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I have a interior landing (a room entrance) sitting at 12 inches from the floor. I'm having trouble running the numbers on the stringer calculation since dividing the total rise by 7 is 1. That means 1 step (tread).

Technically I need 2 steps (treads) including the tread which is at the TOP of the stairs which should sit flush with the landing.

How can this calculation account for this top step? Also, for this short height, I'm assuming I need wider treads -- any tips to calculate this would be excellent.

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    mycarpentry.com this guy has a excellent site. And a whole section on stairs and stair building and layout.
    – Paul Logan
    Commented Nov 20, 2017 at 6:54

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Yes, there is a ratio of height of riser to width of tread. Exterior stairs are not generally as steep as interior stairs. The ICC Code requires a minimum of 4” high riser and allows residential stairs to have a maximum of 8” high risers. All risers in a run must be within 3/8” difference of largest to smallest. (See R311.7.5.1.)

A commonly used formula for calculating the depth of tread to height of riser is established in “Graphic Standards”, commonly used by architects and engineers. On page 7 they give this formula for interior stairs:

Riser plus tread = 17.5”.

Thus 7 1/2” riser + 10” tread = 17.5”

Or

Riser times tread = 75

Thus 7.5” riser x 10” tread = 75

You have two steps 6” high...so I’d use 12 1/2” tread.

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