⚠️ Safety note: Negative-side waterproofing is an engineering-grade application where workmanship directly affects performance. Where the leak source can be addressed from the exterior, that should always be the priority — negative-side treatment is a remedial solution, not a replacement for proper external waterproofing. If the structure shows severe cracking or settlement, have it assessed by relevant professionals first. This article is general guidance only; the actual approach depends on site conditions and the product Technical Data Sheet (TDS).
What is negative-side waterproofing? Why apply from inside?
Waterproofing works in two pressure directions: positive and negative:
- Positive pressure (water-facing side) — water pushes against the coated surface, e.g. bathrooms, water tanks, pools. The coating directly blocks the water — this is the most common approach.
- Negative pressure (dry side) — water seeps in from behind the substrate, and the coating is applied on the opposite (interior) side to resist it. A typical example: a basement wall surrounded by soil, where water seeps through concrete cracks into the interior, but you can only work from the inside.
Negative-side waterproofing demands more from the material than positive-side — the coating must bond to the concrete surface while water pressure pushes against it, without being lifted off. Not every waterproofing product can do this; it requires materials specifically designed for the purpose.
Three common negative-side scenarios
The water sources for negative-side waterproofing fall into three main types. Although the application is always from the interior, the causes and behaviour of the seepage differ:
Above-grade exterior wall: wind and rain seepage
In Hong Kong's older buildings, public housing or densely packed blocks, exterior walls are constantly exposed to heavy rain, typhoons and weathering. When the concrete ages, develops cracks or the window-perimeter sealant fails, rainwater seeps through the exterior wall into the interior. But due to ownership disputes, scaffolding constraints or jurisdictional issues (e.g. public housing exteriors are managed by the Housing Authority), renovation teams cannot carry out exterior repairs. In these situations, applying a negative-side waterproofing coating from the interior can resist rainwater penetrating through the exterior wall.
Below-grade: soil and groundwater seepage
The exterior face of basements, underground spaces and retaining walls is in direct contact with soil and groundwater. Water pressure continuously pushes through concrete cracks and pores into the interior, and the exterior is buried in earth, making excavation physically impossible. This is the most classic and direct technical application of negative-side waterproofing.
Same-unit bathroom seepage: bathroom leaking into the living room
Another common renovation scenario: the waterproofing in your own bathroom or kitchen has aged, and water seeps through the wall or floor slab into the living room or bedroom. The source is within the same unit, but the owner may choose not to renovate the entire bathroom/kitchen due to cost (especially during a partial renovation), or the waterproofing problem cannot be fully resolved in the short term.
In MiC (Modular Integrated Construction) units, this situation is more pronounced: the bathroom and kitchen waterproofing is factory-prefabricated as part of the module and is already completed when the unit leaves the factory. Renovation teams find it very difficult to dismantle and redo it, and the cost is high. If the owner chooses not to renovate the bathroom/kitchen but the living room wall already shows signs of seepage, applying a negative-side waterproofing coating on the affected living room wall can serve as short-term protection — controlling the damage from bathroom seepage to the living room finishes.
⚠️ This is a temporary remedial measure and cannot permanently solve the problem. The source of seepage (failed bathroom/kitchen waterproofing) remains, and the long-term solution is to renovate the bathroom/kitchen waterproofing.
When is it used in Hong Kong?
Negative-side waterproofing is used in several common Hong Kong scenarios, all because exterior access is impossible:
Old buildings and "three-nil buildings"
In older buildings without an Incorporated Owners' Corporation, residents' organisation or property management company (commonly known as "three-nil buildings" / 三無大廈), ownership is fragmented, scaffolding costs are prohibitive, and coordinating exterior wall repairs is simply not feasible. Negative-side waterproofing from the interior is a remedial option that allows the owner to actively manage water ingress.
Public housing and newly completed developments under warranty
Public housing exteriors are managed by the Housing Authority; newly completed private developments have their exteriors under the developer's responsibility during the defects liability period. Interior renovation teams have no authority to touch the exterior — when interior seepage occurs, negative-side waterproofing can serve as temporary protection, controlling seepage while awaiting official repair by the responsible authority.
Slope retaining walls and basements
Hong Kong is built on hills, and many basements, carparks and lower-level spaces are backed directly against slopes. The exterior of the retaining wall is soil and groundwater, and excavation from outside is physically impossible. This is the most typical and direct technical application of negative-side waterproofing.
Modular Integrated Construction (MiC)
As MiC becomes more common in Hong Kong, its modular structure may present exterior-access constraints for future maintenance. When seepage occurs at module joints, negative-side waterproofing may serve as one of the interior remedial options, though the actual approach depends on the specific structure — consultation with relevant professionals is recommended.
👉 Detailed scenario analysis: Negative-side waterproofing product range
Negative-side waterproofing materials are applied to concrete or mortar substrates to form a waterproof layer. If the source of seepage is the window or door frame itself (e.g. aged aluminium window deformation, failed perimeter sealant, rotted timber frame, corroded metal frame), applying a waterproof coating on the adjacent wall will not solve the frame leak. These situations require frame-specific repair — such as replacing the sealant, repairing or replacing the frame, or redoing the window-perimeter waterproofing joint. Negative-side waterproofing is primarily for water seeping through concrete walls or floor slabs (some products such as water-based epoxy types can also be used on timber substrates — refer to the product TDS).
Five negative-side waterproofing products compared
We currently stock five products confirmed by their TDS for negative-side waterproofing, across three different application types. Here is a practical comparison:
| Product | Type | Components | Application | Coating thickness | Pos. + neg. pressure | Mixing tools | Enclosed spaces | Key feature |
|---|---|---|---|---|---|---|---|---|
| Sikagard 720 EpoCem | Epoxy-modified cementitious mortar | 3-component | Trowel | Thicker (render finish) | Mainly negative | Electric mixer required | Ventilation needed | Dual epoxy + cement bonding; trowel-applied render-ready surface |
| SPEC CW100 | Polymer-modified cementitious | 2-component | Brush / roller / trowel / spray | Medium | ✓ Both | Electric mixer required | Standard | Handles both positive and negative pressure; flexible application methods |
| Ardex WPM 300 | Water-based epoxy membrane | 2-component | Brush / roller / squeegee / spray | Thinnest (membrane) | ✓ Both | ✓ No electric mixer needed | ✓ Low odour | Solvent-free, low odour, thinnest coating |
| Fosroc CM210 | Polymer-modified cementitious | 2-component | Brush / roller / trowel / spray | Medium | ✓ Both | Electric mixer required | Standard | Elastomeric coating; handles both pressures; good crack-bridging |
| Weber Elastic Slurry | Polymer-modified cementitious | 2-component | Brush / squeegee / roller | Medium | ✓ Both | Electric mixer required | Standard | Simple brush application; cured surface texture aids rendering and tiling adhesion |
How to read this table:
- Coating thickness: The thicker trowel-applied type can level and waterproof in one step, but uses a bit of space; the thinnest membrane type has the least impact on headroom, making it suited to tight basement spaces.
- Pos. + neg. pressure: If your project has both positive-pressure (e.g. bathroom) and negative-pressure (e.g. basement) requirements, choosing a "Both" product lets you standardise on one material.
- Enclosed spaces: Basements and underground spaces are poorly ventilated — a low-odour water-based epoxy membrane is safer and more comfortable to work with.
Application methods: a big difference
The five products fall into three application types, each with different requirements for the crew and the workflow:
Trowel-applied type (rendering)
Represented by Sikagard 720 EpoCem. After mixing the three components, it is trowelled onto the wall — the feel is similar to rendering. The finished surface is smooth and fine, ready to serve as a substrate for subsequent epoxy systems or decorative finishes. If your team does rendering work day to day, this method will feel natural, and it achieves waterproofing and surface preparation in one step.
Brush/roller-applied type
CW100, CM210 and Elastic Slurry are all cementitious brush/roller-applied types. After mixing the two components, they are applied by brush or roller — the feel is closer to painting. All of these products are cement-based, so the cured surface naturally suits rendering and tiling (cement-to-cement-adhesive bond strength is inherently good). Application is relatively straightforward, suited to teams of all sizes.
Membrane type (roller/squeegee)
Ardex WPM 300 is a water-based epoxy membrane. Applied by roller or squeegee, it forms a very thin waterproof film. The coating is the thinnest of all, with minimal impact on floor-to-ceiling clearance and door-bottom clearance. Solvent-free and low odour, it is safer and more comfortable to use in poorly ventilated enclosed basements. And because both components are liquid (just pour A + B in proportion and stir), there is no need for an electric mixer to process heavy powder — the mixing process is relatively simple compared to cement-based products. Once cured, it can be directly painted, carpeted, or tiled with tile adhesive.
Substrate preparation: the make-or-break step
Regardless of which negative-side waterproofing product you use, substrate preparation is the step that most affects the result. Done well, the coating bonds firmly and holds back water; done poorly, even the best product will delaminate and peel.
- Clean the substrate — remove all loose material, dust, oil and old coatings. A wire brush or high-pressure water wash can be used.
- Dampen the substrate (saturated surface-dry / SSD) — spray-dampen the concrete surface before application, but ensure there is no standing water. Cement-based materials need moisture for hydration; a dry substrate will draw moisture out of the coating, impairing bond strength. This is critical for all cement-based negative-side products.
- Pre-treat cracks — larger cracks or honeycombed areas should be filled with repair mortar first, before applying the negative-side waterproofing coating.
How to choose? Match your situation
When choosing a negative-side waterproofing material, the key is to match your site conditions and subsequent works:
Is the seepage concentrated at individual cracks or joints?
→ First consider chemical injection (grouting) for localised crack sealing. If a full-surface waterproof coating is also needed, combine it with negative-side waterproofing.
Need a full-surface negative-side waterproof coating? Ask yourself a few questions:
- Want to achieve waterproofing + a rendered surface in one step, then board over or apply an epoxy finish coat?
→ Sikagard 720 EpoCem (trowel-applied type) - Want flexible brush/roller application, and the project may have both positive and negative pressure requirements?
→ SPEC CW100, Fosroc CM210 or Weber Elastic Slurry (brush/roller type, handles both positive and negative pressure) - Site is poorly ventilated, tight on space, needs the thinnest coating and low odour, or no electric mixer available on site?
→ Ardex WPM 300 (membrane type, solvent-free)
If you are unsure which to choose, WhatsApp Wah Hing with your site conditions and we will recommend a suitable solution.
FAQ
Can negative-side waterproofing replace exterior waterproofing?
No. Negative-side waterproofing is a remedial measure, used when exterior access is genuinely impossible. Water pressure may be diverted rather than fully eliminated, and the root cause (failed external waterproofing) remains. If the source can be addressed from the exterior, that should always be the priority.
Can I tile directly after negative-side waterproofing?
Cementitious negative-side products (720, CW100, CM210, Elastic Slurry) cure to a cementitious surface that is generally suitable for tiling with tile adhesive or rendering over — refer to each product's TDS for specifics. The water-based epoxy membrane type (WPM 300) can also accept tile adhesive after curing, or be directly painted or carpeted. Check each product's TDS for curing times and overcoating procedures.
What is the difference between 3-component and 2-component?
Two-component (liquid + powder) mixing is relatively simple and is the design most cement-based products use. Three-component (as with 720 EpoCem's Part A resin + Part B hardener + Part C powder) adds a liquid-component pre-mix step, but it is precisely this separate epoxy component that enables the dual bonding mechanism of epoxy resin penetrating pores + cement hydration. Each has its strengths — more components does not automatically mean better.
Why dampen the substrate before application?
Cement-based materials need moisture for hydration to cure and bond properly. If the concrete surface is too dry, the substrate draws moisture out of the coating, preventing the cement from fully hydrating, and bond strength drops significantly. Spray-dampening the surface to saturated surface-dry (SSD) condition — moist but no standing water — is a basic requirement for all cement-based negative-side products.
Can negative-side waterproofing be used for window-edge leaks?
You need to distinguish whether the seepage is from the window frame itself (aged sealant, frame deformation, corroded metal) or from the concrete wall beside the window (concrete cracks). If water is entering through the window or door frame itself, negative-side waterproofing materials will not help — the frame needs repair or replacement, and the sealant needs redoing. If water is seeping through concrete wall cracks next to the window (and the frame itself is sound), a negative-side waterproofing coating can address the wall portion.
How to choose between negative-side waterproofing and chemical injection?
The two methods target different types of seepage: chemical injection pumps PU resin into individual cracks for localised sealing; negative-side waterproofing applies a continuous coating over the concrete surface. If seepage is concentrated at a few cracks, injection may be more direct; if the entire wall face is seeping, a negative-side coating is more comprehensive. Many projects combine both — injecting the main cracks first, then applying a negative-side coating as overall protection.
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