METHOD COMPARISON
Roof Waterproofing in Hong Kong: PU, Polyurea, or Membrane?
Quick answer
In Hong Kong, 'PU waterproofing,' 'polyurea' and 'membrane' are frequently mixed up, but they describe different things: PU injection grouting (打針) pumps PU resin into cracks to chase and seal a water path — used for localized cracks and joints, distinct from surface coating; and for large-area roof waterproofing, the Buildings Department's Minor Works Control System item 29 splits roof-surface waterproofing into 'coating-type' (liquid-applied, including PU and polyurea) and 'membrane/felt-type' — both exempt from registered-contractor requirements as long as no tile or screed layer is opened. Generally speaking, which approach fits depends on an on-site inspection — the breakdown below explains why.
The three terms you may be mixing up
'PU waterproofing' covers at least two completely different processes in Hong Kong: (1) PU injection grouting (打針) — pumping high-pressure PU resin into cracks or joints from the wet side to chase and seal a water path, used for localized ceiling/wall cracks and window-frame joints, a non-demolition technique; and (2) PU waterproofing coating — applying a liquid coating over a large surface such as a roof or bathroom to form a continuous waterproof layer. Polyurea is another liquid-coating material, grouped with PU coating under what the Buildings Department calls 'coating-type' works. A membrane system, by contrast, uses prefabricated felt/membrane sheets laid over the surface — a different application method entirely from coating-type work.
- PU injection grouting — a localized crack/joint injection technique, not large-area surface waterproofing.
- PU/polyurea coating — a liquid coating applied over a surface such as a roof or bathroom, what the Buildings Department calls 'coating-type.'
- Membrane/felt system — prefabricated sheet material laid over the surface, what the Buildings Department calls 'membrane/felt-type.'
How the Buildings Department splits it (the regulatory angle)
Under the Buildings Department's Minor Works Control System, designated exempted works item 29 covers installing or replacing a roof-surface waterproofing layer, coating-type or membrane/felt-type — neither requires a registered minor-works contractor, but the exemption explicitly excludes any work that lays or removes a tile or screed layer (that scope falls under Minor Works items 1.62/2.34 and needs a registered contractor). This regulatory split isn't a technical performance comparison, but it does tell you something useful: pure surface coating or membrane renewal (without opening tile/screed layers) is what qualifies for the exemption. As for internal bathroom renovation: the Buildings Department FAQ confirms that simple in-flat works — plastering, laying tile finishes, repairing or replacing sanitary fitments — are generally exempted building works; see the bathroom waterproofing service page for the exact wording and boundaries.
Which approach generally fits which situation
| Situation | Common approach |
|---|---|
| Localized ceiling/wall crack seepage | PU injection grouting — non-demolition, chases a local water path |
| Roof surface ageing over a large area (sound substrate) | PU/polyurea coating (coating-type) — generally covered by the item 29 exemption |
| Roof waterproofing layer failed / needs replacing | Membrane (felt-type) or full removal — depending on whether tile/screed layers are opened |
| Bathroom surface renewal (sound structure) | Surface re-waterproofing coating — generally within the Buildings Department FAQ exemption |
| Bathroom full strip-and-rebuild | Membrane/waterproofing layer with new screed — a full-removal-tier job |
Generally speaking, which approach actually applies depends on an on-site inspection (substrate condition, seepage source, structural integrity) — this comparison is directional at the method level, not a substitute for that assessment.
Warranty terms, as the market generally frames them
Per published operator data, warranty periods generally scale by system tier (a market convention, not a promise unique to us): injection work around 2 years; surface renewal/coating tiers around 1-7 years (bathroom surface renewal around 1-3 years, roof surface coating around 5-7 years); full-removal/membrane structural work up to around 10 years by market convention. These are observed market ranges, not product specification claims — the actual warranty period and trigger conditions should always follow the written quote.
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Frequently asked questions
Is injection grouting the same thing as PU waterproofing coating?
No. Injection grouting is a localized technique that chases and seals a water path in a crack; PU coating is a large-area surface treatment forming a continuous waterproof layer. Both may use PU-based material, but the application method and use case are completely different.
Is polyurea better than PU?
Both fall under the liquid-coating category (what the Buildings Department calls 'coating-type'). We don't yet have enough technical performance comparison evidence to say one is definitively better — the actual choice generally depends on the on-site inspection, budget, and which application method the contractor is experienced with.
Does roof waterproofing always require removing the tiles?
Not necessarily. If the substrate is sound and just ageing, item-29-exempt coating-type or membrane-type surface renewal is usually enough. Only a failed waterproofing layer or structural-layer damage calls for tile/screed removal — which does require a registered contractor.