Expert: Daniel Humphrey - 9/13/2010

Question
i read your previous post on alternating stairs and tried to figure out a formula based on your answer, though was unsuccessful. i have not build my tiny house yet, so cannot measure but am trying to figure out if alternating stairs would fit. it will be 12x16x10h with a loft beginning at either 7.6 or 8 ft H, and 6fx12. how do i figure out the size of the stairs and the space it would need?

Answer
Dear Beth,

Yes your house is tiny! I have made stairwells that size.

By alternating stairs, I presume you mean the ship's stair design that has a separate rise and tread for each side.

Such a stairway for your application would measure about 30 inches wide and about 6 feet long.


One design objective, using standard lumber, is to have each tread about 11 inches and each alternating rise about 7-3/4 inches. Since the slope of the stairway is for two risers at a time, the optimum slope would be about 11/15.5". This means that for a 93" loft height, you need six treads to total 66 inches (66/93 = 11/15.5). If you have a different height and different length in mind, adjust the ratio.

The left-hand stair would have six treads, but the right-hand stair would have only five.

I have found the only way to get my head around this is to draw each side out separately on a grid paper.

Good luck,

Daniel