I answered this question a couple of years ago on RemodelQA and recently received an email with a few more questions, so I thought I would do a repost:
Q: I want to move my stairs down to the basement. How much room do I need?
There several dimensions of "space" to consider when deciding stair
placement. To understand the space needed you have to think in 3D.
How much space will be determined to an extent by the measurement from
the finished floor of the main floor to the finished basement floor.
However there are a few other considerations.
One "headache" will be enough headroom. The opening to be cut into the
main floor has to be long enough so that the minimum headroom in all
parts of the stairway will not be less than 80" measured vertically from
a sloped line connecting the tread nosing (the rounded front of the
tread) or landing to the lowest part of your ceiling line in the
basement.
Some times you can solve space problems by adding a landing and turning
the stairs in a l-shape or an U shape, sometimes you're just eating up
other space. Another space consideration is the width. Most codes call
for a minimum clear width from finished wall to finished wall of 36". If
they are only service stairs that will be fine. If the basement is
going to be finished or you have to move furniture or appliances up and
down, I would recommend 42" minimum. Remember to add to the clear width
the thickness of wall framing and the finish material of the walls.
Your riser height will determined by the measurement from the finished
floor of the main floor to the finished basement floor. Keep in mind
that most codes determine that maximum riser height should be 73?4
inches. I like a rise of around 7" for a comfortable stairway when
possible.
Tread depth will determine the overall length of the stairs from the
riser of the bottom step to topmost riser. Code says that minimum depth
of tread, where you step, is no less than 10". For service steps that
may work, I try for a depth between 10 1/2" and 11" is more comfortable for many people.
A: A little more
Sorry, this is wordy because of the complex nature of stairs.
To determine the 2 dimensional space required, take the overall
measurement from floor to floor as said above, for a straight run of
stairs. Divide by 7" to get the number of risers you will need. It most
likely won't be even, so adjust the number of risers up or down to get
your number. Say the floor to floor number is 104 1/2". Divided by 7 you
would get approx. 14 15/16 risers. Hard to do so 104 1/2"/ by 15 risers
= approx a rise of 16 15/16". 14 rises would be approx. 7 7/16"
For 14 risers you would need 13 treads at 10" for approx.10'-10" in
length or the 10 1/2" tread preferred for approx. 11' 4 1/2". Now
remember overall stair length won't necessarily be the same length you
have to have for the opening you will cut into the upper floor. That
size may be a little shorter as long as you have the headroom mentioned
above of at least 80"
Clear as mud? There are many factors to be determined to have the vision
to understand how it will all come together before cutting into any
floor joists.
Let me know if I can add anything...or clarify. You may need to have a professional be onsite to give you the best assessment.
Here is the original post from RemodelQA: http://www.remodelqa.com/other-remodels/636/i-want-to-move-my-stairs-down-to-the-basement-how-much-room-do-i-need
