How do I calculate finished floor levels with falls?

  1. First work out what the minimum thickness of the screed bed needs to be.
  2. Next check that the base slab is level by using a laser level. If the slab IS level, skip the next step.
  3. If the slab is not level, use a laser level to find the difference in height of the drainage outlet and the floor at each perimeter wall.
  4. Measure the distance from the outlet to the furthest wall.
  5. Work out what fall you are trying to achieve. 1:100 is the minimum required by AS 3740 for bathroom floors outside the shower, and 1:100 is also required by AS 4654.2 for external balcony decks.
  6. Use the following formula to calculate the height of the finished floor level (H) at the walls. Once calculated, measure up the wall and mark the height.

H = L x r – d + s + t


H = height of finished floor level at walls
L = length distance from outlet to wall.
r = ratio required to be achieved
d = difference in height of slab at wall relative to at outlet.
s = minimum thickness of screed bed
t = thickness of tile & adhesive

Example as follows:
Let’s say the distance from outlet to wall is 6 m, convert this to mm, L = 6000 mm.
r = 1:100
Let’s say the base slab at the wall is 5 mm lower than at the drainage outlet, as determined used a laser level so d = -5
s = 60 mm
t = 8 mm

H = (6000) x (1/100) – (-5) + 60 + 8

H = 133 mm

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