Floor Build-Up Depth and Level Thresholds

The reference entries describe the layers of a floor as interdependent rather than stacked arbitrarily. Each has a position it has to occupy relative to the others, and none of them can be thinned to make the arithmetic work without changing what it does.

The clearest case is the layer above the insulation in a ground floor. Where the thermal layer sits above the slab, the layer over it stops levelling and becomes an unbonded screed carrying loading across something compressible, which means its depth is decided with the layer beneath it rather than adjusted to suit a doorway.

The practical consequence is a sequencing one. If the finished floor level is fixed first and the build-up developed afterwards, any layer that turns out to need more depth than was assumed has to take it from something else — the structure below, the head height above, or a step at a threshold.

The entries approach this from the finishes side and reach the same conclusion: the floor covering is best chosen early enough for the base beneath it to be specified around it, rather than selected once the base has already been built.

A step has to appear wherever two floors of different total depth meet, and in practice that is a small number of predictable places: the external doorway, the junction between a heated area and an unheated one, the boundary between two coverings, and the head of a stair where the top riser has to reconcile with the finished level above.

Where an existing floor is being re-covered, the same arithmetic runs in reverse. A thicker build-up than the original has to be absorbed by the doorway, by trimming a door, or by a transition that was not in the original design.

The floating screed entry names the doorway twice: as the place the isolation is most often bridged, and as the place the covering usually changes. The joint in the screed, the threshold detail and the edge strip all meet in a narrow line that several different trades pass through.

The consequence is disproportionate. A floating screed touching the wall at a doorway, bridged by a pipe sleeve, or pinned to the base by a fixing driven through it has ceased to float at that point, and the whole layer behaves differently for the sake of one contact.

The perimeter isolation strip is a small component with a disproportionate consequence. It has to be continuous around the entire perimeter, including at doorways and behind fitted units, and extend to the full height of the floating layer so that contact cannot occur above it.

The entries also name the moment it is lost. It is installed early and trimmed at the end, and the trimming stage is where it is frequently cut back too far or removed. Cutting it below the top of the floating layer opens a contact path, and a skirting fixed to both wall and floor reconnects them.

The desire for a low or level threshold and the need for the barrier and the isolation to be carried through it pull in opposite directions. At a threshold the layers are compressed into the least available height, and each has to connect to something made of a different material that arrives at a different time.

That conflict is resolved at design stage rather than deferred to the person holding the sealant. Where an accessible threshold is wanted, that is a reason to settle the build-up depth earlier rather than a reason to take depth out of the build-up.

The information that prevents most of this is short: the intended total depth of each build-up, the finished floor level in each area, the position of any change in level, and the detail at each doorway showing how the isolation and any joint are carried through. Fixed early, those four decide the rest. Left open, they are settled by whoever arrives last.