For the life of me I cannot duplicate this angle. Compound angle, at facia corner. I have the two dry rotted pieces to go off of, but I just can't get it right. 

Can someone please teach me how to arrive at the proper compound angle for this? Thanks

 

John, if you have a speed square, you should be able to get your angles off the old pieces.  You need to know your roof pitch to start with if the speed square doesn't work. I'm sure a framing square would work too if you were good with one.  I used to "stick" a lot of roofs but it's just been too long for me to recall off hand.

Do you have one of these

The you just replicate the angle on the old board to the miter saw.

 

This is a compound cut.  Take the old board lay it on the fence along it longest edge.

Mark the angle either by alligning the saw to the cut on the board in the verticle plane or use the angle finder to find the angle.

Then Tilt the saw blade to mimic the rake of the cut along the short edge. 

Always make sure the blade us cutting a peice of wood 90 degrees or greater you do not want to trap a small peice of wood between what you are cutting and the blade.

 

Thanks a heap guys! I already got it last week. It's all buttoned up. I used the calculator I linked above, after I measured the rise:run on my roof from the inside of the home. It worked really well. 

My first go-to, was to use the original pieces to set my miter saw too, but they were too far gone for any accuracy. I tried, but it didn't work.

Thanks again guys, I'll use your suggestions in the future though.

Miter angle tables are handy for this stuff...but if anybody wants the math behind it see here... 

 

http://www.woodcentral.com/bparticles/miter_formula.shtml

Nick Engler (formerly of Shopsmith, and an independent author) did an Excel spreadsheet for this, too.  Generally known as the "hopper problem." https://22293-presscdn-pagely.netdna-ssl.com/wp-content/uploads/calc.xls

 

But, that said, when I'm doing a number of these, I find it easier and less error prone to build a little jig to hold the piece at the splay angle and just adjust the miter saw left and right at the right cut angle (e.g., 45).    It's just too error-prone to set one at 34.5 degrees and another at 30.6 degrees, then get the board on the correct side of the saw.   Besides having to make too many changes.

In my "Book of Things I'd Rather Not" these are up there with jack and hip rafters...  Not the only things in the Book, but they're in there...

3 hours ago, Nickp said:

In my "Book of Things I'd Rather Not" these are up there with jack and hip rafters...  Not the only things in the Book, but they're in there...

Can I check it out to read?

Chapter 1 in my book, "Plumbing."

 

I always say if I have a board that's too thick or too thin, too  wide or too narrow, or too long or too short, I can generally either fix it, or find another in the pile.   If you need a copper 3/4" to 1/2"  90 degree elbow and don't have one, you're headed to the hardware store.

 

Three rules of plumbing that I know

1. Hot on the left; cold on the right
2. Stink goes up, poop goes down
3. Don't chew your fingernails on the job

21 minutes ago, kmealy said:

If you need a copper 3/4" to 1/2"  90 degree elbow and don't have one, you're headed to the hardware store.

 

pair of 45's...

T capped on one leg...