# Thread: 3 edged (of a octogon shape) diagonal stringer help

1. Junior Member
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May 2011
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Boston, MA area
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## 3 edged (of a octogon shape) diagonal stringer help

I have built the desk with steps shaped with 3 edges of an octagon. Using standard trig calculations I am able to figure out the stringers, however in real life nothing is perfect nor is the angle of the diagonal stringers 22.5 degrees exactly....

So the real question is how do I figure out how to measure, mark and cut the diagonal stringers using the real world measurements????

2. ## Re: 3 edged (of a octogon shape) diagonal stringer help

Are these steps headed away from each other or towards each other?

3. ## Re: 3 edged (of a octogon shape) diagonal stringer help

Cut the straight stringers for the three sides, then start at the first set and let the step run long past where the diagonal stringer will be. You will be able to measure directly from the steps themselves to locate the cut lines for the stringer. Duplicate this method for the remaining diagonals

4. ## Re: 3 edged (of a octogon shape) diagonal stringer help

exactly as spruce mentions, any time i get into a deck with such a detail i break every thing down into simple "steps" pardon the pun
by doing so it makes things easier to complete and you can see things more clearly as you get further into it.

do the standard stringer first for each section, from there run a straight edge through and draw layout lines, this will create a template for which you can take measurements from to create the diagonal stringer. just note because its on an angle, the tread can become wider depending on how wide the stair itself will be which means going from 2x10 to a 2x12 to maintain correct "effective depth" which means the amount of material left on the stringer stock after the notch has been made.

other wise the other option is to switch to building small platforms to build the stair

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## Re: 3 edged (of a octogon shape) diagonal stringer help

Hi,
I thought about the best way to figure this, and direct measurement works, but I wanted to figure out what number to use for the "run"of the octagon stringer, and happened to find this handy triangle calculator: http://ostermiller.org/calc/triangle.html
I put in a value of 10 for one side (common run), 90* for the first adjacent angle, and 22.5 for the other adjacent angle, and the calculator spat out the rest. The 22.5* run ends up being 11". Now, your run may be something other than 10. But this calculator gives you the "plan view" (from directly above, IOW) of how your octagon stringer module (one step's worth) would be. The value would be along either side closest to a common stringer, so you still have to extrapolate from there, but it's a start. The rise is always constant on these. You would set your framing square for a 11/8 run/rise instead of the common 10/8 (given that you were using an 8" riser, YMMV)
Casey

6. Junior Member
Join Date
May 2011
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Boston, MA area
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## Re: 3 edged (of a octogon shape) diagonal stringer help

Thanks guys...

I had the standard stringers done and yes I have calculated the run for the diagonals, but they were difficult to figure since the top and bottom (1st and last) run lengths where confusing to determine...

So I finally determined the final rises location, since it had to meet the bottom rises from the other steps. That helped to determine the total run, which them tells me the run of each step on the diagonal stringers. Know I can ignore the calculated hypothetical sizes if it was truly plumb and square. Which is was not, they were 22.6 and 22.4 degrees (it was a little off on each side because of the initial post installations.

jkirk, you are right, I might had to go to bigger lumber depending on the cut. I hope to measure them out this weekend and see how it goes..

Thanks everyone.