Screed

Screed is a thin, level layer of mortar laid over a slab or substrate, either to provide a bedding for tiles or to set falls toward a drain. In residential construction, screeds appear in two main contexts: the bedding screed under tiles in a wet area, and the topping screed that brings a structural slab to finished floor level or sets fall for water drainage.

Who lays it

In residential wet areas, the screed is typically laid by the tiler as part of the tile installation, immediately before the waterproofing membrane goes on. For larger topping screeds outside wet areas (garage floors, sub-floors over underfloor heating), a concretor or specialist screed contractor does the work. On bigger jobs, self-levelling compounds may be poured by a flooring contractor instead of a hand-laid screed.

Mix and makeup

A typical wet-area screed is a sand-to-cement mortar at roughly 3:1 sharp sand to cement by volume, mixed to a damp, just-cohesive consistency, laid 10 to 40 mm thick to AS 3958.1 (ceramic tile installation). For better bond to the slab, a bonding additive (SBR latex or acrylic admixture) is brushed onto the substrate or mixed into the screed itself. Common product categories used:

• Bonding additives: SBR latex or acrylic-based slurries, brushed onto the slab before the screed.
• Self-levelling compounds: pre-bagged cement-polymer mixes for level toppings over a slab.
• Pre-bagged screed mixes: factory-blended sand and cement for consistency, useful on jobs where on-site mixing is impractical.

(Specific brands and product details: see materials/screed-products, to be drafted.)

Where it sits in the wet-area stack

Falls toward floor wastes and grates in showers and bathrooms are set in the screed, with the waterproofing membrane applied over the screed before tiling per AS 3740 (waterproofing of domestic wet areas). The fall, the membrane, and the tile finish are co-dependent: the screed sets the geometry, the membrane keeps water out of the substrate, the tile takes the wear.

Bonded vs unbonded

Outside wet areas, a bonded screed is poured directly onto a clean, primed slab surface. An unbonded or floating screed sits on a separation layer (common over underfloor heating or acoustic insulation).

Common defects

Defects are usually a screed problem before a tile problem: hollow sections, cracks, ponding away from the grate. Hollow-sounding tiles often trace back to a screed not bonded to the slab, not the tile or adhesive.

Also known as: Mortar bed, sand-cement bed, topping.

Category: Materials.