About Fosroc Renderoc CS

Renderoc CS is Fosroc’s answer to the most common defect on any Indian building — the crack that keeps reappearing through a coat of paint. It is a single-part cementitious powder that you simply gauge with clean water on site. There is no polymer emulsion to add, no epoxy to weigh out, and no primer to lay first; the material bonds directly to a prepared, dampened substrate. Because the formulation is engineered to be non-shrink, the mortar does not pull away from the groove edges as it cures, which is precisely what causes ordinary sand-cement fillers to crack again along their own boundaries within weeks.

The product is deliberately narrow in what it claims to do, and that honesty is its strength. Renderoc CS is a crack sealer, not a structural repair mortar and not a movement joint sealant. It is meant for dormant cracks — the kind that have stopped moving — in plaster, masonry and concrete, sealed to keep water and air out and to give a sound, paintable surface. It sets fast, wet-density sits around 1.85 to 1.90 g/cc, and it develops enough compressive strength (22 N/mm² at 7 days, 24 N/mm² at 28 days to ASTM C109) to sit comfortably within the surrounding plaster or masonry. For contractors handling snagging and finishing work across Delhi NCR, it is one of the quietest, most reliable items in the Fosroc range.

The technical insight

The recurring-crack problem is almost always a shrinkage problem. When a plain cement slurry or a rich sand-cement patch dries inside a groove, it loses volume; the restraint from the groove walls converts that lost volume into tensile stress, and the patch tears — usually straight back along the line you were trying to hide. Renderoc CS is formulated to eliminate that drying shrinkage within the repair, so the paste cures dimensionally stable and stays keyed to the groove faces rather than shrinking off them. The V-groove geometry itself does real work here: cutting a 6 mm x 6 mm channel replaces a knife-edge crack (which no material can grip) with two clean, mechanically sound faces and enough cross-section for the mortar to develop a proper bond. Pre-wetting the groove to a damp condition stops the porous substrate from drawing mix water out of the fresh paste, which protects both the setting reaction and the final strength. The result is a seal that behaves like part of the wall, not a foreign strip sitting on top of it.

Where it is used

• Sealing dormant cracks in internal and external cement plaster before repainting
• Filling settlement and shrinkage cracks in brick and concrete-block masonry
• Repairing fine-to-wide static cracks in concrete columns, beams, slabs and walls
• Overhead and vertical crack sealing where a non-sagging paste is essential
• Making good hairline crazing and cracks around door and window openings
• Snagging and finishing crack repairs on facades and boundary walls
• Sealing cracks in RCC surfaces ahead of a protective or decorative coating
• Pre-paint remedial work on ceilings and soffits where cracks telegraph through the finish

Key advantages

• Single component — only clean water is added on site, with no polymer or primer to manage
• Non-shrink formulation eliminates shrinkage cracking within the repair, so cracks stay sealed
• Non-sagging paste works overhead and on vertical faces as easily as on flat surfaces
• Fast set of 15 to 20 minutes keeps finishing crews moving on the same visit
• Can be over-coated with common decorative paints and coatings, so repairs vanish under the finish
• Versatile across plaster, masonry and concrete substrates from a single product
• Good adhesion to a properly prepared and dampened substrate without a bonding coat
• Non-flammable with no health hazards in normal use, suited to occupied-building remedial work

Limitations & considerations

• Not for moving or live cracks — it is a rigid sealer for dormant (static) cracks only
• Maximum crack width 30 mm; wider defects need a repair mortar such as Renderoc GP
• Requires the crack to be opened into a roughly 6 mm x 6 mm V-groove first, not a surface skim
• It is a crack sealer, not a structural bonding agent — it does not re-knit load path across a crack
• Curing is needed where the groove exceeds 6 mm width or the repair is UV-exposed
• Below the 10°C minimum application temperature, set and strength gain are compromised

Selection & best practice

Specifications

Industry use cases

How we position Fosroc Renderoc CS

Renderoc CS sits at the light, paint-and-finish end of the Fosroc concrete-repair family — the crack sealer, not the structural repair. When a crack is wider than 30 mm or you are actually reinstating spalled or damaged concrete, step up to Fosroc Renderoc GP, the single-component patch-repair and reprofiling mortar. When the crack is structural and needs to be re-knit rather than merely surface-sealed, the correct partner is Fosroc Nitofill LV, a low-viscosity epoxy injection resin that penetrates and re-bonds dormant fine-to-medium cracks. And where a crack is live or forms a movement joint — outside what any rigid sealer can do — move to a flexible Fosroc joint sealant such as the Nitoseal or Thioflex range, or to a Dr. Fixit / Sika elastomeric sealant from our wider brand shelf. Space Arc stocks the full ladder, so we can specify the right product for the crack in front of you rather than forcing one to fit.

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