Yes, you can build a raised deck over an existing concrete patio in New Zealand — and it's a popular way to upgrade an outdoor space without demolishing the slab. However, the project involves structural decisions, potential building consent requirements, and drainage considerations that you'll need to get right before you start.
Does Your Deck Need a Building Consent?
This is the first question to answer. In New Zealand, building consent is required for most decks, but there are exemptions under Schedule 1 of the Building Act 2004. As a general guide:
- A single-storey deck no more than 1.5 m above the ground may be exempt from consent, provided it meets all other Schedule 1 conditions.
- Decks attached to the house, or those that could affect the structure or weathertightness of the building, are more likely to need consent.
- If the deck is higher than 1.5 m above ground level, consent is almost certainly required.
Because the rules depend on your specific site, structure, and local council requirements, always confirm with your local council before starting work. You can find general guidance at MBIE Building Performance.
Assessing the Existing Concrete Slab
Before designing your deck, take a good look at the concrete patio underneath:
- Condition: Check for major cracks, heaving, or crumbling edges. A structurally sound slab makes a great base; a badly deteriorated one may need repair or partial removal first.
- Drainage: Concrete patios are usually laid with a slight fall away from the house. Your deck framing must not trap water against the house or create ponding under the deck. Good sub-deck ventilation and drainage are essential to meet the NZ Building Code (particularly clause E1 – Surface Water).
- Thickness: If you plan to core-drill or bolt into the slab for post bases, you'll need to know the slab thickness (typically 75–100 mm for a residential patio).
Fixing Posts and Bearers to the Concrete
This is where building over a concrete patio differs from building on bare ground. You have a few options:
Bolt-Down Post Bases
Bolt-down post bases (also called post anchors) are the most common approach. These steel brackets are fixed to the concrete with chemical anchors or expansion bolts, then the timber post slots into the bracket. This keeps the timber up off the concrete surface, reducing moisture contact and rot risk.
Core-Drilling for In-Slab Posts
For taller or heavier decks, posts can be set into core-drilled holes in the slab and grouted in place, or the slab can be broken out at post locations to allow new concrete footings to be poured to the required depth. This is more involved and may require engineering input.
Low-Profile Framing
If you only want to raise the deck a small amount above the slab (say, 150–300 mm), a bearer-and-joist system sitting on low-profile adjustable feet directly on the concrete is an option. Ensure there is still adequate airflow beneath the deck.
For all fixing methods, the structural requirements should comply with NZS 3604:2011 (Timber-framed buildings) or be specifically engineered. If your deck requires consent, the council will want to see this addressed in your plans.
Choosing the Right Timber
Because the area under a raised deck over concrete can be damp, timber selection matters:
- Use H3 or H4 treated timber for framing members close to or in contact with concrete or the ground. H3 is suitable for above-ground exposed use; H4 for in-ground or concrete contact. Note that direct timber-to-concrete contact requirements should be confirmed with your supplier or engineer, as treatment level requirements can depend on specific exposure conditions.
- For decking boards, H3.2 treated pine is a common and cost-effective choice. Hardwoods such as kwila (merbau) or composite decking are also popular.
- Check treatment levels on the timber label — reputable suppliers at Bunnings, Mitre 10, or PlaceMakers can advise on the right grade.
Drainage and Weathertightness
Water management is critical, especially where the deck meets the house:
- Maintain a minimum clearance between the finished deck surface and any wall cladding or door threshold (check your specific cladding manufacturer's requirements — for fibre cement products, refer to James Hardie).
- Ensure flashing and waterproofing at the ledger board (the board bolted to the house) are correctly installed to prevent water ingress — this is a common source of leaks.
- Leave gaps between decking boards (typically 5–8 mm) to allow water to drain through freely.
- The concrete slab below will continue to drain as it did before, which is an advantage of building over an existing patio.
Do You Need a Licensed Building Practitioner?
If your deck requires a building consent, the restricted building work provisions apply. Structural and weathertightness work on a consented deck must be carried out or supervised by a Licensed Building Practitioner (LBP). The LBP will issue a Record of Work on completion, which is important for your property records and any future sale.
Even if your deck is exempt from consent, hiring an experienced deck builder gives you confidence that the structure is safe, durable, and built to code. You can find qualified local deck builders on tradedirectory.co.nz.
When to Call a Professional
Call a qualified tradesperson or structural engineer if:
- Your deck will be higher than 1.0 m above ground (and especially above 1.5 m).
- The deck will be attached to the house and could affect weathertightness.
- You're unsure about the condition or load capacity of the existing slab.
- The project requires a building consent — an LBP must be involved.
- You need electrical work (outdoor lighting, power points) — this is restricted work requiring a registered electrician under the EWRB.
Licensed Building Practitioner (LBP)
