Overlaying Liquid Screed Over an Existing Screed

Before any overlay work begins, the existing screed should be inspected for cracking, hollow-sounding areas indicating delamination, surface contamination such as old adhesive residue, and general condition. Moisture testing of the existing screed is also important, since a floor that appears dry on the surface can still hold higher moisture levels at depth, particularly if it has never been tested or if the building has had periods of dampness.

Where the existing screed shows significant defects, it's often more reliable in the long run to address those issues properly, whether through localised repair or removal, rather than overlaying and hoping the new layer masks an underlying problem that will likely resurface once the floor is back in use.

A bonded overlay relies on good adhesion between the new screed and the prepared existing surface, typically using a primer or slurry coat as recommended by the screed manufacturer, and suits situations where the existing screed is sound and thin overlay depths are involved. A floating or semi-bonded approach, using a separating membrane, can be more appropriate where underfloor heating pipework is being introduced, or where movement between the layers needs to be accommodated.

Whichever approach is chosen, finished floor levels need checking carefully against door thresholds, staircases and adjoining rooms, since even a modest overlay depth can create trip hazards or clearance issues if not planned in advance of the pour.

Surface preparation of the existing screed is one of the most important factors determining whether a bonded overlay succeeds. Laitance, old adhesive residue, paint, or any other contamination on the surface needs to be removed thoroughly, typically by mechanical preparation such as shot-blasting or grinding, before a primer is applied according to the chosen system's specification.

Skipping or rushing this preparation stage is one of the most common causes of bonded overlay failure, since even a well-formulated primer cannot compensate for a substrate that is dusty, contaminated or otherwise unsuited to receiving a bonded layer.

Where the existing screed contains embedded underfloor heating pipework or other services, these need to be located, checked for condition, and protected before any preparation work or new pour begins, since damaging existing pipework during surface preparation can create a costly and disruptive repair. Historic drawings, if available, should be checked against an on-site survey rather than relied upon alone, since as-built conditions sometimes differ from what was originally designed.

Where services are being retained and simply overlaid, confirming they are still functioning correctly before covering them with a new layer avoids the situation of a fault being trapped beneath a finished floor and only discovered once it becomes difficult and expensive to access.