What are the standard dimensions? - Answers

For the most part a finished width of 36" is the minimum. If not cramped for room, 42" is nice. Length on the other hand has many variables to consider. The degree (angle) of which your stair descends is one. The size of your floor system (joist, ceiling rock and sub flooring) is another. Also, 80" most likely is the minimum headroom you'll need to have from the edge of the nosing of the treads to the bottom of the floor at it's tightest spot. The degree is determined by the rise and tread sizes used. The steeper the stair, the shorter the stairwell can be. Of course there are codes limiting the rise and tread sizes. Stair stringers cut with 7" risers and 11" treads with 2x12 joists will need a stairwell size of 147 1/4". With 7 3/4" riser and 10" tread and 2x12 joists the well would have to be 121". That's more than 2' 2" difference. And switching to 14" I joists you would lose almost 3" of headroom and so the stairwell would have to be enlarged to 151 5/8" or 124 5/8" respectively. All the answers are in a book entitled "Stairwell Lengths For All Situations".