Knee and Toe Clearance (2010 ADA Standards 306)
What makes a lavatory or work surface usable, and what gets built wrong because an open space looks clear. Two stacked zones: TOE from the floor to 9 in, KNEE from 9 to 27 in, both 30 in wide and neither more than 25 in deep. Knee depth is not one number -- 11 in at 9 in above the floor and 8 in at 27 in, tapering no faster than 1 in of depth per 6 in of height, which is exactly what a slanted apron or a hanging bowl violates. And the 27 in figure decides the job: an apron at 26 in has NO knee clearance at all, not shallow clearance, and no depth below can rescue it.
Formula and source
toe clearance = the space from the floor to 9 in, 17 in deep minimum where required and 25 in maximum, 30 in wide minimum. Knee clearance = the space from 9 in to 27 in, 11 in deep minimum at 9 in and 8 in deep minimum at 27 in, permitted to reduce between them at 1 in of depth per 6 in of height, 25 in maximum and 30 in wide minimum. An underside below 27 in provides no knee clearance.
2010 ADA Standards for Accessible Design, 306.2.1 through 306.2.5 and 306.3.1 through 306.3.5. A US federal standard in the public domain, quoted directly.
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