Thread: Need help calculating proper miter angle

time to break out the maths

What are the dimensions (roughly, or exactly if you like)? Basically the key will be the "aspect ratio", the height of the peak from the bottom edge, so that you can start to work out triangles and from that angles. Oh, and we work in mm here, none of that imperial guff... even though my name is one of those

There is an easy way depending what type of saw you have: table saw, mitre (chop)saw or SCM (sliding compound mitre) saw

The attached spreadsheet should sort you out. Sorry it's in metric rather than feet & inches, but if you use inches & decimals you'll be OK.Compound Mitre Angles (version 1).xls

If it’s a 4 sided figure, the angle of the mitre on each side is always 45degrees.

Originally Posted by riverbuilder

If it’s a 4 sided figure, the angle of the mitre on each side is always 45degrees.

Only in plan view, if it was an actual rafter (creeper aka jack) you would mark the plumb cut, set portable circular saw to 45 and cut on the plumb line = perfect fit

Originally Posted by AlexS

The attached spreadsheet should sort you out. Sorry it's in metric rather than feet & inches, but if you use inches & decimals you'll be OK.Compound Mitre Angles (version 1).xls

Won't open for some reason, can you post a PDF?

Originally Posted by rwbuild

Only in plan view, if it was an actual rafter (creeper aka jack) you would mark the plumb cut, set portable circular saw to 45 and cut on the plumb line = perfect fit

So, it’s 45degress, like I said initially. But then those 45 degree cuts are at an equal angle to the base, whatever angle that may be to get the desired pitch of roof, meeting at a point at the apex, forming a pyramid. Did you even look at the little roof over the statues in the photos?

Originally Posted by rwbuild

Won't open for some reason, can you post a PDF?

OK, I'll try & post a version with the formulas showing, so you can set up your own spreadsheet.

Number of sides	4	(cell b1 - data entry)
Angular bend (degrees)	90	(cell c1 - calculation). Formula: =360/b1
Tilt angle (degrees)	75	(cell b4 - data entry)
Cross cut angle (degrees)	14.51 (cell b9 - calculation)	Formula = DEGREES (ATAN(COS(RADIANS(B4)) * TAN(RADIANS(B2/2))))
Blade tilt angle (degrees)	43.08 (cell b10 - calculation)	Formula=DEGREES(ASIN(SIN(RADIANS(B4)) * SIN(RADIANS(B2/2))))

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that's from Alex's spreadsheet (which opened fine for me)

Unless you are cutting by hand you don’t need to know the compound angle.
As Riverbuild and RWbuild have said. Set your saw of whatever variety to 45 degrees and then cut to your pitch angle and you have created your compound angle

Originally Posted by poundy

time to break out the maths

Oh, and we work in mm here, none of that imperial guff... even though my name is one of those

I think you lot scared him off, talking metric. I find it funny how most, not all, Americans hate metric, but they happily use their metric money system.
Rgds,
Crocy.

Originally Posted by poundy

Number of sides	4	(cell b1 - data entry)
Angular bend (degrees)	90	(cell c1 - calculation). Formula: =360/b1
Tilt angle (degrees)	75	(cell b4 - data entry)
Cross cut angle (degrees)	14.51 (cell b9 - calculation)	Formula = DEGREES (ATAN(COS(RADIANS(B4)) * TAN(RADIANS(B2/2))))
Blade tilt angle (degrees)	43.08 (cell b10 - calculation)	Formula=DEGREES(ASIN(SIN(RADIANS(B4)) * SIN(RADIANS(B2/2))))

- - - Updated - - -

that's from Alex's spreadsheet (which opened fine for me)

Thanks Brett. Here's a screenshot. If you type the things in cells F2, F9 &F10 into cells B2, B9 & B10 of your spreadsheet, without the inverted commas, it should work. The things in blue are the numbers you enter, the things in yellow are the numbers you need.

Screen Shot 2020-04-18 at 12.42.26 pm.jpg

Originally Posted by poundy

Number of sides	4	(cell b1 - data entry)
Angular bend (degrees)	90	(cell c1 - calculation). Formula: =360/b1
Tilt angle (degrees)	75	(cell b4 - data entry)
Cross cut angle (degrees)	14.51 (cell b9 - calculation)	Formula = DEGREES (ATAN(COS(RADIANS(B4)) * TAN(RADIANS(B2/2))))
Blade tilt angle (degrees)	43.08 (cell b10 - calculation)	Formula=DEGREES(ASIN(SIN(RADIANS(B4)) * SIN(RADIANS(B2/2))))

- - - Updated - - -

that's from Alex's spreadsheet (which opened fine for me)

so are we pitching a bloody roof or sending it to the moon and back?

neither? Building a birdhouse basically Or sending it TO the moon. I'm sure you need at least two other formulae to get it back as well