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StepCalc

Floor opening and headroom

How long the hole in the floor has to be — the calculation that decides whether a stair fits the house at all.

Units

Why this is the calculation that bites

Headroom is measured vertically from the pitch line. The opening has to be long enough for the stair to drop clear of it.upper floorpitch lineheadroom80″ / 2000mmfloor opening length
Headroom is measured vertically from the pitch line. The opening has to be long enough for the stair to drop clear of it.

Headroom is measured vertically from the pitch line — the line joining the tread nosings — to whatever is above. The opening has to be long enough that by the time a person is under the upper floor, the stair has dropped them far enough to clear it. 

drop needed   = headroom + floor assembly thickness
opening length = drop needed / tan(pitch)

The reason it bites is sequencing. The opening gets framed early, often before anyone has settled the stair. Discovering afterwards that the stair needs another eighteen inches of hole means cutting a joist that is already carrying load — which is a different and much more expensive job than framing it right the first time.