Slab casting day: quantities, the night-before checks, the pour and the dates after
A roof slab gets one day. Once the concrete starts going in, you can't stop to send for more cement, a second vibrator or the cube moulds. So most of the work happens in the two days before the pour, and the engineer's inspection of the steel is part of it. This page goes from the drawings to the strip dates on your calendar.
By Bulan Sarkar · Updated
Take the concrete volume and grade of the slab and beams from the drawings. A 1,000 sq ft roof with its beams is often about 14 m³, which is 112 bags of cement for M20 at CPWD's 8.0 bags per m³ (117 with wastage), and 2 cube samples of 3 cubes each under IS 456 cl. 15.2.2. Two or three days ahead, get the full material lot on site. The day before, have the engineer check the steel, the cover blocks and the shuttering. On the day, start early, run a mechanical mixer (at least 2 minutes a batch, cl. 10.3.1), keep a spare vibrator, and pour continuously up to the joints the engineer has marked. Start curing once the surface has set: 7 days of wet curing for OPC, 10 for PPC (cl. 13.5.1). Then write the strip and prop dates on a calendar, and leave the final call on them to the engineer.
The decision: run slab casting day yourself?
Yes, if you are building on a labour-only or part-material contract and you are the one ordering material and hiring the mixer. This page gives you the quantities, the order of the day, the cube count and the dates to write down. You run the checklist; the engineer gives the go-ahead.
On a turnkey contract the builder runs the day. Skip the ordering, but still be there: check the cube count and labels, take photos of the steel before it is covered, and get the cast date and the strip dates in writing.
The slab thickness, the steel in it, the concrete grade, where a construction joint may go and when the props can come out are for a licensed structural engineer. Have the engineer (or the site engineer they send) inspect the steel, the cover and the shuttering and say yes before the first batch is mixed. Ask before adding water, changing the cement or the mix, or switching to ready-mix.
The steps, in order
Get the volume and grade from the drawings
Have ready:the structural drawings for the slab and beamsthe concrete grade the engineer has specified (M20, M25…)
Ask the engineer for the concrete volume of the slab and the beams cast with it, and the grade. If the drawings only give sizes, run each member through the RCC calculator. For the worked example on this page: a 1,000 sq ft roof is 92.9 m², which at 125 mm thick is 11.6 m³ of slab, plus 2.4 m³ of beams below it, 14 m³ in all.
Then turn the volume into material with the concrete calculator. CPWD's analysis of rates puts M20 at 8.0 bags of cement per m³, so 14 m³ needs 112 bags; our calculator adds 3% wastage and says 117. The same calculator gives the sand and aggregate in cft and brass.
Get the slab-day lot on site two or three days ahead
Have ready:the bag, sand and aggregate figures from step 1a dry place for the cement, off the floor
All the cement, sand and aggregate for the pour should be on site two or three days before, checked at the truck and written in the delivery log. If material runs short in the middle of a slab, the concrete already placed keeps setting while you wait for the next truck.
Count the bags the evening before. The material order for one floor covers the delivery checks for each material.
The day before: steel, cover and shuttering checked
Have ready:the engineer or site engineer, booked for the inspectiona phone to photograph the steel before it is covered
IS 456 asks for every step that the next one will cover to be inspected as the work goes on (cl. 10.1.2), and calls supervision of the steel before it is covered extremely important (cl. 13.6). So the steel inspection happens before the pour, not on the morning of it. Walk the slab with the engineer and check:
- Cover blocks under the bottom bars, the same thickness as the nominal cover on the drawing, and spacers or chairs no more than 1 m apart (cl. 12.3.2). On a 93 m² slab that is about one block per square metre, roughly 90 to 120 of them, more under the beams. The cover can come out up to 10 mm more than the nominal figure, never less. Our cover blocks and chairs guide has the counts and the chair height.
- The top steel sitting on its chairs, not pressed down onto the bottom bars by people walking on it.
- The shuttering clean of sawdust, chippings and wire bits, oiled with release agent but not the steel (cl. 11.2), and tight enough that slurry won't run out of the gaps (cl. 11.1).
- Pipes and boxes for wiring and plumbing in place and tied, so nobody cuts the slab later.
Photograph the steel, room by room. Those photos are the only record of what is inside the slab.
Plan the people, machines and the order of the pour
Have ready:a mechanical mixer and a needle vibrator, plus a spare vibratorcube moulds, a slump cone if the engineer wants slump checks, lights and water
IS 456 cl. 10.3 asks for concrete to be mixed in a mechanical mixer, and cl. 10.3.1 for at least 2 minutes of mixing a batch. Batching goes by the 50 kg bag (Table 9 sets nominal mixes per bag), so 112 bags means about 112 batches, which is 224 minutes, close to 4 hours, of mixing alone before any loading, carrying or placing. That is why slab day starts at first light.
Ask the engineer where the pour starts, which way it moves and where a construction joint may go if the day runs out (cl. 13.4 leaves the joint positions to the designer). Keep a spare vibrator and fuel for the mixer on site; the code wants the concrete compacted with mechanical vibrators (cl. 13.3.1), and a breakdown halfway leaves you with an unplanned joint.
Pour without stopping, and take the cubes as you go
Have ready:6 cube moulds for 14 m³ (more for a bigger pour)a marker and the cube record sheet
The concrete is placed as close as it can be to where it stays, compacted before it starts to set, and not dropped more than about 1.5 m (cl. 13.2). Under-vibration leaves honeycombs; over-vibration and very wet mixes are both called harmful (cl. 13.3.1). No water gets added to make a batch flow better unless the engineer agrees: cl. 10.2.6 bars changes to the set proportions without tests.
Take the cube samples from batches spread over the whole pour, not all from the first ones (cl. 15.2.1). For 6 to 15 m³ of one grade, IS 456 cl. 15.2.2 asks for 2 samples of 3 cubes each. Label every cube with the date and the batch, and fill in the record sheet before the day ends. The cube test guide has the full table and the record sheet.
Start curing, then write the dates on a calendar
Have ready:water and a hose or tanker booked for the curing dayshessian for beam sides and edges, bricks and mortar for ponding bunds
Once the surface has set, keep the concrete continuously wet: at least 7 days for OPC, at least 10 for PPC, PSC or any blended cement, and the code recommends stretching blended cement to 14 (cl. 13.5.1). For a slab cast on Monday 2 November 2026, that is wet until 9 November for OPC, 12 November for PPC, 16 November for the recommended 14 days.
IS 456 cl. 11.3.1 gives minimum times before the formwork and props come out, for OPC above 15 °C with good curing: slab soffit shuttering 3 days (props refixed at once), slab props 7 days up to a 4.5 m span, beam soffits 7 days, beam props 14 days up to a 6 m span. Write them on the calendar as the earliest possible dates. The engineer decides the actual day. The curing and props guide covers both in full.
File the slab-day record
Have ready:the kit's slab-day sheetthe bills, delivery log and steel photos
Before you leave site, fill in the slab-day sheet: cast date and start and finish times, grade and volume, bags used against bags on site, the batches each cube sample came from, any stop and where it happened, and the curing and strip dates. IS 456 lists pour cards and site inspection records among the papers a job should keep (cl. 10.1.3). When the 28-day cube report comes back, it goes into the same file.
What goes wrong
Variants: what changes the steps
Ready-mix concrete and a pump
The plant mixes and the pump places, so step 4 becomes booking trucks at a steady interval and checking each delivery ticket for grade, volume and the time it left the plant. Keep the tickets with the slab-day sheet. Cube sampling for plant concrete can be agreed between supplier and buyer (IS 456 cl. 15.2.2 note), so settle who casts the cubes before the day.
Site-mixed with a lift or head pans
The mixer sits at ground level and the concrete goes up by a hoist or by hand. Count the carrying crew against the mixer's output so batches don't wait. The 1.5 m free-fall limit applies where it is tipped onto the slab.
Hot, dry weather
Pour early in the day or in the evening and keep the surface from drying out. IS 456 cl. 13.1.1 asks for deep containers so less water evaporates on the way, and cl. 13.5.1 raises the minimum curing to 10 days for any cement in hot, dry weather.
Rain likely on the day
Keep polythene sheets and a few people ready to cover fresh concrete, and decide with the engineer beforehand whether a heavy forecast means moving the date. A finished slab under a sudden shower needs covering, not more water.
Tips and tricks
- Book the engineer's steel inspection for the afternoon before the pour, not the morning of it. If something needs fixing, the bar bender has the evening to do it.
- Lay walking planks across the top steel before the crew arrives. It keeps the top bars on their chairs.
- Write the batch count on a board by the mixer. When bags and batches stop matching, you find out at once, not at the end of the day.
- Keep the cube moulds oiled and the 16 mm tamping bar on site the day before, so sampling doesn't wait on someone's errand.
- Ask the curing helper to sign a sheet three times a day. A slab that dries out on the second afternoon doesn't show it until much later.
- Take the photos of the steel with a tape in the frame for the cover blocks and the bar spacing.
What we would do
We would book the engineer's inspection for the afternoon before, start mixing at first light with a spare vibrator on site, and pour without stopping up to a joint the engineer has already named. We'd take the cubes from early, middle and late batches, write the curing and strip dates on the wall of the site shed the same evening, and wait for the engineer to say when the props come out. If the slab is more than about 20 m³, we'd ask the engineer whether ready-mix and a pump make more sense than a site mixer.
Print the working copy
Slab casting day sheet, from the Slab Casting Day Kit. One A4 page with tick boxes and blanks; it prints in black and white.
- 1. From the drawings
- 2. Day before
- 3. The pour
- 4. Dates (earliest; engineer confirms)
Frequently asked questions
How many cement bags do I need for a 1,000 sq ft roof slab?
What should be checked before slab casting?
How many cubes should be taken during slab casting?
How long does it take to cast a house roof slab?
Can the slab casting stop halfway and continue the next day?
When can the slab shuttering be removed?
The source we built this on
Read 18 Oct 2026 from the page images (printed pp. 24 to 29). The bag figure uses the CPWD Delhi Analysis of Rates 2023 item 4.1.2 coefficient behind our cost model (checked Jul 2026). The 14 m³ slab, the bags-per-batch count and the cover-block count are our worked examples, not figures from the code.
This plan covers quantities, ordering, the order of the day and records. The slab's design, its steel, the grade and mix, any joint and the stripping of props are for a licensed structural engineer. Nobody stands under a slab while it is being cast or while its props are being moved.