Wagner book, span table question

Started by dablack, July 21, 2011, 09:27:55 PM

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dablack

Ok, guys.  I'm freaking out because this makes zero sense to me.  On page 25 of the Wagner House Framing book, he has a rafter span rating table for different wood species.  It is broken up by slope (3 in 12 or less vs over 3 in 12).

The problem is, it is saying that the different woods can span a greater distance at the 3 in 12 or less vs the over 3 in 12. 

I thought that the steeper the roof (12 in 12 is definately over 3 in 12), the greater distance the wood could span because there would be less load on the rafter.  When something says 3 in 12, I take that to mean a rise of 3" for ever 12" (basially Rise over Run......geometry), so a 3 in 12 or less roof would be pretty flat. 

What am I missing.  The table seems backwards to me. 

I'm a mechanical engineer and took statics with the civil engineers.  If I can't understand this, I sure shouldn't be building a house!

thanks
Austin

MountainDon

The steeper pitch rafter is longer. If the snow does not slide off there will be more snow load on the steeper pitch roof. Don't count on snow sliding off. I have seen 10:12 metal rib roofing here with a foot of snow on it under the right/wrong circumstances.

Rafter tables and calculators usually do not differentiate between pitches and usually call the rafter span as being one half the building width. Not necessarily correct.



Just because something has been done and has not failed, doesn't mean it is good design.


Don_P

I would caution you to stick to either the codebook span tables, those of an accredited grading agency, or the AF&PA tables or span calc. I've seen old tables and just plain wrong tables in various books.

One way to possibly backcheck their span tables is to calculate dead load on the diagonal rafter length and live load on the horizontal span, technically this is correct, the dead load for a 12/12 pitch would then be 1.414 times the horizontal sum.

MountainDon

I probably mispoke; it is the DL (dead load) that increases as the roof pitch increases. As Don_P pointed out the length of the 12:12 rafter (not including the tail) is 1.414 X as long as the rafter for a flat roof. Therefore the materials that make up the roof will weigh more with a steeper pitch roof. Snow load is always taken from the horizontal ground plane; thus the charts show snow as ground load as a rule.

Just because something has been done and has not failed, doesn't mean it is good design.

JRR

Along with the concerns of compressive loading in the rafters of a steep roof, (as already pointed out) there are also concerns of stiffness.  Shifting wind loads on the steep roof will cause flexing ... over time this can loosen shingles or tiles.  The stiffer the roof structure, the better to prevent this. Roof trusses get the nod over rafter structures for this concern.


dablack

thanks guys.  I really appreciate it.  Last night I went to bed mad and woke up with the answer.  The first thing I thought when I woke up was: The steeper the roof, the longer the rafter, so the less load it can carry.  It isn't about how much load it was putting on the outside walls, it is about how much that individual member can carry.  Now I get it. 

thanks
Austin

MountainDon

Some of my best thinking is done while I'm sleeping.  ;)



Re: Don_P's comments on it is best to use the official code books... The code book charts and tables will always be the latest information. As long as you are using the latest version, that is. Other guides and how to books may have outdated material IF there has been a code revision. Other books may have figments of someone's imagination. Some of you may recall the recall on a certain Rodale Press DIY guide that had incorrect info on some electrical how to. It has been corrected in later editions but who knows how many are in the hands of DIY'ers still?

Just because something has been done and has not failed, doesn't mean it is good design.