Levelling concrete after pouring is one of the most time-sensitive jobs in DIY construction — get the timing right and use the correct technique, and you'll end up with a flat, durable slab. Rush it or leave it too long, and you risk a surface that's uneven, weak, or prone to cracking. Here's what you need to know to do it properly.
Gather the Right Tools Before You Start
Having everything on hand before the concrete arrives (or before you mix) is essential — concrete won't wait for you to find a missing tool. You'll need:
- Screed board (or screed rail) – a straight length of timber or aluminium, typically 50 × 100 mm, long enough to span your formwork
- Bull float – a large, flat magnesium or aluminium float on a long handle for initial smoothing
- Hand float (magnesium or wood) – for working edges and smaller areas
- Steel trowel – for the final smooth finish if required
- Darby – a long hand-held float useful for levelling before bull floating
- Knee boards – to kneel on the slab without leaving marks
- Straight edge or spirit level – to check flatness as you go
- Edging tool – to round off slab edges and reduce chipping
For larger pours, a power screed (vibrating screed) can be hired from most NZ hire centres and makes the job significantly easier.
Set Your Screed Rails or Guides
Before pouring, set up screed rails — these are your height guides that tell you exactly where the top surface of the concrete should sit. Use timber or steel stakes driven into the ground, with the top of your formwork or guide rails set to the finished slab height. Check everything is level with a spirit level or laser level. For slabs near buildings, remember the NZ Building Code requires a minimum fall away from the structure to manage drainage — typically at least 25 mm over the first metre. Check with your local council if you're unsure about drainage requirements for your specific project.
Screeding: The First Levelling Pass
Once the concrete is poured and roughly spread with a rake or shovel, it's time to screed. Place your screed board across the top of your formwork or guide rails and use a back-and-forth sawing motion while pulling it towards you. This levels the concrete to your guide height and removes high spots.
- Work in sections of 1–2 metres at a time
- Fill any low spots immediately with extra concrete and re-screed
- Don't overwork the surface — excessive screeding can bring too much water and cement to the top, weakening the surface
- For larger slabs, a vibrating screed helps consolidate the mix and remove air pockets
Bull Floating: Smoothing the Surface
After screeding, push a bull float across the surface in overlapping passes. This embeds aggregate slightly below the surface, removes screed marks, and produces a flatter finish. Keep the leading edge slightly raised to avoid digging in.
For edges and corners, use a hand darby or hand float to reach areas the bull float can't access. Run an edging tool along the perimeter to create a neat, rounded edge.
Timing Is Everything: Waiting for Bleed Water
After bull floating, you'll notice bleed water — a sheen of water rising to the surface. This is normal. Do not work the surface while bleed water is present. Doing so will weaken the top layer and cause dusting or scaling later. Wait until the sheen disappears and the concrete can support your weight on knee boards without leaving deep impressions — typically 30 minutes to a few hours depending on the mix, temperature, and humidity.
In hot, dry NZ summers (particularly in Canterbury or Hawke's Bay), concrete can set faster than expected. In cooler, humid conditions (think Southland winters), it may take much longer. Keep an eye on conditions and adjust your pace accordingly.
Final Floating and Trowelling
Once bleed water has gone and the surface has stiffened slightly:
- Magnesium float finish – work in circular or sweeping arcs for a slightly textured, slip-resistant surface. Ideal for driveways, paths, and outdoor areas.
- Steel trowel finish – use firm, overlapping passes for a smoother, harder finish. Suitable for garage floors or interior slabs. Multiple passes as the concrete stiffens will produce a denser surface.
- Broom finish – drag a stiff broom lightly across a floated surface for a textured, non-slip result — popular for NZ driveways and pool surrounds.
Curing: Don't Skip This Step
Levelling is only half the job. Curing — keeping the concrete moist for at least 7 days — is critical for strength development. Cover the slab with plastic sheeting, wet hessian, or apply a curing compound (available from Bunnings or Mitre 10). Skipping curing can reduce the final strength of your slab by up to a third. BRANZ has useful guidance on concrete slab construction and curing best practice.
Does Your Project Need a Building Consent?
Many small concrete slabs (like a garden path or small shed base) may be exempt from building consent under Schedule 1 of the Building Act — but this depends on size, use, and location. Always check with your local council before starting. The MBIE Building Performance website is a good starting point for understanding consent requirements.
When to Call a Professional
Some concrete work is best left to a qualified concreter, particularly:
- Large or structural slabs (house foundations, retaining walls) — these may require engineering input and must comply with NZS 3604 or specific engineering design
- Reinforced concrete where rebar placement and cover is critical
- Slabs with in-slab heating or plumbing — any in-slab plumbing is restricted work under the Plumbers, Gasfitters and Drainlayers Act and must be carried out by a licensed tradesperson registered with the PGDB
- Decorative or polished concrete requiring specialist equipment
- If you're unsure about any aspect of the pour, a qualified concreter will save you time, money, and frustration
You can find experienced local concreters on tradedirectory.co.nz.
Sources & Further Reading
- BRANZ – Concrete slab construction guidance
- MBIE Building Performance – Building consents and Schedule 1 exemptions
- Standards NZ – NZS 3604 Timber-framed buildings and related standards
- PGDB – Licensed plumbers for restricted in-slab work
- Bunnings – Concrete tools and curing compounds
- Mitre 10 – Concrete finishing supplies
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