Screed thickness. 35, 40 or 50 mm?

An unbonded screed carries itself. It is not stuck to the floor below, so it has to be thick enough and strong enough not to curl or crack. How thick depends on what it is made of.

Where each figure comes from

Bonded site-mixed screed (AS 3958)

15 mm at any point, 50 mm at most in one layer

Unbonded or floating site-mixed screed (AS 3958)

40 mm or more, reinforced

Wet mortar bed over a liquid membrane (AS 3958)

40 mm or more

Unbonded screed over an external membrane (AS 4654.2 guide)

50 mm with light mesh

SCREED X, unbonded over a FORSPEC membrane (FORSPEC)

35 mm minimum

SCREED X, floating over insulation or an acoustic layer (FORSPEC)

45 mm minimum

The 40 mm is a requirement for a sand and cement screed mixed on site. The 50 mm is in a note, so it is guidance. Neither was written for a pre-blended engineered screed.

Three things that matter more than 5 mm

• Bonded or unbonded, not half of each. A screed on a slip sheet, a dirty slab or a membrane that does not bond is unbonded. Design it that way.
• The slip sheet. Over an external membrane the screed sits on two slip sheets, so it never touches the membrane.
• The falls. Know which layer carries the fall before you set the thickness. See screeds on balconies.