Calculate height differential, slope percentage and screed thickness range for floor drainage in showers, wet areas and balconies.
Longest distance from drain to wall
Enter distance and gradient to calculate fall height.
NZS 4858 / AS 3740 minimum 1:80 fall for shower floors. External balconies typically 1:40. Screed range assumes 25 mm minimum at drain.
NZS 4858 and AS 3740 require a minimum 1:80 fall to the drain for shower floors and wet areas. This means for every 80 mm of horizontal distance, the floor drops 1 mm. A 1:60 fall is recommended for improved drainage, especially for floor-waste drains.
External balconies and terraces typically require a steeper fall to prevent ponding. NZS 4858 specifies a minimum 1:80 fall but most waterproofing specifications call for 1:40 on balconies, particularly for tiled surfaces where the tile surface itself may be slightly less drainable than an internal shower.
The minimum screed thickness is typically 25 mm at the drain (lowest point), with the screed building up to create the required height differential. For a 2.5 m floor with 1:80 fall, the height differential is 31 mm. Screed would taper from 25 mm at the drain to 56 mm at the wall edge.