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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.
 
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.
 
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.
 
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.
 
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.
 
wallmaxx--so is the rafter span the horizontal distance or do you have to apply the conversion factor? My framing inspector bases his rafter spans on horizontal distance, not actual rafter length. This book would seem to indicate that rafter span is actually the length of rafter and not horizontal distance. Thanks .
 
I am helping out with the roof and stairs on a commercial building. They trussed the 46 foot clear span...but i will be stick framing the middle entry area and helping with ALL THE OVERFRAMING (that trusses don't do). We have been sent 20' 2x6s to span 18 for rafters. The pitch is 3.5:12. So, I will be putting in beams perpendicular to the rafters every 5" to support the load as per the span charts for 2x6. I'll try and get some pix up soon.
 
wallmax--so is the rafter span the horizontal distance or do you have to apply the conversion factor? My framing inspector bases his rafter spans on horizontal distance, not actual rafter length.
That's because the rafter length is not the span. The rafter span is horizontal distance from the outside plate to the outside plate. The rafter run is half the span minus half the thickness of the ridge.

For example if you have a rafter span of 20' with a 5/12 pitch and a 2x ridge, half of 20' is 10'. Subtract 1/2 the thickness of the ridge which is 3/4" and that gives you a rafter run of 9' 11-1/4". With a 5/12 pitch your rafter length is10' 9-3/16", not including an overhang.


This book would seem to indicate that rafter span is actually the length of rafter and not horizontal distance. Thanks .
What book is that?
 
joe,
sorry, i need to make my question a little more clear. I understand what a rafter span is, but i meant the span listed in the "allowable spans" chart of a code book. The "book" that i mentioned is the one that Wallmaxx included a photocopy of on post # 33.

Read the section under "allowable spans". The part that I was wondering about is this statement. "For sloping members such as roof rafters, a factor must be applied to the horizontal rafter span to determine the actual clear length or sloping distance" This would sure seem to indicate that one must multiply the span by the factor given in the chart to determine the rafter length for code purposes. Am I reading and interpreting this correctly?
 
The part that I was wondering about is this statement. "For sloping members such as roof rafters, a factor must be applied to the horizontal rafter span to determine the actual clear length or sloping distance" This would sure seem to indicate that one must multiply the span by the factor given in the chart to determine the rafter length for code purposes. Am I reading and interpreting this correctly?
No, that does not indicate anything about multiplying for load calculations.

What that statement means is the multiplier for the cut length of the board at whatever pitch given a known horizontal span.

Way back in the late 80's, we were doing nothing but hand cut 12/12 pitch roofs all over the east side of Seattle/Bellevue Washington. And that mutiplier of 1.414 came in handy, and this was in the days before Construction Master Calculators were common.


Ever play with 24' and 28' 2x12's, or set a triple 2x12 48' long hip. Thats how big these roofs were with the houses averaging 6,000 sq. ft.

This is one of the smaller ones at 5,000 sq. ft., with a simpler roof.
Image
 
kgmz,i understand what the conversion chart does. However, the this book sure seems to differentiate between horizontal spans and "sloping member" spans. This statement about applying the conversion table is under the section of the book for allowable spans, so why include the conversion chart if rafter length has nothing to do with allowable span? I am not trying to beat a dead horse here, but I am obviously not reading this the same way you are. Thanks for the help and patience!!
 
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