For most New Zealand decks, the minimum recommended fall (slope) for adequate drainage is 1:100 — that's 1 mm of drop for every 100 mm of horizontal run, or roughly 1%. This slope should direct water away from the house and any structures, helping prevent moisture damage, slippery surfaces, and premature timber decay. Depending on your decking material and surface finish, a slightly steeper fall may be advisable.
Why Deck Drainage Matters
New Zealand's climate — particularly in wetter regions like the West Coast, Northland, and Wellington — means decks are regularly exposed to heavy rain. Poor drainage leads to pooling water, which accelerates timber rot, promotes mould and algae growth (making surfaces dangerously slippery), and can cause moisture to track back into the house structure. Getting the fall right from the start is far cheaper than remediation later.
The NZ Building Code Requirements
The NZ Building Code sets out performance requirements for drainage and weathertightness. Clause E1 (Surface Water) requires that surface water — including rainwater landing on a deck — is directed away from buildings and does not cause damage or nuisance. Clause E2 (External Moisture) is also relevant for any deck that is attached to or sits over habitable space.
For timber-framed decks built to NZS 3604, the standard provides guidance on construction details, but the specific drainage fall is typically addressed through good practice and manufacturer specifications rather than a single prescriptive number in the standard itself. Always check with your local council to confirm what applies to your specific project.
Recommended Falls by Deck Type
| Deck Surface | Minimum Recommended Fall | Notes |
|---|---|---|
| Open-jointed timber boards | 1:100 (1%) | Water drains between boards; fall still needed |
| Composite decking | 1:100 (1%) | Check manufacturer specs — some require up to 1:60 |
| Solid timber (close-jointed) | 1:60 to 1:80 | Less gap for water to escape |
| Waterproof membrane (over living space) | 1:60 to 1:50 | Steeper fall critical to protect structure below |
| Tiles or pavers on deck | 1:60 minimum | Grout joints reduce drainage capacity |
Always refer to your decking manufacturer's installation guide for their specific fall requirements, as these can vary and will affect your warranty.
How to Achieve the Right Fall
The fall is typically built into the deck frame at the joist or bearer level, not by angling the decking boards themselves (which would look odd and create trip hazards).
Practical methods include:
- Tapering the joists — cutting a slight angle along the top edge of each joist so the decking surface naturally slopes.
- Packing the posts — using different post heights or adjustable post bases on one side to create the overall fall across the frame.
- Adjustable deck feet — products available at Bunnings or Mitre 10 allow fine-tuning of height at each support point.
- Angling the ledger — where the deck attaches to the house, the ledger can be set slightly lower on one end to encourage water to run toward a drain or off the open edge.
For a 3-metre-wide deck with a 1:100 fall, you only need 30 mm of total drop across the width — subtle enough to be barely noticeable underfoot, but effective for drainage.
Decks Over Living Spaces
If your deck sits above a habitable room, garage, or enclosed space, drainage becomes critical. A waterproof membrane system is required, and the fall must be sufficient to move water quickly to a drain or outlet before it can pond and find a path through the membrane. In these situations, BRANZ recommends consulting a professional, as the consequences of getting it wrong are significant and costly.
Do You Need a Building Consent?
Many ground-level decks in NZ are exempt from building consent under Schedule 1 of the Building Act, but this depends on the deck's height, size, and proximity to boundaries or other structures. Always confirm with your local council before starting work — exemptions have specific conditions, and building without consent when one is required can cause serious problems when you sell your home. More information is available at MBIE Building Performance.
When to Call a Professional
While setting a deck fall is within the capability of a competent DIYer, there are situations where you should engage a qualified deck builder or Licensed Building Practitioner (LBP):
- The deck is more than 1.5 metres above ground (restricted building work requiring an LBP — see LBP).
- The deck is over a habitable space and requires a waterproof membrane system.
- You are unsure about subsoil drainage, stormwater connections, or the deck's interaction with the house's weathertight envelope.
- The project requires a building consent.
For decks that include any electrical work (outdoor lighting, power points), this must be carried out by a registered electrician — see EWRB for licensing requirements. Similarly, any connection to stormwater drains is restricted drainlaying work under the Plumbers, Gasfitters and Drainlayers Board.
You can find qualified local deck builders and LBPs on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
