Expert: Bruce E. Johnson - 11/13/2010

Question
QUESTION: Hola,  I am writing from Costa Rica. I have just added a Yoga floor and need some stairs. The rise is 285 cm (112") so 28.5 cm gives a nice 7 1/4" rise.  The space is 144cm wide (41") and I have 244 cm (8')of run.  You told Jim: "your steps will be twice as far apart so you will be using 6.5 as your divider for the treads"  You lost me there. But I understand the rest.  If the widest treads are 26 cm (10") deep, how much run will I need?

ANSWER: Hi Elena, this type of stairs with the numbers you give me doesn't work out. Even with a 7 1/2" riser height you will need 14 treads which computes to 140" of run.  To fit this stair into the space you have available you would need treads of 6 7/8".  Please write again regarding this situation.. Sincerely bruce e johnson

---------- FOLLOW-UP ----------

QUESTION: I made a mistake with the metric conversion, an 18 cm is closer to the standard rise. OK, change the rise to 106", with an 8' run; with the alternate tread type stairs, what size treads, and rise? Thanks,  Elena

Answer
Hi again Elena, I'm sorry, I misread your original email. An alternating stair would work in your case however this type of stair is not normally recommended for residential use.  By splitting the stair case in half you can cut down on the size of  the treads needed by making each alternating tread narrower than the previous one. Check out the following article:

http://www.treehugger.com/files/2008/03/alternating-tread-stairs.php

Basically the alternating 'wider' step is two steps higher than  same size  step directly below it allowing the person to bypass the narrower step as they alternate their way up.  In your case the tread on the right of the first step is 7 1/4" off the floor and 10" deep whereas the other half of the tread is only 6 7/8" deep.  This pattern would alternate so that in reality you are building a staircase with 6 7/8" treads to accommodate your run but by alternating the tread widths it allows your foot to bypass the narrower side.

This type of step takes some getting used to and is not generally allowed in residential settings but is allowed in industrial settings where the personnel are trained in the use of this type of stair. I have attached a simple drawing to illustrate. I hope this information helps.  Sincerely bruce e johnson