also, keep in mind the species of wood has a lot to do with what you can and cannot do.

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hatchet

Where do you find 30' 2x6's?

__________________
Rich
WIT Construction Co.
Superior MT Contractor

KentWhitten

read that one just a little closer hatchet LOL. Maybe I shoulda put a comma there for clarity.

tom, that's what I meant. The measurement the span charts give is not the diagonal measurement of the rafter, it's meant for the horizontal (run) measurement.

Span wasn't the correct word. Span would be full distance. My mistake.

__________________
If you correct your mind, the rest of your life will fall into place ~Lao Tzu

Custom Cabinetry - Portland, Cape Elizabeth, Scarborough, Kennebunkport, Yarmouth, Falmouth, Cumberland, Ogunquit, Maine

Salmon Falls Cabinetry

hatchet

I was pretty sure I read it correctly - you're comparing a 30' - 16" TJI to a 2x6. And when comparing things I would assume the same span, of which was 30'. I understand what you're saying - but the span on the 2x6 would be much smaller spans to get decent floor performance.
Just for clarification - a 30' 16" TJI 210 @ 16" o.c. doesn't meet specification due to load ends up with L/252 for deflection which also doesn't meet minimum floor performance.
A HemFir #1 2x6 @ 16" o.c. will span about 12' with similar performance (still noting that it doesn't meet minimum floor performance).
Not really trying to start a flame war here - just clarifying the performance differences between spans of those 2 particular members you mentioned.

__________________
Rich
WIT Construction Co.
Superior MT Contractor

KentWhitten

OK dude, whatever.

__________________
If you correct your mind, the rest of your life will fall into place ~Lao Tzu

Custom Cabinetry - Portland, Cape Elizabeth, Scarborough, Kennebunkport, Yarmouth, Falmouth, Cumberland, Ogunquit, Maine

Salmon Falls Cabinetry

festerized

Pick up your self a construction master calculater,
its the only way to frame roofs.
I have an employee who went out on his own
he calls me for roof calcs. ,he also sends me a few bucks for helping ,nothing is for free!!!!!!!!!
All i ask is 2 questions
1 span? 26'
2 pitch? 6 pitch

Answer 14' 6 7/16" diag. Thats long to short along top of rafter,just deduct for ridge and your plump cut and over hang.
The calculer is real good for "basterd roofs"
spot on everytime

Jcllc

Hey there fellow carpenter, sometime back i had to start figuring roofs and complex cuts for hips etc..I started searching and found a site by joseph fusco. he's very detail oriented and thourough in his explanations and diagrams.I hope this hekps you as much as it did me. Let me know how it turns out.
http://www.josephfusco.org/Articles/...after%20Square

Joe Carola

He's not asking how to figure the length of a rafter. He's asking what size rafter to use. The length is easy.

__________________
Joe Carola

mighty anvil

Both the code tables and online calculators for rafters take into consideration the Modulous of Elasticity (E) and the Maximum Fiber Stress in Bending (Fb) [modified by the appropriate Size Factor (Cf), Repetitive Member Factor (Cr), and Duration of Load Factor] of the particular wood species and grade you are using. Then you enter the horizontal "on center" spacing of the rafters and the projected horizontal distance from the support wall to the ridge (span).

The only reason to fuss with all that information yourself is to use a deflection limit stiffer than L/360 in order to reduce bounce in floor joists (can't think of any reason to do it for rafters). Some engineers just increase the floor live loading to 60 PSF to be sure of adequate stiffness. An even easier way is to use the code table for exterior deck joists to design interior floor joists.

Of course, this is only for uniformly distributed loads.

loogout1

How about this, 8 on 12, width of building is 30' 9", simple gable with 1' overhang. Arch. has spec'd 2x12s. Seems like overkill no? I live 40mi. north of nyc.

K2

Are you putting insulation in that 2x12 space?? May be the reason . That's a pretty big rafter.

r-30 with an inch and a half of airflow = 11.25 or so.

loogout1

Wasnt planning on ins. just a attic. Ridge vent and vented soffits. I thought a 2x10 would be more than enough.

K2

Could it be future living space? Easier to put them 2x12's in now than later.

K2

What does he have for ceiling joists? That would give you a clue if he is thinking living space... Actually 8/12, someone is going to be thinking living space somewhere down the road.

loogout1

I'm talking about roof rafters, not ceiling rafters. Maybe I misunderstood the thread. Anyway it just seemed excessive to have a 2x12 for a pretty steep roof thats not that wide. Each rafter is going to be roughly 20' or so. A 12 would definitely do it but money IS an issue, so economics is going to play a part also.