Engineered openings and drainage paths

Engineered gaps allow air into building elements to provide ventilation and drying, moderate air pressure and prevent transference of moisture. Drainage paths allow water that has penetrated the cladding, window or other detail to drain out.

It is vital that these gaps and paths are not blocked during construction. Builders will need to ensure that other tradespeople such as painters are aware of these gaps.

Examples of engineered gaps include gaps that allow air in behind claddings (to moderate air pressure), openings in foundation walls to provide subfloor ventilation and the separation between flexible roof underlay and roof insulation.

Examples of engineered gaps that must remain open:

  • Drainage and ventilation slots in the bottom of brick veneer and ventilation openings at the top – for example, every third perpend in a full-height course must be left open to allow ventilation and drainage.
  • The 5 mm gap between the bottom of a cladding and a window head or inter-storey flashing, to allow a drained and vented cavity to work.
  • The 5 mm gap between the sill window flange and the sill flashing for windows installed in direct-fixed claddings.
  • 3,500 mm2 of clear opening for each 1 m2 of floor area to ventilate under a suspended floor.
  • 25 mm minimum gap between a flexible roof underlay and roof insulation.
  • The 5 mm separation between a notched apron flashing downturn and the roofing profile.
  • Expressed open drained joints in flat sheet (fibre-cement, plywood) cladding panels.

Updated: 28 July 2022