Rafter Calculations - Help Me Understand, Please. - Building & Construction - DIY Chatroom Home Improvement Forum

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04-18-2010, 07:27 PM   #1
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## Rafter Calculations - help me understand, please.

Hi.

I'm building a porch addition - essentially a new porch from scratch including a new roof.

I've been doing lots of reading on roof framing - "measuring, marking and layout," "a roof cutter's secrets..." "simplified roof framing," and all sources online such as fusco's site, etc...

I've built a gable roof over a 10X12 shed. This was easy because I knew the pitch I wanted, cut the end rafter pairs first, then slid the ridge up to meet them.

Here's what I'm confused about: In this case, I will have an existing ledger on the house (will need to relocate it in a certain position below a window).

I know how to punch in rise and run and get the hypotenuse on a construction calculator, but this "line length" is completely useless. I have yet to find a SINGLE source in all my reading that tells you how to mathematically (and simply) calculate pitch and "actual line length" of the rafter after determining HAP and seat cut. Almost all the sources either ignore this completely, assume the ridge will be put in after the end commons are cut, or they imply you can arbitrarily pick your HAP and the rafter will magically plane in with the top of the ridge/ledger.

I like Joe Fusco's "block and string" method, but I would much prefer to figure it all out from the ground using a calculator.

How about this: can I simply choose an arbitrary HAP, then reduce my rise by that number, get the line length and pitch, lay it out with a framing square on the rafter and see how much seat cut it leaves, and basically go through iterations of this process until I have the full bearing on the seat cut, and know exactly the HAP, the pitch, and my actual measuring line length?
Is it that easy?

Am I missing something? Am I making it harder than it needs to be?

PS: this will be a hip roof with spans of 5' 10 1/2" using 2X6 rafters.

Any help or clarification would be appreciated.

Thank you.

PS: so far "measuring, marking and layout" i've found to be a great book, but it hasn't quite given me the info i need." i'm very disappointed in "a roof cutter's secrets," and am returning it.

Last edited by wombosi; 04-18-2010 at 07:31 PM.

04-18-2010, 07:40 PM   #2
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It doesn’t have to be that complicated. If you already have your ledger and you know were you want the rafter to end up then just hold your board up there, scribe your angles and there you go.

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04-18-2010, 07:44 PM   #3
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Quote:
 Originally Posted by kwikfishron It doesnt have to be that complicated. If you already have your ledger and you know were you want the rafter to end up then just hold your board up there, scribe your angles and there you go.
thanks.
still, i would like to know how to calculate this mathematically.
even holding the board up doesn't help me calculate HAP/seat bearing.
and to hold it up i would need the proper plumb cut on the rafter already, no?
if this were a ridge, i could hold it up and shoot the top of the rafter past the ridge and scribe plumb, but in this case i'd be hitting the house.
i don't want to scribe this. i want to be at least semi pro and calculate.

 04-18-2010, 07:56 PM #4 Member     Join Date: Mar 2010 Location: Kansas City/Oregon Coast Posts: 10,206 Rewards Points: 818 If you hold your board up against the outside of your ledger and the beam it will sit on you can scribe it all. When I was a young buck learning from the old timers they would say “just scribe the dam thing your, wasting to much time thinking” and that reminds me of another thing they drilled into me that I can’t forget, “shut up and nail”. __________________ A Picture Is Worth A Thousand WordsEspecially In The DIY Chatroom
04-18-2010, 08:03 PM   #5
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Quote:
 Originally Posted by kwikfishron If you hold your board up against the outside of your ledger and the beam it will sit on you can scribe it all. When I was a young buck learning from the old timers they would say just scribe the dam thing your, wasting to much time thinking and that reminds me of another thing they drilled into me that I cant forget, shut up and nail.
i hear your point, and i don't want to turn this into a petty argument. i still don't see how to scribe the rafter as it would sit on top of the beam. holding the rafter flush with the ledger at the top would then give me the wrong angle.

anyway, there's a time and a place for doing it this way, namely something like a shed roof. with multiple hips and planes involved, i don't want to mess around with guesswork and looking like a total amateur. at this point i have more confidence in the block and string method, or trying to figure it out mathematically.

the bottom line is, i want to UNDERSTAND this and LEARN why and how.
which brings me back to my original point: can you tell me how to do it with a calculator? can anyone??

 04-18-2010, 08:16 PM #6 Old School     Join Date: Jan 2009 Location: St. Petersburg, FL Minds of moderate caliber ordinarily condemn everything which is beyond them. Posts: 3,634 Rewards Points: 2,000 The common rafter length can be determined by the use of the Pythagorean Theorem . . .c2 = a2 + b2 . Where a² is the building rise and b² is the building run in inches. __________________ "True eloquence consists in saying all that is necessary, and only that which is." François Duc de La Rochefoucauld Willie T
 04-18-2010, 08:17 PM #7 Member   Join Date: Jul 2008 Location: NW of D.C. Posts: 5,990 Rewards Points: 2,000 Post a diagram or link to what dims & angles are known and what dims & angles you want to know. I think we can reverse engineer this link's method http://www.sbebuilders.com/tools/hap.php and this http://www.builderbill-diy-help.com/roof-framing.html Last edited by Yoyizit; 04-18-2010 at 08:22 PM.
 04-18-2010, 08:18 PM #8 Member     Join Date: Mar 2010 Location: Kansas City/Oregon Coast Posts: 10,206 Rewards Points: 818 You hold it to the outside of the ledger and beam in the position as it would be if it was butting the ledger and sitting on top of the beam, then scribe. The block and string works fine too, just a few extra steps. I apologize if I came off argumentative. __________________ A Picture Is Worth A Thousand WordsEspecially In The DIY Chatroom
04-18-2010, 08:19 PM   #9
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Quote:
 Originally Posted by Willie T The common rafter length can be determined by the use of the Pythagorean Theorem . . .c2 = a2 + b2 . Where a² is the building rise and b² is the building run in inches.
willie, come on man. did you not read anything i wrote?

if i wanted my rafter to end at a point at the building line, with no overhang, than your method would be fine.

but as soon as there is a bird's mouth, that measurement is no longer correct, RIGHT?

 04-18-2010, 08:24 PM #10 Member   Join Date: Apr 2008 Location: Western Masschusetts Posts: 575 Rewards Points: 500 the only known dimension right now is the span of 5' 10 1/2". there is still the original roof which will soon be gutted. i'm guessing 2X4 rafters with no ledger and no bird's mouth. the house is my neighbors and is of the same style and vintage as mine. i found the same thing in my roof. i'm pretty sure the existing beam is a 4X8, and i think there will be 1X roofers for s heathing, then a layer of cedar shingles, then asphalt. so when i go to rebuild it, the pitch may have to change slightly. kwikfish - no worries man. just saying i'm not interesting in the scribing method at this point. so, if the block and string method works, can't i just say: "ok, i want a 3.5" HAP, then calculate rise from that imaginary line, punch in the run, and then i get angle, pitch, and line length. then i can lay it out with a framing square, and if that HAP doesn't give me the seat cut bearing i want, start over with a different number. this will work right? but it also seams cheesy. it would be nice to say: "OK. i want 4" bearing on my top plate, what's my angle and pitch and length?" this is the essence of what i'm getting at here. Last edited by wombosi; 04-18-2010 at 08:28 PM.
04-18-2010, 08:37 PM   #11
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Quote:
 Originally Posted by [B Am I missing something? Am I making it harder than it needs to be?[/B] Any help or clarification would be appreciated. Thank you. PS: so far "measuring, marking and layout" i've found to be a great book, but it hasn't quite given me the info i need." i'm very disappointed in "a roof cutter's secrets," and am returning it.
Yep.

Good night.
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04-18-2010, 09:01 PM   #12
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Quote:
 Originally Posted by schmolze willie, come on man. did you not read anything i wrote? if i wanted my rafter to end at a point at the building line, with no overhang, than your method would be fine. but as soon as there is a bird's mouth, that measurement is no longer correct, RIGHT?
Visualize a line straight up from the vertical cut of the bird's mouth. From there to the centerline of the ridge (minus half of the ridge thickness) is the basic run. Add the horizontal run of any overhang from that point.

This is 5th grade math. (See the diagram attached.)
Attached Files
 Rafter diagram.doc (70.0 KB, 240 views)
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Last edited by Willie T; 04-18-2010 at 09:12 PM.

 04-18-2010, 09:11 PM #13 Member   Join Date: Jan 2009 Location: South of Boston, MA Posts: 17,248 Rewards Points: 2,000 I've held the board up & scribed the lines on everything I have built Not easy with a 20' 2x12....I placed nails into hold it up My last house had 5 different roof levels on it This place - greenhouse, shed, dormer, sunroom & great room roofs All scribed --actually my last house my neighbor (lifetime carpenter) did the calcs on the 1st rafter for the main roof...I did the rest Yes - but Willie - have you seen the show.."Are you smarter then a 5th grader ?" I did do the math to get me to approx heights & to calc roof slopes before building
04-18-2010, 09:14 PM   #14
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Quote:
 Originally Posted by schmolze the only known dimension right now is the span of 5' 10 1/2". "ok, i want a 3.5" HAP, i want 4" bearing on my top plate
In addition to the 3.5", the 4" and the 70.5", if you can give me angle or a pitch or a max overall length I can tell you all the other lengths and angles. It's tedious but it seems to work.

E.g., with a rise:run of 1:2, the rafter is 26.6 degrees above the horizontal and the top edge of the rafter is 78.82" long, but the max. overall rafter length is slightly longer than this, by 1.57".
The rafter end would go down 3.5", over 4" and then down again until the 5.5" width is used up.

Draw what you want reasonably to scale and label each length and angle. I should be able to fill in the numbers. An Excel-ready general formula is a little more complicated.

Last edited by Yoyizit; 04-18-2010 at 09:23 PM.

04-18-2010, 09:15 PM   #15
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Quote:
 Originally Posted by Scuba_Dave I've held the board up & scribed the lines on everything I have built Not easy with a 20' 2x12....I placed nails into hold it up My last house had 5 different roof levels on it This place - greenhouse, shed, dormer, sunroom & great room roofs All scribed --actually my last house my neighbor (lifetime carpenter) did the calcs on the 1st rafter for the main roof...I did the rest Yes - but Willie - have you seen the show.."Are you smarter then a 5th grader ?" I did do the math to get me to approx heights & to calc roof slopes before building
That's what the Smiley Face was about. I never saw a third of the stuff those TV kids tackle till I was in High School.

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