Stair Well Opening Calculator

Spike's Calculators

This calculator determines the clear opening length needed in a floor for a straight-run staircase and identifies which tread first satisfies the required headroom

Note: Fractions are entered as separate numerator and denominator for quick entry of values such as 7 ½" or 1 ¼".

Stair Well Opening

Rise in n
d
Run in n
d
Floor Thickness in n
d
Required Headroom ft in
Dimension of Nosing in n
d

Results:

Opening Size ft in
Tread # at head-room

Calculator

  1. enter the riser height (unit rise) in inches
  2. run depth (unit run) in inches
  3. thickness of floor structure (floor finish material plus joist plus plywood plus ceiling finish)
  4. required headroom
    Note: Keep in mind that the required headroom must allow for finished surfaces.
  5. nosing, the amount the stair tread projects past the vertical riser

Results

  1. Opening Size - the length of the rectangular hole to cut in the floor, shown in feet and inches.
  2. Tread # at head-room - The first tread whose nosing sits directly under the required head-room line
"Tread # at head-room" equals the number of steps down from the landing (the floor itself is not counted) to the first tread whose nosing falls wholly below the required head-room clearance line.
Note - Why the total floor-to-floor rise is irrelevant to the opening length?
The opening only has to clear the portion of the stair that passes under the floor sheathing.
That clearance is reached as soon as the vertical distance beneath the floor equals the required headroom + floor thickness.
Because this point is always well above the bottom of the staircase, the unit rise (R), unit run (T), head-room (H), floor thickness (F), and nosing (N) fully determine the opening; the staircase can continue any additional distance down without affecting the size of the hole.

Stair Framing

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