Threshold (doorway)

A threshold in residential construction is the horizontal element at the base of a doorway, where the door closes and where the floor or paving on one side meets the floor or paving on the other. The height of the threshold is the rise (in mm) the occupant must step over to pass through the doorway. NCC 2022 Volume Two Housing Provisions Part H8 (Livable Housing) regulates threshold heights to prevent trip hazards and to ensure mobility access (verified 2026-05-16).

The three NCC H8 threshold options at a Class 1a livable-housing entrance:

Level threshold (under 5 mm): maximum 5 mm height, with a rounded or bevelled profile on the rise. The preferred option; no step required.
Ramped threshold (5 to 56 mm): maximum 56 mm height only if the rise is achieved via a 1:8 maximum gradient ramp, with the ramp on the exterior side of the door (so water sheds outward).
Square-edge sill (up to 15 mm): permitted only where weatherproofing requires it, such as a flush external door on a wind-driven-rain exposure. In that case, the higher sill is the trade-off and the design must show why the alternative options were rejected.

Where it applies under NCC 2022 H8:

The primary entrance of a Class 1a dwelling (per Livable Housing requirements).
Internal doorways on the path of travel within a livable-housing-rated dwelling.
External doors on accessible-rated features (verandahs, courtyards, etc.).

Where it doesn’t apply (existing residential not in livable-housing scope):

Non-livable-housing Class 1a where the H8 election was not made (older builds, pre-NCC 2022).
Internal doors not on the accessible path of travel.
Doors to spaces specifically excluded (e.g. a small store room, an external wet area).

Builder execution detail:

Sill flashing under the threshold is independent: even a 5 mm threshold needs a sill flashing per sill flashing install to keep water out of the wall.
Ramped threshold on the outside is the most common practical choice: a 50 mm wedge of solid timber or a proprietary ramp piece, placed external to the door, creating a 1:8 outward slope.
Setting the slab and the doorframe at install is where this gets locked in. A concrete slab poured 50 mm too low for the threshold cannot be raised; the design freezes at slab level.

Common defects:

Threshold over 5 mm with no ramp and no documented weatherproofing justification.
Ramped threshold steeper than 1:8 (e.g. a wedge providing 80 mm rise over 500 mm length = 1:6.25, non-compliant).
Internal threshold (e.g. between living and laundry) at 10 mm step where path of travel needs 5 mm or ramped.
Bevelled threshold where the bevel slope is on the wrong side (interior, not exterior).

Also known as: doorway threshold; door sill; doorstep (loose).

Category: Building science.

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