Overlaying an Existing Old Screed with New Liquid Screed

Before deciding to overlay, the old screed needs a proper condition survey — sounding for hollow or debonded areas, checking for active cracking, and confirming there's no ongoing moisture issue from below. Areas that fail this check should be cut out and repaired rather than simply built over, since a weak patch under a new bonded screed will eventually show through as the new floor is loaded.

A tapping survey with something as simple as a hammer or a length of steel bar can reveal hollow-sounding areas that aren't visible from the surface. These patches often correspond to places where the original screed lost its bond to the slab years ago, perhaps from an earlier leak or simply from age, and they're exactly the areas that need attention before anything new is poured over the top.

Overlaying adds to the existing floor build-up, so thresholds, skirtings and any adjoining rooms need checking against the new finished level. Where the old screed is a calcium sulphate type, priming compatibility with whatever is poured over it needs following the manufacturer's guidance, since not all screed types are designed to bond directly to every other type.

It's also worth checking whether the added depth affects door swings, staircase nosings or the height of fitted kitchen units, particularly in a renovation where these elements were originally set to the old floor level. Discovering a mismatch after the pour is far more disruptive than allowing for it at the design stage.

Any section of old screed identified as loose, cracked or debonded during the survey should be cut back to sound material and repaired using a compatible patching mortar, rather than left in place under the assumption that the new screed will bridge the defect. A bonded overlay poured over a hollow patch simply inherits the same weakness, and it can be considerably harder to diagnose once buried under a new floor.

Where cracking in the old screed is linked to an underlying slab movement joint or structural crack, this needs identifying and carrying through into the new floor's design, typically as a matching movement joint, rather than papering over a joint that will continue to move regardless of what's poured on top.

A bonded overlay onto a sound old screed keeps the added depth to a minimum, which suits refurbishments where floor-to-ceiling height or threshold levels are tight. It does, however, demand a higher standard of surface preparation and a compatible primer, since the bond depends on the condition of the old screed's surface as much as the slab beneath it.

A floating build-up over insulation sidesteps most of the bonding and compatibility questions, at the cost of extra depth. Where insulation is being added for thermal performance anyway, or where underfloor heating is planned, floating over the retained old screed can be the more straightforward and lower-risk route.