Getting the overlap right when joining sheet membrane sections is one of the most critical steps in any waterproofing job. Too little overlap and you risk water tracking through the join; too much and you waste material and create unnecessary thickness. As a general rule, most manufacturers specify a minimum overlap of 50 mm to 100 mm, but the exact figure depends on your specific product, the application area, and the requirements of the NZ Building Code.
Why Overlaps Matter So Much
Sheet membranes — whether self-adhesive bituminous, torch-on, or peel-and-stick polymer types — rely on continuous, unbroken coverage to keep water out. A join is inherently a weak point. If the overlap is too narrow, capillary action can draw water between the layers, especially under hydrostatic pressure or in high-rainfall NZ conditions. A correctly lapped and bonded join effectively becomes a double layer of protection at the most vulnerable point.
The General Overlap Minimums
While you must always defer to your product's technical datasheet, the following are widely accepted minimums across the industry:
| Application | Typical Minimum Overlap |
|---|---|
| Horizontal (flat/low-slope) surfaces | 100 mm |
| Vertical surfaces and upstands | 75–100 mm |
| End laps (where rolls meet end-to-end) | 150 mm |
| Internal corners and details | Full coverage + 150 mm minimum flashing strip |
End laps — where one roll ends and the next begins along the same run — generally require a larger overlap (150 mm or more) because they run perpendicular to water flow and are more exposed to lifting.
NZ Building Code Considerations
In New Zealand, waterproofing work in wet areas (bathrooms, showers, laundries) is governed primarily by Clause E3 (Internal Moisture) of the NZ Building Code, while external waterproofing falls under Clause E2 (External Moisture). These clauses don't always prescribe exact overlap dimensions — instead, they require that the system as a whole achieves the performance standard. This means the membrane, its joins, and all flashings must work together to prevent moisture ingress.
For compliance, your membrane system should ideally follow an Acceptable Solution (such as E2/AS1 for external moisture) or be supported by a specific Alternative Solution with documented evidence. MBIE Building Performance publishes the Acceptable Solutions and Verification Methods for each clause — worth bookmarking if you're doing any serious waterproofing work.
BRANZ also provides detailed guidance on waterproofing best practice for NZ conditions, including lap requirements for various membrane types.
Always Follow the Manufacturer's Datasheet
This cannot be stressed enough: the manufacturer's technical datasheet is your primary reference. Some products specify 50 mm as adequate for side laps in sheltered vertical applications; others require 100 mm as a minimum everywhere. Deviating from the datasheet can void the product warranty and may mean your installation doesn't meet the Building Code.
Products from suppliers such as Sika and Ardex include detailed installation guides with specific overlap requirements for each membrane in their range. Mapei similarly provides product-specific technical data sheets. Download and read these before you start — not halfway through the job.
Preparing the Overlap Zone Correctly
A correctly sized overlap is only half the battle. The bonding within that overlap must also be sound:
- Clean the surface — dust, oil, or moisture will prevent adhesion.
- Prime if required — many sheet membranes require a compatible primer on the substrate and sometimes on the membrane surface itself before lapping.
- Apply consistent pressure — use a roller or seam roller to firmly press the overlap down, working out any air bubbles from the centre outward.
- Check for fish-mouths — any wrinkles or lifted edges at the lap must be cut, re-bonded, or patched before covering.
- Stagger end laps — where possible, offset end laps so they don't all fall on the same line, reducing the risk of a continuous weak point.
Torch-On vs Self-Adhesive Membranes
Torch-on (heat-welded) membranes require the overlap zone to be fully melted and fused — you should see a small bead of bitumen squeeze out along the edge of the lap as confirmation of a good bond. Self-adhesive membranes rely on pressure and sometimes a primer; the release liner must be fully removed from the overlap area before bonding.
For liquid-applied membranes used in conjunction with sheet reinforcement, overlap requirements are different again — typically governed by the reinforcing fabric width and the number of coat applications.
When to Call a Professional
Waterproofing is one of those trades where mistakes are expensive and often hidden until serious damage has occurred. Consider engaging a qualified waterproofing tradesperson if:
- You're working on a deck, balcony, or roof where failure could cause significant structural damage.
- The area involves complex details such as drains, penetrations, or internal corners.
- A building consent is required — check with your local council, as some waterproofing work on habitable spaces may need consent.
- You're unsure which membrane system is appropriate for your substrate or exposure conditions.
A Licensed Building Practitioner (LBP) may be required to carry out or supervise certain restricted building work. Check the Licensed Building Practitioners register to find qualified tradespeople, or search for a local waterproofing specialist on tradedirectory.co.nz.
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