Laying a Floor on a New Screed

Ask the builder for the screed's designation and you will get a short code. Under EN 13813 a screed material is named by its binder and its strength classes: CT is a cementitious screed and CA a calcium sulfate one, followed by C and the compressive strength class and F and the flexural strength class, so a common cement-based levelling screed is written EN 13813 CT-C12-F2. The code tells you what the screed is made of, and what it is made of decides how it dries and how it is tested, which is why it is worth having in writing before anyone quotes for the floor.

BS 8204-1 gives a drying allowance for levelling screeds of one day per millimetre of thickness for the first 50 mm, with an increasing time for each millimetre above that. A 50 mm screed drying under good conditions can therefore be expected to be dry in about two months. Two things in that sentence do the damage on site. The first is "good conditions": a sealed new house with the windows shut and the heating off is not drying under good conditions, and the calendar says nothing about it. The second is the thickness: the allowance is not linear above 50 mm, so a thicker screed takes disproportionately longer, and the thickness is often not what the drawing says.

The drying allowance is a planning figure. It tells you when to start testing. It does not tell you the screed is dry.

The test in UK practice is a hygrometer sealed to the surface of the screed inside an insulated box with butyl tape, left until the air trapped under the box reaches equilibrium with the base, and then read. Where the damp-proof membrane sits between the base and a 50 mm screed, at least four hours pass before the first reading; where the membrane lies below the base slab, the wait is 72 hours. A reading taken sooner measures the air in the room, not the screed.

The figure that counts as dry follows BS 8203: readings not exceeding 75 % relative humidity for resilient floor coverings, and 65 % for solid timber. The Contract Flooring Association's guide sets the same 75 % as the maximum before a floor finish goes down, alongside the other conditions it puts on the base: clean, sound, level, strong enough to carry the preparation and the finish, mechanically prepared and dust free. A floor laid on a screed that has not been read is a floor laid on an assumption.

Approved Document C requires a floor next to the ground to resist the passage of ground moisture to its upper surface, not to be damaged by moisture from the ground or by groundwater, and to resist the passage of ground gases. A new ground floor therefore has a damp-proof membrane in it, and it is tempting to read that as permission to skip the hygrometer. It is not. The membrane deals with water coming up from the ground; the water in a new screed is the water it was mixed with, and it is above the membrane, drying upwards into the room. The test is about that water, and the membrane does nothing to it.

BS 8204 grades the surface regularity of a screed in three classes, measured as the greatest gap under a 2 m straightedge laid on the surface: SR1 allows 3 mm, SR2 5 mm and SR3 10 mm. Which class was specified is a question for the builder, and whether it was met is a question for a straightedge, not for the eye. What the class means for the floor depends on how the floor is fixed. A glued board follows the screed and shows every hollow as a hollow; a floating floor bridges the hollow, and a bridged hollow becomes a soft spot under a click joint. Either way the answer to a screed outside its class is a levelling compound before the floor, not a thicker underlay.

Where the heating pipes are in the screed, the Contract Flooring Association is unambiguous: the system must be run through a full drying and commissioning cycle of heating up and cooling down before any subfloor preparation or floor covering goes down, and that cycle usually takes around 21 days. The guidance calls it a drying and commissioning cycle, and the order of the two words matters: the screed is taken through its working temperatures while there is still nothing on top of it to take the consequences. A heated screed that has not been through the cycle is not ready, whatever the hygrometer says on the day.

The last condition is the room, not the screed. The CFA's guide says the room should be at its in-service temperature 48 hours before installation, so that the flooring can be kept in the room and brought to room temperature before it is laid. For multilayer modular floors, rigid vinyl among them, the MMFA puts the same 48 hours on the boards themselves: stored flat and cross-stacked in their unopened boxes, in the same conditions as the installation area, at roughly 15 to 25 °C and between 35 and 65 % relative humidity. A pallet that came off a cold van the same morning has not met either condition.

Once the screed is dry and flat, the choice is the usual one. The BML rigid vinyl in the range arrives with its underlay backing attached and clicks together as a floating floor. Laminate floats over a separate underlay, and on a screed that underlay carries a moisture barrier. Engineered oak can float over an underlay with a barrier or be bonded to the screed; solid oak is fixed, and it is the covering the stricter 65 % figure was written for. None of them changes the order of events: read the screed, level it, run the heating, let the boards settle, then lay.

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