Waterproofing Membranes Are Common Property — Unless Your Plan Says Otherwise

Reviewed and updated 14-10-2026. We removed the law-firm quotes and the planning-approval section, which we could not verify against the legislation, and now explain the rule from the Act and the Court of Appeal's decision in Seiwa. The position itself has not changed: by default the membrane is common property. We changed the heading because, under the default rule, the membrane sits outside your lot, not inside it.

If water is coming through a floor, balcony, courtyard or planter box, here's the rule to remember: the waterproofing membrane underneath is, by default, common property — in a townhouse or villa just as in an apartment. That makes it the owners corporation's job to repair. What can change this is your registered strata plan or a registered by-law.

What's a membrane and why does it matter?

A waterproofing membrane is the layer that stops water getting through a floor or slab into whatever's underneath. You don't usually see it because it sits under your tiles, pavers, soil or screed.

You'll find membranes:

• Under bathroom and laundry floors
• Under balcony tiles or paving
• Under planter boxes and raised gardens
• Under courtyard paving and outdoor entertaining areas
• Beneath flat roof finishes

When membranes fail, water finds its way through, and that's when ceilings stain, walls bubble, mould appears and timber starts to rot.

The legal position — common property by default

Lot boundaries are set by section 6 of the Strata Schemes Development Act 2015 (NSW). Where the strata plan doesn't say otherwise, the lower boundary of a lot is the upper surface of the floor and the upper boundary is the under surface of the ceiling (s6(1)(a)(ii)). Everything below the floor surface — the slab, the screed and the waterproofing membrane — is outside the lot, so it is common property.

This is the same in townhouses and apartments, and it applies whether or not there is another home underneath. In The Owners – Strata Plan No 35042 v Seiwa Australia Pty Ltd [2007] NSWCA 272, the NSW Court of Appeal applied this rule to a membrane under tiles and held that both the tiles and the membrane were common property.

In a two-storey townhouse the same rule applies to the floor between your own two levels. The joists, sheet flooring, screed and membrane under a first-floor bathroom sit outside the lot's airspace and are common property unless the plan says otherwise.

When the answer can be different

• A notation on the registered strata plan. A boundary described on the floor plan overrides the default (s6(1)(b)) — for example, a notation that the lot includes the building structure. Many townhouse plans carry notations like this, so the plan must be read before anyone says who pays.
• A common property rights by-law. A registered by-law made under section 143 of the Strata Schemes Management Act 2015 can make an owner responsible for maintaining a particular area. It needs the benefiting owner's written consent.
• Approved alterations. Where an owner was approved to alter common property, the special resolution under section 108 must say who maintains the altered area.

What about the tiles on top?

The boundary is fixed when the strata plan is registered and does not move later (Seiwa [2007] NSWCA 272 at [39]). Tiles laid before registration are part of the floor and are common property too ([37]–[39]).

Where finishes have to come up so a membrane can be repaired, who pays to replace them depends on when they were laid and on your scheme's by-laws. We work that out with you before the work starts.

So who pays?

The owners corporation has a strict duty under section 106 of the Strata Schemes Management Act 2015 to maintain and repair common property. "Strict" means there's no "we didn't have the money" or "we're getting around to it" defence.

If an owner caused the damage, the owners corporation still repairs the common property and can then recover the cost — by doing work the owner failed to do and recovering it (s120), or through an NCAT order (s132).

What to do if you've got a leak

1. Document it. Photos with dates, water stains, any expert reports.
2. Report it in writing to the strata manager. Email is fine — you want a record.
3. Don't fix the common property yourself and send the bill. The NSW Supreme Court has held that an owner can't recover the cost of common property work done without the owners corporation's approval or an NCAT order (Colman v The Owners – Strata Plan 61131 [2025] NSWSC 63).
4. Allow access. Most membrane problems need a building consultant or waterproofing specialist to find the actual source.
5. If the owners corporation doesn't act, NCAT can make orders, and an owner can claim damages for losses caused by the owners corporation's failure to repair (s106(5)), within six years of becoming aware of the loss (s106(6)). See our article on Section 106(5) damages claims.