Bonded screed: definition and principle

What is a bonded screed?

A bonded screed is poured directly onto its substrate, forming a single, unified structure. There is no insulation, film, or intermediate layer: the mortar is in direct contact with the slab and adheres to it both chemically and mechanically.

This bond changes its entire behavior. While a floating screed acts like a slab resting on a cushion, a bonded screed behaves like an additional layer of the substrate itself. It inherits the substrate's strength, but also its flaws.

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How is adhesion achieved?

The result depends on the preparation of the substrate before pouring. The surface must be sound, clean, and free of laitance, dust, plaster residue, or grease. A closed or overly smooth substrate must be mechanically opened by shot blasting or scabbling. An absorbent substrate is treated with a bonding primer, which limits the fresh mortar's water absorption and creates a bridge for adhesion.

This step is the primary cause of failure for this type of screed. It is also why a screeding contractor always begins by assessing the substrate before discussing thickness.

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When should you choose it?

A bonded screed is appropriate when available height is limited and there is nothing to be placed in between. It allows for thin layers, dries faster than a thick screed, and can withstand heavy loads because the substrate works in tandem with it. It is commonly used in partial renovations, utility rooms, garages, or anywhere that insulation is not a requirement.

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Its limitations

Three things disqualify it. No insulation can be placed underneath, which rules out thermal comfort. No acoustic decoupling is possible, so impact noise travels directly into the structure. And any cracks in the substrate will eventually transfer to the screed, as the two become one.

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Bonded, unbonded, or floating?

The three are distinguished by what separates them from the substrate. A bonded screed rests on nothing: it sticks. An unbonded screed is poured over a simple separation film, without insulation: it slides but does not float, which protects it from substrate cracks without providing any insulation. A floating screed adds a compressible layer and becomes a full-fledged insulation system.

The choice is made during the assessment. We carry out these three configurations depending on the condition of the substrate and the project goals, as detailed on our screedpage.