Yes, pouring a concrete shed floor is one of the more achievable DIY concrete projects for a capable homeowner. It doesn't involve restricted licensed work, but it does require solid preparation, the right materials, and a realistic understanding of what's involved before you mix a single bag. Get it wrong and you'll be living with a cracked, uneven slab for years — so read on before you start.
Do You Need a Building Consent?
Before you lift a shovel, check whether your project needs a building consent. In New Zealand, many small shed slabs fall under Schedule 1 exemptions of the Building Act 2004, meaning no consent is required — but the exemption depends on factors like the shed's size, use, and proximity to boundaries or other structures. Always confirm with your local council before starting. You can find general guidance at MBIE Building Performance.
What You'll Need
Tools
- Tape measure, string lines and pegs
- Spade and wheelbarrow
- Timber formwork (framing timber works well)
- Screed board (a straight length of timber)
- Bull float or darby
- Steel trowel and edging tool
- Concrete mixer (hire one) or access to a ready-mix truck
- Tamping rod or vibrating screed (hireable)
- Safety gear: gloves, safety glasses, rubber boots, and knee pads
Materials
- Compactable gravel or AP65 hardfill (at least 100 mm deep)
- Polythene DPM (damp-proof membrane) — minimum 0.25 mm thick (confirm current specification with your supplier or BRANZ guidance)
- Concrete: either bagged mix or ready-mixed (see below)
- Steel mesh reinforcing (typically 665 mesh for a shed floor)
- Bar chairs to hold mesh at mid-slab depth
- Curing compound or plastic sheeting
Step-by-Step Overview
1. Plan and Mark Out
Decide on your slab dimensions and mark them out with string lines. A typical shed floor is 100 mm thick for light use. Allow for a slight fall (around 1:100) away from the shed if drainage is a concern.
2. Excavate and Compact
Dig out topsoil and organic material — you want to be sitting on firm subgrade. Lay and compact your hardfill in layers. Poorly compacted base material is the number-one cause of cracked slabs.
3. Set Your Formwork
Nail together your timber forms to the exact perimeter of the slab. Check they're level (or on your intended fall) and well-staked so they won't move when concrete is poured.
4. Lay the DPM and Mesh
Lay your polythene membrane over the compacted base, lapping joins by at least 200 mm. Place your reinforcing mesh on bar chairs so it sits roughly in the middle of the slab depth. BRANZ has useful guidance on slab construction best practice.
5. Order or Mix Your Concrete
For anything larger than a small slab, a ready-mix truck is strongly recommended — it's faster, more consistent, and less back-breaking than mixing bags. Specify 20 MPa concrete for a standard shed floor (25 MPa if you'll park a vehicle on it). For small slabs, bagged concrete from Bunnings or Mitre 10 is fine.
6. Pour, Screed and Float
Pour the concrete into the forms, working it around the mesh with a rake. Screed off the surface using your straight-edge timber, then use a bull float to close up the surface. Work quickly — concrete sets faster on hot or windy days.
7. Finish the Surface
For a shed floor, a broom finish (dragging a stiff broom across the surface) gives good grip and is forgiving for a first-timer. A steel-trowelled finish is smoother but harder to achieve without experience.
8. Cure Properly
Curing is critical and often skipped. Keep the slab moist for at least 3–7 days by covering with plastic sheeting or applying a curing compound. Rushing this step leads to surface cracking and a weaker slab.
Concrete Quantities: A Quick Guide
| Slab Size | Thickness | Approx. Volume |
|---|---|---|
| 3 m × 3 m | 100 mm | 0.9 m³ |
| 4 m × 6 m | 100 mm | 2.4 m³ |
| 6 m × 9 m | 100 mm | 5.4 m³ |
As a rough guide, ready-mix concrete in NZ is typically ordered by the cubic metre — your supplier can help you calculate quantities. Always order a little extra to allow for wastage.
Safety First
Wet concrete is caustic and can cause serious skin burns with prolonged contact. Always wear rubber gloves, waterproof boots, and safety glasses. If concrete splashes on skin, rinse immediately with clean water. Check WorkSafe NZ for safe handling guidance.
When to Call a Professional
While a shed floor is DIY-friendly, there are situations where hiring a qualified concreter makes sense:
- Large slabs (over 25–30 m²) where timing and crew numbers matter
- Sloped or unstable ground requiring engineered fill or a thickened-edge slab
- Vehicle loads — garages for heavy vehicles may need an engineer-specified design
- If you're unsure about consent requirements — a Licensed Building Practitioner (LBP) can advise
- Poor results on a test mix — if your trial mix isn't working, stop and get advice
A professional concreter will have the equipment, experience, and crew to get the job done efficiently and to a high standard. You can find qualified local concreters on tradedirectory.co.nz.
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