Levels and Datums in a Building Project

The formation level entry describes the first link plainly: it is the surface reached when excavation stops, the plane on which a foundation, slab, pavement or drainage run is subsequently built, and its significance is that it is the point at which the ground stops being an unknown and starts being a construction surface. Everything above is set out from it.

It is also a condition rather than a number. Two excavations to the same depth can arrive at very different formations, and an unprotected formation can deteriorate to the point where the level has to be taken lower, which changes the chain rather than just the surface.

Several systems say the same thing in their own language. A strip foundation is expected to present a level, continuous surface from which the wall above can be set out and built accurately. The tops of pile caps and ground beams become the datum for everything above them. The first course of an insulating concrete formwork wall sets the plumb and line the rest of the wall inherits. The lowest joint in a precast structure sets the geometry for every storey.

A volumetric building makes the point most starkly: the accuracy of the plane the first row is set on is inherited by the whole stack and cannot be recovered higher up, because a finished unit cannot be adjusted internally to correct what it is standing on.

At the wall base, three levels have to be settled together: the external ground, the damp-proof course in the wall and the internal floor. The continuity entry is explicit that the position of the course relative to those two decides whether the barrier is interrupting an upward path or holding back water standing against the wall, and that whether that relationship has been fixed on the drawings is a decision constraining everything else.

In a suspended floor the same set of decisions arrives even earlier. At ground level the units usually bear on foundation walls, so floor level, ventilation openings and the damp course are all settled by decisions taken in the foundation work long before any floor member arrives on site.

Between the structural plane and the surface people walk on sits a build-up, and every layer in it consumes level. Where insulation sits above a slab, the layer over it becomes a floating screed rather than a bonded topping, and the perimeter, the isolation and the movement joints all follow from that arrangement.

That is why the floor build-up is a levels decision as much as a thermal or acoustic one. A build-up settled late has to fit within a level that was fixed early, and something has to give.

At a threshold the barrier, the frame, the floor finish and the external surface meet at the level where water gathers. The desire for a low or level threshold and the need for an upstand pull in opposite directions, and the entries are clear that this conflict has to be resolved rather than deferred.

Level access reduces the height available for the detail, which makes it tighter rather than simpler. That is a design-stage conversation, because by the time the threshold is being built there is nowhere left to find the height.

A stair is an inclined structure between two levels, and a landing belongs to the stair rather than to the floor it happens to sit level with. It is also the datum from which the flight arriving at it and the flight leaving it are set out, which is why moving a landing is never a local change: the platform moves, the structure carrying it moves, and the arrangement of both flights reopens at the same moment.

Where a stair is made off site, the entry puts the levels question directly: at what point are the levels it is made to confirmed, and by whom. A stair manufactured to a level that later moves is a stair that no longer fits.

The chain does not stop at the wall. Where treatment or paving stops at or near the building line, external paving frequently sits on untreated ground next to a building that does not, and the step in behaviour appears at the threshold, which is exactly where it is most noticeable.

The finished external level also decides what the damp control at the wall base is being asked to do, and whether the openings serving any ventilated space beneath the floor stay clear. A level fixed on a drawing and a level built in a garden are the same number only if somebody checks.

The practical remedy in the entries is unglamorous and consistent: the relationship between levels should be drawn rather than described, on a drawing that shows structure, damp control, thermal line and external surface together, because each of those is otherwise owned by a different package.