A slab pour is the one step on a building site that cannot be undone. Once the concrete is placed, every mistake in the formwork, the steel or the mix is locked into the structure. Ten minutes of checking before the first truck arrives is the cheapest insurance a site has.

This is a general routine, not a replacement for your drawings, specification or the supervising engineer's instructions. Where they differ, they win.

1. Formwork: level, tight and propped

Walk the whole deck before anything else. Check that the soffit is at the level shown on the drawing, that the joints between sheets are tight enough not to leak cement paste, and that the edge forms are fixed so they cannot be pushed out by the weight of the concrete.

Then look underneath. Props should stand on firm, even ground or on a base plate, be plumb, and be braced against each other. A prop resting on loose bricks or soft soil is a settlement waiting to happen.

2. Reinforcement: spacing, cover and cleanliness

Compare the bars against the drawing: diameter, spacing, laps, and the extra bars at openings and edges. Then check the cover.

  • Cover blocks should be in place under the bottom bars and at the edges, spaced so the steel cannot sag between them.
  • Top bars need chairs or supports strong enough to carry a person walking on them.
  • Remove loose rust, oil, mud and binding wire offcuts from the steel and the deck.

Cover is not decoration. It is what protects the steel from moisture for the life of the building.

3. Clean and wet the form

Sweep or blow out sawdust, nails and standing water from the deck. Timber and brick that are dry will pull water out of the fresh concrete, so wet them down before the pour, without leaving puddles. If you use a release agent on steel or plywood forms, apply it before the reinforcement goes in, not after.

4. The mix and the pour plan on site

Confirm the mix with the supplier or the site batching record: the grade, the water-cement ratio you are aiming for and the workability. When the first load arrives, check the consistency yourself, for example with a slump test, and compare it with what the specification allows. Do not let anyone add water at the chute to make the concrete easier to place.

Agree the pour plan before you start: where concrete enters, in which direction you pour, where the construction joint goes if the pour has to stop, how many vibrators are on hand and who is responsible for each. Keep a spare vibrator and enough lighting if the pour may run into the evening.

5. Curing: decide it before you pour

Curing is the step most often forgotten, because the work looks finished once the surface is levelled. Plan it in advance: who will start curing, with what (wet hessian, ponding, or a curing compound as specified), how often it will be wetted, and for how many days. Make sure water is available and that someone is named for it, including on holidays.

A last look

Before the pump or the first wheelbarrow starts, stand at one end of the deck and look along it once more. Level, props, steel, cover, a clean form, a plan for the mix and for curing. If any of the five is not ready, wait. A delayed pour costs an afternoon; a poor slab costs much more.