rafter length

[ken ray]

 Although I appreciate the response, the book on framing I have and the calc. Ican't use. I'm also using this item called easy rafters but I'm not sure I'm reading it right. Could someone figure this and see if they get what I get. 12 in 12 pitch wqith a total span of 16' with a run of 8' I guess this is how you describe it.

[n74tg]

So what exactly are you looking for; the length of the rafter, the location of the birdsmouth cut, what?

Simple geometry says that for your 12:12 pitch with an 8 foot height and a 16 foot total span the rafter will be 8 feet times the square root of 2 (1.414) = 11.31 feet.  To that you add how much eave overhang you want (I would suggest 16" minimum); so overall rafter length is now 12.64 feet (12' 7-5/8").  Now are you using a ridge board made of 2x stock, then subtract 3/4" (half the thickness of the ridge) if you want.

To calculate the rafter length another way use the old a squared plus b squared = c squared formula; where a and b are 8' each, so c becomes the square root of 64 + 64 = 128, which is 11.31 feet. 

Let us know what you decide to do.

[MountainDon]

One of the handiest and completely free rafter calculators I've seen is...

http://www.blocklayer.com/Roof/GableEng.aspx

I used it for our gazebo and our cabin. Measured twice, drew the cut lines, checked the measurements again, then cut one, used it for a pattern, cut the second. Left the tails long to trim off later. Test fitted them to the ridge board that was temporarily mounted/supported. Everything was fine.

[John_C]

If you don't trust the calculations and can't visualize it lay two sheets of plywood on the ground and draw it full size.  You will be able to draw the ridge board and wall thickness as well and get the true length and an accurate modeling of the bird mouth cut. Leave the tails long and give them a haircut later.

8' x 8' makes lofting it quite eay.

[Don_P]

Assuming it has a 2x ridgeboard, both calculators and a longhand check came up with 11'2-23/32" from plumb to plumb.

Here's one way longhand;
16 foot span = 192"
Subtract the thickness of the ridge 192"-1.5"=190.5
Divide that in half for the rafter horizontal run length, 190.5/2=95.25" (3/4", or half the thickness of the ridge, under 8')

On a 12/12 the rise and run are the same so for A squared + B squared= C squared we can sustitute
(95.25x95.25)+(95.25x95.25)+ C squared
(9072.56+9072.56)=18145.125
The square root of 18145.125=134.70"... 11'2-11/16"

or
Run/Sin 45= Hypoteneuse
95.25/.7071= 134.70"

That is the plumb to plumb length without overhang yet. Remember on a 12/12 for every inch of overhang out, it drops down 1" think about windows and screen doors.

[ken ray]

thanks to all for replies.

[Jens]

Where they enough to figure it out?  Would a diagram be helpful?  Just kinda sounds like you might be getting confused (which is totally nothing to be embarrassed about with rafters).

[rwanders]

If you are enjoying rafter calculations, you will LOVE laying out staircases!

[Don_P]

They're more of the same thing 

I was thinking the same thing as Jens and took the camera today. Don't let that preclude a diagram, my bulb lights to different approaches at different times.

this is how I use the measurement from above to layout a rafter. First I make a plumb mark at the top edge of a piece of crowned rafter stock.


then I hook the tape on the long point of that


Pull the tape down along that edge to the length and make a mark


Make another plumb mark across the board. From the bottom edge of the rafter (towards us in the picture) I've mad a 3-1/2" wide seatcut square to the plumb line. When installed the seatcut will sit atop the top plate.


Rather than overcut the birdsmouth its easy to finish it with a handsaw or even a sharp chisel.

[Jens]

if using a chisel, knock the piece out with a hammer, swing up.  Swinging down, to where it is not cut all the way, will make tear out.  Now the stupid triangle isn't in the way of the chisel.  Handsaw is better, IMO, or jigsaw too.

[Redoverfarm]

To keep things accurate pu a set of stair guages or buttons to mount on your square.  This will insure that there is no deviation in the placement of the square.  Of course this does not compensate for mismeasurements

http://www.swansontoolco.com/sg0020.cfm