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Old 07-19-2011, 10:34 PM   #16
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Rafter length mistakes. Help


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Originally Posted by Selectadon View Post
First of all how did u come up with the 4.09

I'm pretty sure the hap is 2" the seat Is 1 1/2 I know it's 2x4 rafter but Its a long shed.

So Please explain that if I always use full bearing seat ie: in this situation it's 51/2 how do u calc that in

Thank u u are educating a good carpenter that always struggles with rafters I always wrk it out but I kno it aint so complicated
142.3175 Span - 1.5= 140.8175

140.8175/2= 70.40875 (rafter run)

70.40875/12= 5.8673 ft.(rafter run in feet)

24"/5.8673= 4.09 (rise / feet of run= rise per foot of run)

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Old 07-19-2011, 10:36 PM   #17
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Rafter length mistakes. Help


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It won't change the rafter length, but if your HAP is not correct, it will have the same effect.

Imagine if you took 1/2" out of the seat cut. It would drop the rafter and effectively bring the plumb cut of the birdsmouth closer to the wall. The opposite is true if you put 1/2" shim under the rafter.


So please explain to me how to factor the hap in when figuring rafter lengths
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Old 07-19-2011, 10:50 PM   #18
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Rafter length mistakes. Help


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So please explain to me how to factor the hap in when figuring rafter lengths
It's more difficult when you have a ridge height that is set, because you have to work backward to some degree.

If you are matching another roof, it's best to find the existing HAP, then calculate the new roof pitch while taking the existing HAP into consideration.

Let's assume your ridge is exactly 25.5" above the plates that your rafter will bear upon.

Using the calculations I've already used for your span, the HAP would be 1.5"

(24+1.5=25.5)

Now, suppose the top of ridge is 26" above the plates. Using the same rafter, it would fall 1/2" below the top of ridge.

If you increase the HAP by 1/2", it will fit perfectly. The rafter length stays constant, but the HAP either increases or decreases the overall height of the ridge.

Think of the HAP as if it were adding or subtracting shims under the seat cut.

I've had some framing jobs that had me cross-eyed at times. This was one of them. Trying to get the shed dormer walls set to the proper height, getting the length of the shed dormer rafters so they tie into the steeper rafters properly.....
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Old 07-20-2011, 12:06 AM   #19
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Rafter length mistakes. Help


Wow lone framer I can only imagine what u thought when u first got those plans. Thank you very much u have helped alot. The one thing I left out that it is a reroof. No venting so the rafters and the ridge completely dryrotted . To top it off the building narrows 2" in 24'. Each one needed it's own calc. Out of level and plumb all the way. But yes I always am a half inch off when I do it regularly. The hap info was my missing link. I know it's late in NJ

Thank you!!!!!! I owe u a beer
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Old 07-20-2011, 06:42 PM   #20
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Here's a real simple way of doing it.
Cut the ridge end of your rafter to the angle you need(5:12 6:12 whatever).
Measure from the top edge of the ridge board to the outer edge of your wall.
Now transfer that measurement from the point of your rafter cut to wherever it hits on the other end of the rafter and mark it all the way across the rafter (it will be a slight radius mark).
Now measure up from the lower edge of the rafter 1/3 of its thickness (2 3/8 for a 2x8, 3 1/8 for a 2x10).
Now put the inside corner of your framing square on that point and adjust it to match your pitch.
Mark the angles and that will be the birdsmouth.
It's simplistic (if you understand my explanation), and I don't use it myself, but it could solve your problem
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Old 07-20-2011, 07:27 PM   #21
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Rafter length mistakes. Help


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Originally Posted by Selectadon View Post
When cutting rafter i always have the samw problem,

Measure span
- 1 1/2 for ridge
divide by 2 for my run, my rise is set
That's your mistake right there. If you don't know how to figure the EXACT rise your rafter will always be off.

How did you figure your rise and set the ridge height first?
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Old 07-21-2011, 12:07 AM   #22
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Like I said the ridge height was set it's a addisition to existing ridge so I put my tape measure on the top plate pulled it up to the ridge read the number and determined that that is the height of the existing ridge.
Is ther a better way that I missed?
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Old 07-21-2011, 12:11 AM   #23
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Originally Posted by mrgins View Post
Here's a real simple way of doing it.
Cut the ridge end of your rafter to the angle you need(5:12 6:12 whatever).
Measure from the top edge of the ridge board to the outer edge of your wall.
Now transfer that measurement from the point of your rafter cut to wherever it hits on the other end of the rafter and mark it all the way across the rafter (it will be a slight radius mark).
Now measure up from the lower edge of the rafter 1/3 of its thickness (2 3/8 for a 2x8, 3 1/8 for a 2x10).
Now put the inside corner of your framing square on that point and adjust it to match your pitch.
Mark the angles and that will be the birdsmouth.
It's simplistic (if you understand my explanation), and I don't use it myself, but it could solve your problem


Seems like the block and string method to me. Which also works if you are not trying to be exact.
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Old 07-21-2011, 05:38 AM   #24
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Rafter length mistakes. Help


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Like I said the ridge height was set it's a addisition to existing ridge so I put my tape measure on the top plate pulled it up to the ridge read the number and determined that that is the height of the existing ridge.
Is ther a better way that I missed?
It's a good idea to pull a string from bearing plate to bearing plate and measure from top of rige to the string, especially in an older building where the floors may not be level. This will give a more accurate rise.
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Old 07-21-2011, 01:13 PM   #25
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Seems like the block and string method to me. Which also works if you are not trying to be exact.
Call it what you want, it's simple and accurate
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Old 07-21-2011, 01:28 PM   #26
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Rafter length mistakes. Help


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Call it what you want, it's simple and accurate
I remember using a method very similar to that years ago. We were framing in a house with hip roofs with a foundation that was 6" out of square. I was up on the wall calling down individual measurements. My buddy down below had a mock up of the tail end of the rafter with a birds mouth cut in it. I would measure from the top of the ridge to the outside of the wall where the birdsmouth sat. He would transfer that measurement and use the mock up to trace out the birdsmouth and plumb cut on the lower end. It was simple, quick, and accurate for what we were up against.
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Old 07-21-2011, 02:00 PM   #27
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If you know your wall narrows by 2'' in the length of the place, then your run will be inconsistent, and you may be working against yourself doing the math.

Unless you want to measure and redo the math for every time the run changes, I would think it might be faster to cut the plumb cut for the ridge, throw the rafter up, mark for the birdsmouth and have a guy cutting for you, or measure each one as mentioned. IMO will be faster, more efficient and more accurate. Just in this case when the building your working with is out.

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Old 07-21-2011, 02:45 PM   #28
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Also keep in mind if you are doing existing work, the original pitch might not be exact. The string and block method comes in real handy in these situations.
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Old 07-21-2011, 11:03 PM   #29
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For sure the block and string is solid. And unfortunately nothing is ever as easy as it should be
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Old 07-22-2011, 07:58 AM   #30
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Pay attention to this "measuring what is ACTUALLY there" advice. Math is impressive, but the real world (especially in old existing structures) will soon have it biting you in the butt.

What you're building has to work.... it DOES NOT have to precisely match numbers on a calculator screen.

And remember, in an old building, nothing says all the measurements on a full set of new rafters are going to be identical.

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