What is taken on site before anything is ordered

Attendance is at the address. Nothing is specified from a drawing alone.

Levels are taken on a grid across each area. A 2 m straightedge is laid in two directions at every station. Deviation under the straightedge is recorded to the nearest millimetre. Falls to gullies and to thresholds are recorded as a fall over a stated run.

Existing coverings are opened up at three positions per room as a minimum. Openings are taken down to the structural base. Adhesive residue, bitumen, old smoothing compound and separating layers are logged by layer and by thickness.

The survey report carries the level grid, the opened-up sections, the marked hygrometer positions and the defects found. It states the preparation required, the tolerance the base is to be brought to, and the reading to be met prior to ordering. No product is named in it.

Section through the build-up

A cut section, seen edge-on, each material hatched to its own convention. Bands read from the seal coat down to the structural base. Select a band for its clause.

Cut section, seal coat uppermost. Hatching denotes material, and band depth is not to scale.

Defects in the base, and what each one stops

Laitance is removed by mechanical preparation. Grinding, scarifying or shot blasting is specified against the residue found. Priming is withheld until the base is sound and free of dust.

Curling at slab edges and at joints is measured under the straightedge. Curled edges are ground back where the deviation exceeds the tolerance the finish demands.

Cracks are recorded by width, and by whether they are live. Live cracks are carried up through the build-up as movement joints. Dead cracks are cut out, filled with a resin repair, and dressed flush.

Contamination is recorded by substance and by area. Bitumen, old adhesive, curing compound, plaster droppings and oil are removed prior to priming.

Each defect gates one stage. Laitance gates priming. Curling gates the levelling tolerance. Live cracking gates the finish. Contamination gates adhesion through the whole build-up.

The hygrometer box, and how long it stays down

Readings are taken with a surface hygrometer box, sealed to the base at its rim.

Boxes are sited away from heat sources, away from external walls and away from openings. One box is placed at the centre of each area. Positions are marked on the level grid prior to the boxes being left down.

A box stays down for 72 hours as a minimum. A reading lifted before 72 hours is discounted. Where readings are still falling, the box is left in place and read again at 24-hour intervals.

The reading is recorded as a percentage of relative humidity at the base surface. Box reference, position and time down are recorded against it.

One reading releases one stage. At or below the figure the specified finish requires, the finish is ordered and laid. Above that figure, the base is left to dry further. In the alternative, a surface membrane is specified, and its own release condition then applies.

Readiness read off the depth rule and the reading

Two figures are printed below, and they are printed apart. The first is a day count taken from the laid depth. The second is the distance, in percentage points, between the reading recorded and the reading the finish requires.

The day count is arithmetic on laid depth, and it forecasts nothing about the reading. The percentage-point figure is a position recorded on the day the box was lifted. The two are never added together, and neither stands in place of a hygrometer reading taken on the floor in question.

Primers, and compounds laid to a tolerance

Primer type is set by the base. An absorbent base takes a diluted acrylic primer. A non-absorbent or sealed base takes an epoxy primer, blinded with kiln-dried sand while wet.

Coverage is taken from the data sheet and calculated against the surveyed area. Diluted acrylic primers are ordered on a coverage in the region of 8 to 12 square metres per litre.

Levelling compound is ordered by powder weight. Powder is calculated at the rate the data sheet gives for one millimetre over one square metre, multiplied by the mean thickness read off the level grid. A rate in the region of 1.5 kilograms per square metre per millimetre is common.

Compound is poured inside the primer's over-coating window. Where that window has closed, the primer is re-applied prior to pouring. Finished compound is laid to SR2 as a minimum, and to SR1 where the finish is a thin sheet or a resin.

Screed types, thicknesses and surface regularity

Sand-cement screed is laid at 25 mm minimum where fully bonded. Unbonded screed over a membrane is laid at 50 mm minimum. Floating screed over insulation is laid at 65 mm minimum under domestic loading, and at 75 mm minimum under heavier loading.

Calcium sulphate flowing screed is laid to the thickness its bond condition requires. It is sanded to remove surface laitance prior to priming.

Surface regularity is stated as a class. SR1 permits a deviation of 3 mm under a 2 m straightedge. SR2 permits 5 mm. SR3 permits 10 mm. The class is set by the finish, and it is stated in the survey report.

Drying time follows laid thickness and the conditions held in the building. Force drying is withheld until the screed has cured. Underfloor heating is commissioned through a stated heat-up and cool-down cycle prior to any reading being accepted.

Resin, tile and sheet, and what each demands of the base

Resin demands the driest and the soundest base. Bond strength is tested by pull-off where the resin system calls for it. Resin is applied over a base at or below the reading its data sheet states, unless a surface membrane has been specified.

Tile is laid to a bed thickness set by the tile and by the adhesive class. Movement joints in the base are carried through the bed and through the tile. Wall tiling is set out from a datum line struck at the survey.

Sheet is the least tolerant of deviation. Every ridge and every hollow in the base reads through a thin sheet. The base is taken to SR1 prior to laying. Sheet is conditioned in the room, laid, then welded at the seams where a welded joint is specified.

The base decides the finish. Where the base cannot be brought to the tolerance the finish demands, the finish is changed, or the base is replaced.