Ladies and Gentlemen-
Could someone settle a small inhouse fued for us.
When figuring your rafters. My leads have always used run from outside of top plate. Rise to top of ridge. The hypotenuse – 1/2 ridge thickness+ tail = rafter length.
I am arguing that they are not getting the true pitch of the roof unless they account for the width of the rafter and measure from a point above the top plate to the ridge.
Further complicated by the birdsmouth which would change the angle slightly.
Now in true fighting spirit they have let me know that I am exceptionally tight in the back end and that it all amounts to very minor differences in the end and that their roofs have shed water since before I was even a zygote.
What is the definintive, most accurate method of calculating rafter length? And what is the formula to give the true roof pitch adjusted for the birdsmouth and rafter width?
Thanks
Replies
yeah.. your leads suck...
any framing book will give a diagram that shows allowing for H.A.P
( Height Above Plate ) is part of the basic calculation
they are probably the same guys that don't know how to shorten the first riser on a stair run
they probaly get away with it because no one is checking the results and they have a long enough diagonal so it might not show up if they measure it with a 2' level
I read your reply while thinking of how I would respond about his hacky leads getting their hackles up, but you are just so fluent - what could I add!
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
<Further complicated by the birdsmouth which would change the angle slightly.>
Now I may be wrong(not that, that has ever happened before)
As Mike pointed out in your H.A.P., Your angle doesn't change
just the height of the rafter and consequently it does the same
thing at the ridge, so it is parallel. I guess in other words
you don't change the angle you just change the height.
So if your working with a 12/12 and you have 5" hap, that doesn't
change the pitch just the height of the seat cut and the height
of the ridge(in other words parallel)
Your crusty old leads don't know their H-A-P from their A-S-S. Run is figured from outside of top plates to center line of ridge. Deduct half the width of your ridge stock and you have your "actual run". The rise is always the rise, but you need to account for your HAP or heel stand, as I call it, to calculate your "actual rise". A birdsmouth or seat cut doesn't change the angle of anything.
Edited 3/12/2006 3:05 pm ET by dieselpig
These guys shouldn't be leading anything.
Sounds like they're from the 'close-enough' school.
"Citius, Altius, Fortius"
Romero, I guess I'm in the minority here but I don't see anything wrong with your leads explanations. Perhaps I'm not understanding something but I think you are mixed up, not them. I think you might be confusing two terms.
I really can't make a firm decision regarding your question, unless you also include the method that your framers use in determining the length of the rafter. There are several ways: 1) multiplying the unit run x the unit length or 2) deriving the total legth using total height and total run. If they are using the first method, then they are right and you are confusing the issue by bringing up the total height of the ridge.
I also don't understand why you think you need to incorporate the "width of the rafter" in your formula. That particular assertion leads me to believe that you are somewhat confused....because that confuses me!
I think we will need to put some numbers to a roof to clarify what each is thinking.
Lets do a real easy roof: Lets cut some common rafters for a garage that is 20' - 1 1/2" wide (that would be the total span). The slope would be 5/12. The heel (HAP) is 4". The ridge is a 2 x 8.
The actual run would be 10'. That already deducts 1/2 the thickness of the 2x ridge.
The actual length of the rafters would be 10' - 10". The actual height of the ridge would be 54" (50" rise + 4" heel).
Note that the theoretical lengths of this roof would be different. Perhaps your argument is confusing the theoretical lengths and the actual lengths.
I just don't see what is wrong with your lead's methods, if he is using unit multiplyers rather than total rise and run methods.
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