Need help cutting jack rafter - Anglely/mentally challenged!

Solar Burrito · July 15, 2010, 12:13:36 PM

Please help!

I need to cut a jack rafter and am having problems because the bevel settings on my circular saw doesn't go past 45 degrees. I need 53 degrees.

The first cut is 45 degrees on both ends that's no problem. then I have to bevel the saw plate thingy and cut it 53 degrees on one end so it can attach to the valley rafter and connect to the main ridge beam.

What's the easiest way to do this for a Newb with limited tools and brain cells?

Isn't there a way to subtract the angle you want to cut from 90 then take the remainder and cut the board flipped over? Say 90-53=37 degrees cut upside down?

I have a 10" Dewalt chop saw but it doesn't go to 53 degrees either. Below are the plans to show you what I'm working with.

Any help would be appreciated!

ScottA · July 15, 2010, 02:53:12 PM

OK it's not that complicated. Your saw has a range of 90 degrees to 45 degrees. Think about it. So 53 degrees is 8 degrees back from the full 45 degrees. Depending on which direction you make the cut you'll either get 53 degrees or 37 degrees. Savy?

July 15, 2010, 03:28:29 PM

Thanks Scott, this really helps me but can you confirm my ramblings below....?

Oh so I guess I was on the right track but instead of cutting on a different side of the board I should cut on the same side but in the opposite direction with my bevel set at 37 degrees to get a cut at 53 degrees??

Is that correct?

ScottA · July 15, 2010, 03:38:59 PM

The short side of the cut will be on top of the board as you make the cut.

Edit: Sorry I got it backwards. Fixed.

Don_P · July 15, 2010, 04:43:45 PM

Nope, that works if you cut along the end grain of the mitered stick, not the other face or the other direction. You can also rip a shim and fasten it to the shoe of the saw that lays it over the appropriate amount.

July 15, 2010, 05:46:33 PM

I think I got brrain damage from the heat today. Ignore what I said, Don's right. Make a shim to tilt your saw. I know I did this before but I can't remember how atm.

Don_P · July 15, 2010, 09:31:35 PM

LOL, I hear that. After wheeling concrete today

Got home fried, returned a call and left a nearly incoherent message on a prospective client's answering machine. If I didn't blow it, they are people I want to work for.

JRR · July 17, 2010, 05:16:03 AM

A lot of wasted wood had taught me the following trick when making complicated angular cuts on costly sticks of wood: Make a "dummy stick" of wood for the cutting/fitting.

Using wood screws, assemble a "stick" of wood with scrap wood attached to the ends of a 1x or a 2x ... long enough to match the span. Make trial cuts on the scrap wood ends. Taking advantage of the screws, adjust the angle of the cuts and the length of the assembly ... or the wood cuts ... 'til the dummy-piece is "right". Then transfer the cuts and angles, and the length, to new wood.

Solar Burrito · July 19, 2010, 11:28:38 PM

Any good illustrations on how to make and position a shim on my miter saw?

I tried to practice cutting this rafter today in my driveway and found it hard to figure out which side of the board was cut at 37 and which was at 53. Since it was a compound cut with 45 degrees I couldn't tell how to measure it with my framing square...

I'm sure this is carpentry 101 still greek to me. Any one know of a good video about this stuff?

July 20, 2010, 06:49:07 PM

This was quick and dirty down in the shop...
The shim doesn't need to be anything fancy, you still have the bevel setting ability in the shoe so all it has to do is get you in the ballpark and then you can adjust the bevel like normal to fine tune it. 2 small holes in the shoe, use small screws to attach the uniform thickness shim in a straight line along the shoe.

Waalaa 53 at a 45

I'll call down to the bench "I need a 53 bevel at a 45 angle" and then give length measured from the long point to a reference point on the other end of the stick, often the notch of the birdsmouth... the outside corner of the top plate, if we are measuring in air.

It is hard to visualize at first, whack up some scrap and play with it a bit and it should start to make sense. JRR's advice is spot on, mock up the ends and trial fit with them screwed to a 2x4. Cut your long jacks first, if you mess up they become the next length down. This is geometry and trig put to real use. Homeschoolers, here is a real world use for this stuff, build a cross gable doghouse

edit:
Rise/run ratios can be converted to angles by dividing the rise by the run and taking the arctangent of the result
12/12=1, atan of 1=45 degrees
10/12=.833...atan=39.8 degrees (we call it 40*)

The other way around. I have an angle and only a framing square to lay it out
40* is asked for, tan of 40 is .839, I want to put one leg of the square on 12", .839x12=10.06... put the other leg of the square on 10-1/16" for a true 40*.

Solar Burrito · July 20, 2010, 10:29:29 PM

Thanks for the pics Don, I finally got it tonight. It took buying this little protractor thing at Woodcraft store so I could verify what angle I was cutting.

This thing: http://www.woodcraft.com/Catalog/ProductPage.aspx?prodid=8396&ss=fa2dbb0c-f2f7-4af5-be82-0d1b81f1f2af

Don_P · July 21, 2010, 11:42:51 AM

That will work I'm sure. If you look up at my pics you'll see an aluminum triangle, a speed square. It measures angles in degrees and in x/12 pitches. It also has a scale and slots for scribing. They are available at any building supply in the 6" version, that one is a 12", and are pretty tough. That one is a retiree after breaking the pivot corner dropping it on a slab. The 6" speedy will fit in your bags and is handy for running the saw along for perfect 90 degree cuts as well. It also comes with a good roof framing book.

I oughta get a kickback for that ad