About Fosroc Renderoc ST 05

Fosroc Renderoc ST 05 is a high-performance, multi-purpose cementitious protective coating and levelling mortar engineered for thin-section work – from a feather edge up to 5 mm in a single pass. It belongs to Fosroc’s well-established Renderoc concrete repair family and is the go-to product when a surface needs to be made smooth, sound and protected rather than structurally rebuilt. The product is supplied as two components: a 20 kg powder bag (a blend of cements, graded aggregates with a maximum grain size of 0.5 mm and chemical additives) and a 5-litre liquid based on modified acrylic polymers. The polymer liquid is what gives the mixed mortar its creamy, easy-spreading consistency and its strong barrier characteristics against water, chlorides and carbon dioxide.

In the Indian context, Renderoc ST 05 fills a very specific and common need. Across Delhi NCR and most Indian cities, RCC structures suffer from honeycombing, blowholes, form-tie holes, shutter marks and shallow carbonation-driven spalling – the kind of defects that are too shallow for a heavy patch-repair mortar but too important to ignore before painting or waterproofing. Renderoc ST 05 is purpose-built for exactly this gap. Because it can be feathered to almost nothing and built to 5 mm, contractors use it as a fairing coat to true up uneven concrete faces, fill imperfections and create a uniform, paint-ready surface, while simultaneously adding a measurable chloride and carbonation barrier to slow future reinforcement corrosion.

Functionally, the material does three jobs at once. First, it is a thin-section repair and reprofiling mortar, achieving a 28-day compressive strength of about 50 MPa (Class R4) despite its low build. Second, it is a protective barrier – 1 mm of cured Renderoc ST 05 is reported to be equivalent to about 13 mm of 0.45 w/c concrete in terms of chloride resistance, which is why it is specified to restore effective cover to steel at far lower thickness than concrete. Third, it serves as a moisture and chemical barrier offering both active and passive water-pressure resistance, making it suitable for basement linings, construction-joint over-banding, potable-water tanks and swimming pools (before tiling).

It is fully compatible with the wider Fosroc system. It bonds directly to concrete without a separate primer in normal conditions, is compatible with other Renderoc repair mortars (for deeper repairs done first) and can be over-coated with the Dekguard range of anti-carbonation and decorative coatings to bring the surrounding structure up to the same protective standard. For Indian projects this makes it a clean, single-source repair-and-protect specification rather than a patchwork of unrelated brands.

The technical insight

The performance of Renderoc ST 05 hinges on the modified-acrylic polymer dispersion that replaces most of the gauging water. Because the powder is gauged at a low liquid:powder ratio of about 0.22, the cement matrix cures dense with very low capillary porosity – the route through which water, chloride ions and CO2 normally migrate into concrete. This is why a sub-millimetre to a few-millimetre coat behaves as a genuine barrier rather than just a cosmetic skim: laboratory data show CO2 permeability of Sd = 57 m (requirement Sd greater than 50 m), liquid-water transmission of 0.08 kg/m2.h^0.5 and capillary absorption of 0.2 kg/m2.h^0.5, all well inside BS EN 1504-2 limits. The polymer also drives the strong bond – 2.8 MPa by pull-off, comfortably above the 2.0 MPa Class R4 requirement – which is what allows priming to be skipped on sound, pre-wetted concrete. The 0.5 mm maximum aggregate size is the other key design choice: it is what permits a true feather edge and a sponge-smooth finish, but it also caps single-layer build at 5 mm and total multi-layer build at 15 mm. Anything deeper must first be brought up with a coarser mortar such as Renderoc Patch (over 5 mm) or Patchroc GP (over 12 mm) before fairing with ST 05. The practical corollary for site teams: saturated-surface-dry substrate preparation and proper Nitobond AR curing are not optional niceties – because the section is so thin, premature drying or rain before final set causes water uptake that severely degrades the hardened barrier.

Where it is used

• Filling blowholes, form-tie holes and surface imperfections on fair-faced RCC before painting
• Fairing and reprofiling coat to true up uneven or honeycombed concrete faces over large areas
• Thin-section patch repair of shallow spalled or carbonated concrete (feather edge to 5 mm)
• Restoring effective cover to reinforcement as a chloride and carbonation barrier
• Barrier coating over cut ends of rebar in core holes and service penetrations (20 mm overlap onto concrete)
• Protection and repair of concrete-encased steel beams and girders, with a thick-section Renderoc mortar
• Basement wall linings and construction-joint over-banding as a moisture barrier
• Swimming pool shells prior to tiling, and potable cold-water tanks (consumer premises)
• Marine and aggressive-groundwater structures requiring chemical and mild-acid resistance
• Wear-resistant skim on lightly trafficked balconies and walkways

Key advantages

• Builds from a true feather edge to 5 mm in a single layer – ideal for shallow defects too thin for patch mortars
• Acts as both a chloride and a carbonation barrier; 1 mm equates to roughly 13 mm of 0.45 w/c concrete for chloride resistance
• Excellent bond to concrete without a separate primer in normal conditions (2.8 MPa pull-off)
• High Class R4 compressive strength of about 50 MPa at 28 days despite thin application
• Applies vertically, horizontally and overhead by brush, roller or trowel – flexible on site
• Non-combustible Euroclass A2-s1,d0 fire rating
• WRAS / BS 6920 approved for contact with cold potable water when correctly cured
• Good abrasion and impact resistance for light foot traffic; foot-trafficable in 12 hours at 20C
• Fully compatible with other Renderoc mortars and over-coatable with Dekguard anti-carbonation coatings

Limitations & considerations

• Single-layer thickness limited to 5 mm and total multi-layer build to 15 mm – deeper repairs need Renderoc Patch (over 5 mm) or Patchroc GP (over 12 mm) first
• Not suitable for heavily trafficked floors – it is a thin protective/fairing coat, not an industrial floor screed
• Should not be applied when substrate/air temperature is below +5C and falling
• Exposure to rain, mist or high humidity before final set causes water uptake and severe loss of hardened performance
• High humidity or a falling thermometer can cause harmless white surface discolouration
• Must not be applied over elastomeric, epoxy, PU, polyurea, polyaspartic or MMA coatings, or any coating subject to traffic; over acrylic coatings limit to 2 mm after bond testing
• Potable-water use is cold-water, consumer-premises only – the product is not DWI listed
• Requires diligent saturated-surface-dry prep and Nitobond AR curing; cured material can only be removed mechanically

Selection & best practice

Specifications

Industry use cases

How we position Fosroc Renderoc ST 05

Within the Fosroc range, Renderoc ST 05 sits at the thin-section, finishing end of the Renderoc concrete-repair family. Use it after – not instead of – the structural patch mortars: rebuild lost section with Renderoc HB, Renderoc S or Patchroc GP (or fill 5-12 mm depressions with Renderoc Patch), then fair and protect with ST 05, and finish the whole element with a Dekguard anti-carbonation/decorative coating. It bonds to concrete typically without priming, but on very porous substrates pair it with Nitobond AR. Against competitor systems, Renderoc ST 05 competes with Sika’s MonoTop / Sika levelling and fairing mortars (e.g. MonoTop-723 / 612-type fairing coats), MBS Murexin / MYK ranges, Dr. Fixit’s repair and microconcrete fairing products, and STP (Krishna Conchem) polymer-modified repair mortars. Its differentiators are the combination of a true feather edge to 5 mm build, an EN 1504-3 Class R4 strength of about 50 MPa, a documented chloride/carbonation barrier (1 mm equivalent to roughly 13 mm of concrete), WRAS potable-water approval and a non-combustible A2 fire rating in one product. Honestly, where the requirement is a heavily trafficked floor topping, a deep structural rebuild, or a DWI-listed potable application, ST 05 is not the right choice and a dedicated flooring screed, microconcrete or alternative system should be specified instead. As an authorised Fosroc distributor, Space Arc Engineering positions ST 05 as the specification-grade fairing-and-protect layer in a single-source Fosroc repair system rather than as a standalone fix for every concrete defect.

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