How to cut stair stringers when the stairs flair out?

[Dumb_Polack]

Hi,

I'm helping someone out on their deck and they want the stairs (red lines)  to "flair" out towards the bottom.  I realize I need to maintain 16"OC @ the bottom of the stairs, so towards the deck (blue box) the stringers will sit closer than 16"OC...no big deal.

The rise will be consistent for the stairs and all the runs will be the same width (they like wide stairs so I'm thinking I'd do 12" treads).  My question is this: Since the stringers (black) will be set on an angle, the runs will get longer as you head down the stairs, but since they'll be angled out, they'll still be 12" front to back.  What's the trick for cutting the risers?

I realize that at the deck they'll be angled too, but I'm having a tough time figuring out the runs for each step.  (I realize the center one will be cut with a 12" run since it is perpendicular to the deck, but as you walk down the stairs, the runs will be progressively longer because of the flair, (see my 2nd drawing)

What's the math to do this (funny, but I did something similar on our pool deck when I built that 15 yrs ago, but I forgot how I did it!!)


(Ok, I saw this video https://www.youtube.com/watch?v=NyZrCwUFDY0  where he talks about a 5 degree flair (mine will flair more than that), but I suppose I could do his trick, but do you have a different way to do it?)

[Dumb_Polack]

...or...should I cut most of the stringers @ 90 degrees and only flair the outside 1 (or 2) while maintaining 16" OC at the bottom??

[Snipe Hunter]

Make your life easy, don't flare interior stringers. You won't see them anyway because the risers have to be filled in with a riser.

I've only done this once and it was easier than I would expect. All my interior stringers were 90 degrees to the house, 16" oc. No different than any other staircase. I installed those before cutting the two end stringers. Then I cut the two outside stringers without notching the treads and risers and tacked them into place. I used a straight edge across the interior risers and marked the cuts on the end stringers, including the angle of the cut for the risers. I just set my circular saw at the angle of the pencil line for the end stringer risers.

If you want a wider flare, just add an extra stringer (at 1/2 the angle) between the last 90 degree stringer and the flared stringer at each end. That way you can maintain the 16" oc.

[MstrCarpenter]

August 31, 2020      

Snipe HunterMake your life easy, don't flare interior stringers. You won't see them anyway because the risers have to be filled in with a riser.

I've only done this once and it was easier than I would expect.  All my interior stringers were 90 degrees to the house, 16" oc. No different than any other staircase. I installed those before cutting the two end stringers. Then I cut the two outside stringers without notching the treads and risers and tacked them into place. I used a straight edge across the interior risers and marked the cuts on the end stringers, including the angle of the cut for the risers. I just set my circular saw at the angle of the pencil line for the end stringer risers.

If you want a wider flare, just add an extra stringer (at 1/2 the angle) between the last 90 degree stringer and the flared stringer at each end. That way you can maintain the 16" oc.

I watched the video, and I agree with Snipe Hunter too except I think I would keep the extra stringer the same as the center ones only shorter as necessary. Either put blocking at the appropriate height to hang the second to last stringer or cut it to fit against the inside of the last stringer. Would that be called a "jack stringer"?