About STP No. 1

STP No. 1 is one of the oldest and most recognised names in Indian integral waterproofing. Manufactured by STP Limited (the former Shalimar Tar Products, now part of the Berger group), it is a plasticizing waterproofing admixture that is blended straight into concrete or cement mortar at the time of mixing. Rather than sitting on the surface as a coating or membrane, it works from within the mix, filling capillary voids and improving cohesion so that the hardened cement matrix resists the passage of water. It is supplied in both a liquid and a powder grade so that site teams can choose whichever is easier to batch accurately for the job in hand. The product conforms to IS 2645, the Indian standard for integral waterproofing compounds for cement mortar and concrete.

For engineers and contractors, its appeal is simplicity and reach. STP No. 1 needs no special applicators, priming or curing regime beyond good concreting practice, and it is stocked across the country, which makes it a dependable default for routine waterproofing on roofs, plaster and general RCC work. It should be understood, however, as a permeability-reducing admixture rather than a stand-alone waterproofing system. On structures with hydrostatic pressure, movement or exposure to aggressive water, it is best treated as one layer of a properly detailed strategy that also addresses mix design, compaction, joints and, where needed, an external membrane or crystalline coat.

The technical insight

Water moves through hardened concrete mainly along the interconnected capillary pores and micro-channels left when excess mix water evaporates. STP No. 1 attacks this pathway in two ways. As a plasticizer it improves the workability of the mix at a given water content, which lets the crew place and compact concrete that is less porous to begin with. Its water-repellent and pore-blocking constituents then line and partially fill the finer capillaries in the set matrix, breaking the continuity of the channels that would otherwise wick water inward. The net effect is lower water absorption and reduced permeability, along with a secondary benefit in freeze-thaw and efflorescence resistance because there is less free water held in the pore network. Because the action is integral, the protection is distributed through the full thickness of the pour rather than concentrated at a surface film that can be punctured or worn away. The liquid grade (specific gravity around 1.00-1.05) and powder grade (specific gravity around 0.70-0.80) deliver the same fundamental mechanism; the choice is about batching convenience, not performance philosophy.

Where it is used

• Waterproofing of RCC roof slabs, terraces and chajjas
• External and internal cement plaster where dampness resistance is required
• Non-potable RCC water tanks, sumps and underground reservoirs
• Basement raft, retaining walls and other below-grade masonry
• Bathroom, balcony and wet-area screeds and rendering
• Brick and block masonry mortars for parapets and boundary walls
• General site concrete where a low-cost permeability reducer is specified
• Repair mortars and patch renders needing improved water resistance

Key advantages

• Conforms to IS 2645, giving specifiers a recognised compliance benchmark
• Available in both liquid and powder grades to suit site batching preferences
• Improves plasticity and workability while reducing water demand
• Reduces capillary water absorption and overall permeability of the mix
• Integral action protects the full thickness of the pour, not just a surface film
• Needs no specialist applicators, primers or extended curing beyond good practice
• Widely stocked across India and economical for large or routine volumes
• Long-established, contractor-familiar product with decades of field use

Limitations & considerations

• A permeability reducer, not a stand-alone membrane; will not bridge cracks or resist standing hydrostatic head on its own
• Not recommended for potable (drinking) water tanks, where a crystalline or approved cementitious system should be used instead
• Performance depends heavily on correct dosage, thorough mixing, compaction and curing at site
• Cannot compensate for poor mix design, honeycombing, cold joints or inadequate cover
• Offers limited protection against active or moving cracks, expansion joints and construction-joint leakage

Selection & best practice

Specifications

Industry use cases

How we position STP No. 1

STP No. 1 sits at the value end of the integral waterproofing shelf and is the natural pick when the brief calls for an IS 2645-compliant, widely available admixture for routine roof, plaster and RCC work. Within the STP (Shalimar) family, step up to ShaliPlast IW for a dedicated liquid integral waterproofer, or ShaliPlast IW CI where RCC also needs corrosion inhibition; for water-retaining and potable-water structures, move to STP’s crystalline waterproofing (ShaliCrystal 200) or two-component elastomeric coatings (ShaliCem EWP) rather than relying on an admixture alone. Where a project standardises on Fosroc, Conplast WP90 is the closest equivalent. Space Arc supplies all of these alongside membranes, crystalline systems and joint treatments, so the admixture can be specified as one part of a complete, correctly detailed waterproofing package.

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