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07-18-2011, 10:38 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

07-19-2011, 09:47 AM   #2
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Wow 53 views and no replies???????

07-19-2011, 05:51 PM   #5
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Quote:
 Originally Posted by Selectadon 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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 07-19-2011, 06:27 PM #6 NACE Coating Inspector     Join Date: Apr 2011 Location: Florida Posts: 524 Rewards Points: 500 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 06:29 PM.
 07-19-2011, 08:36 PM #7 Newbie   Join Date: Jul 2011 Posts: 12 Rewards Points: 10 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.
07-19-2011, 08:39 PM   #8
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Quote:
 Originally Posted by mustangmike3789 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.

 07-19-2011, 09:18 PM #9 KemoSabe     Join Date: Mar 2009 Posts: 644 Rewards Points: 500 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. __________________ It's not that it took him an hour to make 100 bucks, it's that it didn't take you 10 hours to save 100 bucks.
 The Following User Says Thank You to loneframer For This Useful Post: BigJim (07-19-2011)
 07-19-2011, 09:41 PM #10 Ole Wood Worker     Join Date: Sep 2008 Location: Chattanooga, Tennessee Posts: 10,257 Rewards Points: 118 Blog Entries: 1 Maybe because the ridge board doesn't come to a point on top, could that be what is throwing you off? __________________ New members: Please consider adding your location to your profile. He is no fool who gives what he cannot keep, to gain what he cannot lose. http://www.woodworkingtalk.com/forum.php Jim
07-19-2011, 10:07 PM   #11
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Quote:
 Originally Posted by jiju1943 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

07-19-2011, 10:28 PM   #12
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Quote:
 Originally Posted by Selectadon 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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 07-19-2011, 11:13 PM #13 Newbie   Join Date: Jul 2011 Posts: 12 Rewards Points: 10 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
 07-19-2011, 11:16 PM #14 Newbie   Join Date: Jul 2011 Posts: 12 Rewards Points: 10 And if I think about it how does the hap change the length of the rafter? Thanx again
07-19-2011, 11:27 PM   #15
KemoSabe

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Quote:
 Originally Posted by Selectadon 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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