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Discussion Starter · #1 ·
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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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.
Mike Hawkins:)
 

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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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KemoSabe
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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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Discussion Starter · #7 ·
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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KemoSabe
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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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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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Discussion Starter · #11 ·
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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KemoSabe
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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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Discussion Starter · #13 ·
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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KemoSabe
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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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KemoSabe
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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
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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Discussion Starter · #17 ·
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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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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Discussion Starter · #19 ·
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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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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