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Old 07-18-2011, 09:38 PM   #1
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Rafter length mistakes. Help


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
Put all the info into calc and angle is right but my seat cut is always about a half inch off the wall, ridge is level. So i have no idea why i need to minus a half inch off my rafter lenght to get the seat to be touching the wall, any help?
Yes im a carpenter

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Old 07-19-2011, 08:47 AM   #2
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Rafter length mistakes. Help


Wow 53 views and no replies???????

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Old 07-19-2011, 09:04 AM   #3
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Rafter length mistakes. Help


Select,
Not sure what you are doing wrong. I don't use a calculator though. I lay my rafters out with a framing square. Is you 1/2" not touching the vertical surface of the wall or the horizontal surface of the top plate? Also, when figuring the height of your ridge, are you taking into consideration your 'height above plate' where your birdsmouth sits on the plate? There was a good article on this that came from one of my email subscriptions, can't remember which one though. The HAP dimension adds to your overall ridge height and should be subtracted from your initial ridge height to get the true height. The HAP is that little add on that has you scratching your head when you are trying to set your ridge height going by your rise that you calculate from your pitch numbers. Sounds confusing, but it may be the difference that's causing your seat cut to be off a bit.
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Old 07-19-2011, 10:52 AM   #4
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Rafter length mistakes. Help


Are you setting your ridge board firm then nailing your rafters to it? You can check the rafter length by math, A squared + B squared = C squared. This is an explanation of the way I figure my rafter lengths I posted before for a collage student writing a report, I hope it helps.

The run and rise and rafter form a triangle as you know, A squared + B squared = C squared. Say you have a 5 and 12 pitch, cut the plumb cut at the top end of the rafter, hook your tape on the top edge of the rafter at the top and measure down the length of what the rafter is and make a mark on the top edge of the rafter. This is called the line length.

Now using your framing square make another plumb mark from the mark you made. Now before you go any further you will need to take half the thickness of the ridge board off which is 3/4 inch provided your ridge board is 1 1/2 inch thick. Square off the plumb line you just made and make a mark at 3/4 inch, now make a plumb mark at the 3/4 inch mark you just made, that will be the back side or outside of your bird's mouth.

Take a square and from the bottom of the plumb mark with the square inline with the plumb mark slide the square up the plumb mark from the bottom of the rafter to where you have 3 1/2 inches on the square, mark on top and this will be the bird's mouth.

That can't be very clear, if you had a 2X4 block you could just place it so the 1 1/2 inch edge is square with the plumb line and slide it to where the top edge of the 2X4 is inline with the bottom edge of the rafter which will form the bird's mouth. I know I didn't explain this very well and hopefully someone else can but I will give you and example.

You have a building with a 26.5 foot span, the run will be 13 feet 3 inches. Lets say we have a 5 and 12 pitch. "A" being 13.25 feet (run) to get the rise multiply 13.25 X 5 (which is the pitch) this will = 66.25 inches, ("B") this is how tall the ridge board is from the top of the plate minus the thickness of the bird's mouth cut.

To make this easier we will convert everything into inches. "A" 162 inches squared = 26,244 + "B" 66.25 squared = 4,389.0625 . Add these two together = 30,633.062 inches, now get the square root of this and you have 175.02303 inches. Take 3/4 inch from this = 174.27303 inches. This will be the line length of the rafter.

Line length measurement is this, make a mark on the top edge of a blank rafter, measure down the top edge of the blank and make another mark on the top edge of what the rafter length is. You will make the plumb marks or 5 and 12 on these two marks. OK, I'm done now, I have confused you enough. Just one more thing, to get the rafter tail or overhang place the framing square on the bottom plumb mark and square out the amount you want for the tail.
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Old 07-19-2011, 04:51 PM   #5
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Rafter length mistakes. Help


Quote:
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
Put all the info into calc and angle is right but my seat cut is always about a half inch off the wall, ridge is level. So i have no idea why i need to minus a half inch off my rafter lenght to get the seat to be touching the wall, any help?
Yes im a carpenter
Are you deducting half the thickness of the ridge before calculating your ridge height?

If not, depending on the pitch of the roof, it could easily push the rafter away from the wall a half inch.

I always figure simple rafters in this order.

1. Measure full span in inches

2. Deduct ridge thickness

3. Divide by 2 (This is the rafter run)

4. Divide by 12 and multiply by rise (inches per ft. of run) This is the rafter rise.

5. Add the rafter rise to the HAP, or height above plate, at the wall line and this is your ridge height.

6. As mentioned above, run squared + rise squared + rafter length squared.
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Old 07-19-2011, 05:27 PM   #6
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Rafter length mistakes. Help


should be minus 3/4" for the thickness of the ridge if you are using 2x material. is the gap bigger toward the tail of the rafter?

Last edited by mustangmike3789; 07-19-2011 at 05:29 PM.
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Old 07-19-2011, 07:36 PM   #7
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Like I said I measure the span deduct the 1.5 for the ridge divide by 2 and that is my run. This time the ridge height is set because it's a addition It's a 4/12.

It's just odd that it's exactly a half inch off the calc.
Yes I always sting the ridge and jack it level and straight before I start.

It's a real odd one.

Keep the suggestions coming pls.
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Old 07-19-2011, 07:39 PM   #8
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Originally Posted by mustangmike3789 View Post
should be minus 3/4" for the thickness of the ridge if you are using 2x material. is the gap bigger toward the tail of the rafter?
Yes the gap is at the back of the seat.
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Old 07-19-2011, 08:18 PM   #9
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Something doesn't jive. Either the ridge is too close to the wall, by way of leaning, the wall is out of level, causing the ridge to be lower than calculations would lead you to believe, or the rafter length is wrong.

If your rise, run and hypotenuse measurements are good, the site conditions are not.

I suggest plumbing the ridge down to the plate and measure back to check your run.

Check the wall to make sure there is not a sag in it where the post for the ridge sits.

I've had to adjust the ridge more times than I could possibly recall due to the bearing walls sitting higher or lower than the spot where the ridge posts sit.

I also never set my rafters to the ridge. They are always pulled tight to the wall and the rafters set the ridge height.
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Old 07-19-2011, 08:41 PM   #10
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Rafter length mistakes. Help


Maybe because the ridge board doesn't come to a point on top, could that be what is throwing you off?
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Old 07-19-2011, 09:07 PM   #11
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Quote:
Originally Posted by jiju1943 View Post
Maybe because the ridge board doesn't come to a point on top, could that be what is throwing you off?
I guess theoretically it would come to a point if there was no ridge but I know that the deduction of the ridge is for that reason.

I dunno I'm stumped.

Calc this for me. 142 7/16 span 24" to the top of ridge ( it's 2x)
Remember it's a addition so my ridge height is set.
Thank you all so much
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Old 07-19-2011, 09:28 PM   #12
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Quote:
Originally Posted by Selectadon View Post
I guess theoretically it would come to a point if there was no ridge but I know that the deduction of the ridge is for that reason.

I dunno I'm stumped.

Calc this for me. 142 7/16 span 24" to the top of ridge ( it's 2x)
Remember it's a addition so my ridge height is set.
Thank you all so much
74.386, or 74 3/8", at 4.09/12. Now, what do you have for a HAP?

The only other monkey wrench in the works could be an improper HAP.
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Old 07-19-2011, 10:13 PM   #13
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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
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Old 07-19-2011, 10:16 PM   #14
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And if I think about it how does the hap change the length of the rafter?

Thanx again
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Old 07-19-2011, 10:27 PM   #15
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Quote:
Originally Posted by Selectadon View Post
And if I think about it how does the hap change the length of the rafter?

Thanx again
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.

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