Roofing paint is designed for metal or membrane roofs — not for trafficable concrete decks — so while it may offer some short-term water resistance, it is generally not a suitable waterproofing solution for a concrete deck that people walk on. For a durable, code-compliant result you'll need a product specifically formulated for trafficable, horizontal concrete surfaces.
Why Roofing Paint Falls Short on Concrete Decks
Roofing paints are typically formulated for non-trafficable surfaces — they're not designed to handle foot traffic, furniture, or the flexing and thermal movement that a concrete deck experiences. On a horizontal deck they will:
- Wear through quickly under foot traffic
- Peel or blister as water gets trapped beneath the coating
- Crack as the concrete expands and contracts with NZ's temperature swings
- Potentially void any manufacturer warranty
More importantly, a concrete deck that sits over a habitable space (like a garage or room below) must comply with NZ Building Code Clause E2 (External Moisture), which requires the waterproofing system to prevent moisture ingress into the structure. A roofing paint is unlikely to meet this performance requirement.
What the NZ Building Code Requires
If your concrete deck is above an enclosed space, waterproofing is a building performance issue, not just a cosmetic one. MBIE Building Performance provides guidance on Clause E2, which sets out acceptable solutions for keeping water out of the building envelope.
For decks over habitable or enclosed spaces, you may also need a building consent — always check with your local council before starting work. Some straightforward re-coating of an existing waterproof deck may fall under Schedule 1 exemptions, but this depends on the scope of work and your council's interpretation.
Suitable Waterproofing Products for Concrete Decks
For a trafficable concrete deck, look for products in these categories:
Polyurethane Membrane Systems
Polyurethane-based waterproofing membranes are one of the most common solutions for concrete decks in NZ. They are flexible, UV-stable, and designed to handle foot traffic when a suitable topcoat or aggregate is applied. Brands like Sika offer trafficable deck waterproofing systems specifically rated for this application.
Cementitious Waterproofing
For decks that will be tiled, a cementitious waterproofing slurry (applied under tiles) is a well-established method. Products from Ardex and Mapei include systems designed for wet areas and external concrete surfaces. These are rigid systems, so they suit stable substrates.
Acrylic Deck Coatings
Heavy-duty acrylic elastomeric deck coatings are another option for lower-traffic areas. These are more flexible than standard paint and are formulated for horizontal concrete. Check that any product you choose is specifically labelled for trafficable, exterior concrete decks.
Key Factors Before You Start
| Factor | What to Check |
|---|---|
| Deck condition | Cracks, spalling, or movement joints need repair first |
| Drainage | Deck must have adequate fall (typically min. 1:60) to drain water away |
| Existing coatings | Old paint or sealers must be removed or confirmed compatible |
| Below the deck | Is there a habitable space? If so, Building Code E2 applies |
| Building consent | Check with your council — may be required |
Surface Preparation Is Everything
No waterproofing system will perform well on a poorly prepared surface. For concrete decks:
- Remove all old paint, sealers, oil, and loose material — mechanical grinding or scarifying is often needed
- Repair cracks and holes with a suitable concrete repair mortar before waterproofing
- Ensure the surface is clean and dry (moisture content matters — check the product datasheet)
- Prime as directed by the manufacturer — skipping the primer is a common DIY mistake
Always follow the manufacturer's application instructions, including minimum and maximum recoat times and temperature requirements. NZ's UV levels are high, so UV stability is important for any exposed topcoat.
DIY vs. Professional Application
Some trafficable deck coatings are available to homeowners at Bunnings or Mitre 10, and a competent DIYer can apply them to a small, straightforward deck. However, for decks over enclosed or habitable spaces, multi-coat membrane systems, or any work requiring building consent, professional application is strongly recommended.
Waterproofing failures on decks are one of the most common and costly building defects in NZ — getting it right the first time saves significant expense down the track.
When to Call a Professional
You should engage a qualified waterproofing tradesperson if:
- The deck is over a habitable or enclosed space
- There is existing water damage or structural concerns
- The deck area is large or has complex details (penetrations, upstands, drains)
- A building consent is required
- You are unsure which system is appropriate for your substrate
A professional waterproofer will select a system that meets MBIE Building Performance requirements, apply it correctly, and often provide a warranty on their work. You can find qualified local waterproofing tradespeople on tradedirectory.co.nz.
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