How many bags of cement are needed to deck 100 square metres?

About 116 bags for a 150 mm slab with 225 by 450 beams on a 4 m grid, cast in a 1:2:4 mix — 18.4 cubic metres of concrete at roughly 6.3 bags per cubic metre. The familiar site rule of one bag per square metre is the slab on its own; the beams under it add about a fifth again. Both figures are net quantities, so add your own allowance for wastage before ordering.

How much reinforcement goes into a deck?

Roughly 15 to 20 kilogrammes per square metre of floor for an ordinary residential deck — about 13 kg of it in the slab and the rest in the beams. It is a sense check rather than an order: the real quantity comes from the bar bending schedule, which counts laps, bobs and links, and it can sit outside that range for good reasons. A figure far outside it usually means a bar size or a spacing has been misread.

How long before the props come out after decking?

BS 8110 gives 10 days for props to slabs and 14 days for props to beams, for ordinary Portland cement concrete at a surface temperature of 16 degrees, and Nigerian temperatures shorten those. The soffit forms may come off sooner — 4 days for a slab — but the props are what carry the load, which is why they outlast the boards they are holding up. The heaviest day in a slab's life is often the week after striking, when the blocks for the next floor are stacked on it.

Can a deck be cast in two pours?

Yes, but the joint has to be planned before the day rather than declared when the mixer stops. A construction joint belongs near the point of least shear in the span — around a fifth to a third of the way along, never over a support — it is formed vertically with a stop end rather than left as a ragged slope, the reinforcement runs continuously through it, and the surface is roughened and cleaned before the second pour. An unplanned cold joint in the wrong place is a plane of weakness across a member designed as continuous.

Decking a floor — quantities per square metre, and the day itself

Decking is the day the design becomes a building. It is also the one item most often estimated from a half-remembered figure, which is a pity, because the quantities are simple arithmetic and the thing that actually costs money is not the concrete.

This page gives quantities, not prices. Nigerian rates move month to month, and a page quoting them would be wrong before it was read — which is why Structura holds prices on your device, in naira you set yourself, and publishes none.

Per square metre of deck

For the common case: a 150 mm slab with 225 × 450 beams on roughly a 4 m grid, grade 25 concrete at a 1:2:4 nominal mix.

Item	Per m²	Where it comes from
Concrete, slab	0.15 m³	the thickness itself
Concrete, beam webs	0.034 m³	0.5 m of beam per m² on a 4 m grid, 225 × 300 below the slab
Concrete, total	0.18 m³	the number to order against
Cement	1.2 bags	6.3 bags per m³ at 1:2:4
Sand	0.08 m³	0.44 per m³
Granite	0.16 m³	0.88 per m³
Reinforcement	15–20 kg	~85 kg/m³ in the slab, ~120 in the beams
Formwork contact area	1.3 m²	slab soffit, plus both sides and the soffit of the beams
Props	~1 no.	about 1 m centres under the slab, closer under beams

Notice that a square metre of deck needs 1.3 square metres of formwork. Formwork is measured by contact area, it is usually a bigger item than the concrete it shapes, and what determines its cost is how many times the plywood and props are reused — a programme question, not a drawing one.

Worked: a 100 m² deck

About the footprint of a three-bedroom flat, at the quantities above:

Concrete 100 x 0.184 = 18.4 m3
Cement 18.4 x 6.3 = 116 bags
Sand 18.4 x 0.44 = 8.1 m3
Granite 18.4 x 0.88 = 16.2 m3
Steel 100 x 17 = 1700 kg (from the schedule, not from here)

Formwork 100 x 1.3 = 130 m2 contact area
Plywood 130 / 2.98 = 44 sheets per use (2440 x 1220)
Props say = 100 no.

The site rule of one bag per square metre is the slab on its own — 0.15 × 6.3 = 0.95. It is a good rule and it quietly leaves out the beams, which is where the missing fifth of the order goes.

Every figure here is net: the quantity in the finished floor, not the quantity to buy. Add an allowance — often around 5% on concrete and 3 to 5% on steel — and say on the bill which of the two the bill is showing, or nobody comparing two estimates can tell whether one is cheaper or just measured differently.

Before the mixer starts

An hour of checking here is worth more than anything that can be done afterwards. In the order they are worth doing:

Formwork level, tight and propped. Level across the deck, joints tight enough not to leak grout, props on firm bearing and not on soft fill, and the whole of it able to carry wet concrete plus the people standing in it.
Bar sizes and spacings against the schedule, not against memory. Count the bars in one panel and one beam and compare with the drawing before believing the rest.
Cover blocks under everything, at close enough centres that the bars do not sag between them. Cover is what makes the steel last, and it is lost on the day, not over time.
Top steel over the supports, and something to keep it up there. Chairs or spacer bars. Top steel that has been trodden to the middle of the slab is doing nothing at all, and it is invisible an hour later.
Links in the beams at the spacing on the sheet, closed and properly lapped, tighter near the supports where the shear is.
Openings, cast-in pipes and stair connections set out and boxed before, never chopped after.
Cube moulds on site and somebody named to fill them. Cubes taken on the day are the only evidence of what was poured; see cube tests.
Enough material for the whole pour. Running out is what turns a planned deck into an unplanned cold joint.

The pour, and the fortnight after it

Cast continuously, working away from one end, compacting each layer into the one below. A poker fits between bars that were spaced properly and does not between bars that were not — which is the real reason the bar spacing rules exist.
Plan any joint in advance. Near a fifth to a third of the span, never over a support, formed vertically with a stop end, steel running through, surface roughened and cleaned before the next pour.
Cure for at least seven days — ponded, covered or kept wet. In Nigerian sun a deck left to itself loses a serious fraction of the strength the design assumed, and the first week is when almost all of it is won or lost.
Strike to the periods, not to the programme. Slab soffit 4 days, props to slabs 10, beam soffit 9, props to beams 14, at 16°C and for ordinary Portland cement — see striking formwork for what shortens them and what back-propping is for.
Do not load it early. The heaviest day of a slab's life is often the week after striking, when the blocks for the next floor arrive and are stacked in one place on a floor designed for them spread out.

What moves the cost without changing the drawing

Two identical decks can differ substantially in price, and the variables are almost never the ones people argue about:

Formwork reuse. The same plywood over four floors is a quarter of the cost of it over one.
How the concrete arrives. Mixed on site by headpan, site mixer, or ready-mix and a pump — different labour, different pace, and a different risk of a cold joint.
Access and storey height. Getting 18 m³ to a second floor is a different job from placing it at ground.
Wastage and pilferage, which is what the allowance on the bill is really covering.
Rework. The cheapest deck is the one that did not have to be chipped out for a pipe nobody boxed.

Get the quantities out of the design, not off this page

Structura bills every design it does: concrete by member, formwork, reinforcement by the kilogramme, packed into the 12 m lengths you actually buy. You price it with your own naira rates, set once in Settings and stamped with the date they were set — and the bill states whether it is showing net or gross quantities, because a client comparing two estimates cannot otherwise tell.

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