A budding future Santa Rosa, California stair builder ,and maybe a future handrail installer,(a 22 year old framer who needs to build his first set of rough stairs) asked me how to determine the rise and run of a staircase. I wrote him a short explanation that follows.
For both the rise and run of a stair there are a few things to take into account before actual mathematical calculations take place.
Rise – What is the distance from finish floor to finish floor? To determine this one needs to know the exact dimension of the finish flooring at the bottom of the stair and at the top (these are often different thicknesses). Once a physical and accurate measurement is made from the rough floor to the rough floor taking into account possible discrepancies in how level the floor is at the stair area, then with the finish flooring thickness in mind, one can determine the finish to finish floor height. Standard average rise is around 7 1/2″ so if you take the 7.5 and divide it into that floor to floor measurement you will get a number that will represent the amount of individual risers that the stair has. Once the number of risers are determined it is simply a matter of dividing that number of risers into the finish to finish dimension.
Example -rough floor to floor rise = 108.00″
lower floor finish flooring thickness = 3/4″subtract .75″
upper floor finish flooring thickness = 1/2 add .50″
net finish to finish total rise = = 107.75″
divide finish total rise by average rise of 7.5 = 14.3666 risers (you would have 14 risers each a tad over 7 1/2″)
so then we divide total finish rise by 14 and we get 7.696″ which would be a riser height of 7 11/16″+ (new California 2010 codes set the maximum riser height at 7 3/4″)
Run – The first consideration is the actual length of space that the stair can occupy. If there is a door way or walk way around the first riser then this is a restriction that needs to be factored in.
. The average stair tread,considering walking comfort, is around 10 1/2″ plus or minus. Since your total number of risers in the example above is 14, that would leave us with 13 walking treads.
13 treads multiplied by 10.5″ = 136.5″ for overall stair length from face of top finish rise to face of the bottom finish riser.. If you have unlimited space figure that you are in the clear to build your stair. If there is a space restriction, remember that at the top of your stair, your top finish riser (be it a hardwood covering or a carpet), will encroach into your space by it’s thickness from your header and at the bottom of your stair, you need to add for the stair nosing if there is to be one (typically 1 1/4″). If there is a door or through way at the bottom of your stair, don’t forget to allow for any casing or finish trim that might be needed. If there are space restrictions you can reduce the tread dimension but pay attention to local codes (new California 2010 codes set the minimum tread depth to be 10″). For laying out a shop built housed and wedged staircase these rules apply but it is actually easier to figure being that there is no flooring material (added dimension) to be attached later for the stair to become a finished product unless there is to be a carpet runner.