Barrier-free thresholds: the flush detail

What flush actually changes

A standard slider carries a raised threshold: the track sits on top of the floor, and the upstand is what keeps the driven rain out. Flushing the track into the floor removes the upstand — and, if it is done carelessly, the water resistance with it. The detail that works does three things: it recesses the track into the structural build-up, it keeps a concealed drainage path under the sill, and it holds a seal line between the door and the exterior plane. That is why the flush threshold is a drawn detail with a track depth and a seal, not just a lower floor.

The standards it answers

• DIN 18040 (Germany) for step-free access to living space.
• The ADA line in the US for the accessible route, where the project is public or the client wants it.
• The local equivalent where the market has its own access code — the detail is drawn to the level the reviewer checks.
• The leaf height and the track depth come from the chosen level, so the standard is named on the schedule, not assumed.

The weathertight question

The concern about flush thresholds is water, and the answer is the drainage, not the upstand. A drained and ventilated sill carries the water that reaches the track down a concealed path to the outside face, so the opening stays barrier-free and the room stays dry. On the exposed elevations — the coastal and driving-rain cases — the detail pairs the flush track with the same gasket and drainage logic the raised sill uses, which is why the coastal schedule specifies the drained sill rather than a simple level floor.

What to put on the schedule

• The access standard and the level it requires, per opening.
• The track depth and the recess the builder has to form, so the frame and the floor agree.
• The drainage and seal detail, especially on the exposed elevations.
• The leaf height change, because the flush track takes height the standard sill did not.