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-S...55396/ref=pd_bbs_sr_1/002-1569638-3326418?ie=UTF8&s=books&qid=1179433789&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.

 

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.

 

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.

 

....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.

Where do you find 30' 2x6's?

 

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.

 

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.

 

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.

 

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.

 

OK dude, whatever. :thumbsup:

 

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

 

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

 

re:educate me

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/Roof_Cutting/raftercutting.htm#What is a Rafter Square

 

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.

 

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.

 

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.

 

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

 

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

 

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.

 

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.

 

I'm talking about roof rafters, not ceiling rafters. .

Ceiling joist size would give you an indication if there is a possibility of living space. Never mind.

 

It all depends on a number of factors that we would need to know first before we can say fine or not.

What's the live load? dead load? deflection? rafter spacing? species of lumber? grade of lumber?

What I am used to is rafters 24" OC, but here in the NE, I have seen many 16" OC roofs. You say you're 40 miles N of NYC, there could be a snow load factor there. I can't remember how close the Adirondacks are, but they are somewhat close aren't they?

A 2x12 for the span of your rafters being 15' or so doesn't sound off by any means in a snow load area with 2' OC's. 2x10's might get you by, but you honestly, really, need to find the answers to these questions first before you say "ah, it'll be fine"

In a scenario where you have a simple building with a carrying beam down the center that can support the ridge, you can also cut down on your rafter sizes to 2x6 IF you have a support for the mid span of the rafter itself. It would look similar to a simple common truss.

 

In a scenario where you have a simple building with a carrying beam down the center that can support the ridge, you can also cut down on your rafter sizes to 2x6 IF you have a support for the mid span of the rafter itself. It would look similar to a simple common truss.

in prior construction projects, I've used a 2x10 LVL, which had to be stamped by the engineer (courtesy of the lumber yard)-the requirements were that it had to be supported on either end-then the lvl carries the weight versus no ridgebeam and the rafters carrying the weight down to the wall plates.

So... you may get away w/ using 2x10's if you use a LVL ridgebeam, which would need support on either end. I would hazard a guess deeper lvl's are made which could be used for longer spans.

 

I figured there was a lot more that went in to it. I just cant for the life of me remember ever seeing a 12" roof rafter in this neck of the woods. Hell, the roof that I'm planning on replacing is a hip with 2x4 constr. I'm pretty far from the daks by the way, probably 3 hrs northwest of me. The arch. also calls for rafters 16"o.c.

 

Would that affect roof rafter size though? I'm not being a smart ass, I really dont know.

 

There are a number of considerations involved in sizing rafters besides load and span. It would be easy to call your architect and ask him to explain.

 

2x12 rafters sound right. insulate them and who cares about the ceiling joists insulation. it's a good practice to insulate the entire shell instead of just the living area. That way all the mechanicals are in insulated space.

It could be a cathedral ceiling for all we know

 

It may require 2x12's for the insulation factor, up here in Northern Michigan, they are requiring R-38 ceiling insulation which wont fit in a 2x10 cavity. Also there is a split between 60# and 70# snowloading up here, with the latter requiring 5/8" osb roof sheeting.

 

You can't determine if a rafter is adequate without knowing the Modulous of Elasticity and the Max. Fiber Stress of the wood species, the spacing of the rafters, and the snow loading conditions (side wall drifts, etc.). There are combinations of these factors where 2x10's would be just barely inadequate.

Be sure the architect intends to put the insulation in the attic floor. Today it is generally considered a better practice by most architects to put it in the rafters and include the attic space within the conditioned space of the house. That's what I would do and that's why I normally use 2x12 rafters.

 

oops

 

I appreciate the comments guys. I just wanted to make sure I wasnt spending money I didn't need to. Sounds like I should just go with the 2x12s and be done with it. Thanks again for all the help.

 

2x6, 16"oc, 14' span okay?

I've been asked to unofficially inspect a house for friend in contract and was amazed to see 2x6 rafters and ridges without collar ties on 16" centers over a 14' span (the house is 28'wide).

This was done ca 1968, the wood looks like good southern yellow pine, and when walking on the roof there is no "spongyness".

Anyone want to take a shot at those variables and see if this could be legit?
We're in central Virginia, some snow but not huge amounts at all.

Meanwhile I'll try to decipher this stuff too.

 

Here's a sample from a GREAT book I use every day.

The Visual Handbook of Building and Remodeling
-Professional Edition-
by Charlie Wing

ISBN 0-87596-808-2 Hard cover

The List price was $52...got it for $10 on Amazon used