Does heel height determine ridge height?
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| Dear “superframers”, I know that you guys can carry more studs than 5-ton flatbed, rough in a house faster than a van full of Amish and walk 2×4 plates better than spider man , but can you….. tell me this. Doesn’t the heel height of the rafter adjust the ridge height? And if that is true why doesn’t the rafter length change? I came across this question while playing around with the pythagorean theorem; Hypothetically, if all I have is the roof pitch and the building width, I should be able to calculate the ridge height and the rafter length with the Pythagorean Theorem. But I can’t set my ridge height unless I know the heel height- correct? And if the heel height does in fact adjust the rafter height, how is it that the rafter length does not change? this forum rocks…Semper Fi….. Paulewog
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Replies
The Heel height, Icall it the Hap cut(Height Above Plate)will always determine your ridge height because it has a Vertical Rise and that means that it does not change your rafter length.
Lets say you have a 10' run to the back of your ridge with a 8/12 pitch that means that your rise would be 80" if it was a triangle. Now take a 10' triangle with a 80" rise and a 12'1/4' hypotenuse and sit the end of that on your top plate where the seatcut in the birdsmouth would go and you would just have a point.
Your rise would still be 80" and your seatcut would be 10' because you have no birdsmouth yet and no Heel cut yet. Now draw the birdsmouth in with a 3-1/2" seatcut which would for example give you an 8" heel cut. That 8" Heel cut is vertical and is also 8" of rise which would raise your triangle up 8". So add that 8" to your 80" rise and you will have 88" for a total rise.
So all your doing is just raising the rafter 8" higher. That doesn't change the Run of the rafter at all it's still 10'. You just lifted your triagnle up 8" more.
Why do you need to figure the ridge height anyway? Are you sitting these rafter on a structural beam?
If it's just a gable you you never have to figure ridge rise. You can figure close enough for a scaffold but never to set the ridge first and then put your rafters in. All you need to do is just nail two rafters on each end of the building and then slide the ridge up in between the rafters and the rafters set your ridge.
I hope this makes sense.
Joe Carola
Edited 11/6/2005 11:23 pm ET by Framer
Who's better than you with the illustrations?
Man.... tough act to follow. ;)
Thanks Brian. I hope it helps him.Joe Carola
Joe what program did you use to draw thatwith? Simple but very clear.
I'm just using Paint.Joe Carola
Thanks Joe- what you said makes perfect sense, keyword for me was "vertical rise", What I do at the bottom, adjusting hap to a certain height I also do at the top, which doesn't change the pitch at all. I guess I knew that but was having a brain fart and posted the question without really thinking through it.
You asked why I need to know this-- I am an assistant teacher at a vo-tech school and was working with the guys on the Pythagoran theorem. I was telling them how you can find the rafter length and the ridge height and all this cool stuff with a little geometry and a calculator, i also showed them the C.M IV, rafter tables, step off, etc. Just touching the surface really but want to show them there is more than one way to skin a cat,and to learn the one way that works best for them. All that said; I was showing them the rise button on the const.mstr calc and also setting up proportions to find the rise or the ridge height, and then it dawned on me- that the numbers I was getting for height is the mathematical ridge height not actual ridge height. Anyway that's why, I didn't want to be telling anyone the wrong stuff, if they learn it wrong they will spend the rest of their life trying to get it right.
thanks for your time!
To illustrate the point, it might help to set up a really obvious example. Like maybe a 2' wide building at a 12/12 pitch with 2X12 rafters. Sketch it up to scale without allowing for the heel height, and you've got an almost flat roof. All for the heel height and you're back to 12/12.Just a thought...........
When the going gets tough, the tough take a coffee break.
Simply, yes.
When you adjust the heel height you are adjusting the whole assembly up or down.
Imagine you cut a piece of OSB where the legs are 12" and the hypotenuse is 16.97". If you set it on your garage floor (heel height=0") your angles and rafter lengths are a constant. If you set it on top of a 2x (heel height=1.5") or 4x4 (heel height=3.5") the angles and lengths are the same.
Jon Blakemore
RappahannockINC.com Fredericksburg, VA