STP TarPlastic
A ready-to-use, cold-applied bituminous putty that plugs cracks, gaps and sheet-fixing holes without a flame, a torch or a heating pot on site.
What is it?
A cold-applied bituminous caulking compound in putty form, blended with mineral fillers and reinforcing fibres to a non-sag, thixotropic consistency.
Where is it used?
Cracks and gaps in terraced roofs, masonry walls, and around J-hooks on asbestos-cement (AC) and galvanised-iron (GI) roofing sheets.
Why use it?
It seals and waterproofs minor cracks and sheet penetrations instantly, stays flexible to absorb movement, and needs no heating or specialist equipment.
How is it applied?
Trowel, putty knife or gloved hand straight from the tin into a clean, dry gap; press firmly to fill the full depth and tool the surface smooth.
About STP TarPlastic
STP TarPlastic is one of those quiet workhorse products that most site engineers have used at some point without giving it much thought. It is a cold-applied bituminous caulking compound, supplied as a black, putty-like paste that you apply straight from the tin. There is no melting kettle, no gas torch and no hot-work permit involved, which is exactly why it remains a go-to for roof maintenance and small remedial sealing jobs where dragging out hot-applied bitumen would be overkill. STP Limited blends the bitumen with mineral fillers and reinforcing fibres, so it sits thicker than a paint-grade sealant and holds its shape on a vertical face instead of slumping.
Where TarPlastic earns its keep is in the day-to-day sealing that every building throws up: hairline cracks opening in a terraced roof screed, gaps around the J-hooks that hold down AC and GI sheets, open joints in parapet masonry, and the small holes that appear once fasteners loosen and let water track in. It conforms to IS 1580, the Indian standard for bituminous compounds used in this kind of caulking and waterproofing work. For contractors and facility teams across Delhi NCR and the rest of India, it is a low-cost, keep-it-on-the-shelf answer to leaks that do not justify a full membrane system but still need stopping before the next monsoon.
The technical insight
The reason a cold bituminous caulk works at all comes down to how the binder behaves. Bitumen is a viscoelastic material: it never fully hardens the way a cement mortar or an epoxy does, so it stays permanently plastic and can flow slowly to accommodate the thermal expansion, contraction and vibration that roofs and sheet cladding experience every day. TarPlastic exploits this by keeping the bitumen soft while using mineral fillers and structural fibres to stiffen the mass into a non-sag putty. The fillers give it body so it packs into a gap and stays put on a slope or a vertical wall; the fibres bridge across the crack and resist the compound being pulled apart or squeezed out as the joint moves. Because it is a solvent-cutback, cold-applied formulation rather than a hot pour, it wets onto concrete, asbestos-cement and metal without any priming heat, and the surface skins over on exposure while the bulk stays flexible underneath. That combination of permanent plasticity and a tough, filled body is what lets a single putty seal survive years of movement without cracking off.
Where it is used
- Sealing gaps and holes around J-hooks and fasteners on AC and GI roofing sheets
- Caulking overlap joints between corrugated roofing sheets
- Filling minor cracks and gaps in terraced and RCC roof slabs
- Plugging gaps around roof tiles and at tile-to-flashing junctions
- Sealing open joints and cracks in masonry and parapet walls
- Bedding and sealing around pipe and duct penetrations through roofs
- Localised leak-stopping repairs on old bituminous felt roofs
- General waterproof caulking where light thermal movement is expected
Key advantages
- Cold-applied straight from the tin, so no torch, kettle or hot-work permit is needed
- Stays permanently plastic to absorb expansion, contraction and vibration without cracking
- Non-sag, thixotropic body holds its shape on vertical and sloping surfaces
- Bonds to concrete, asbestos-cement, GI and corrugated sheet without priming heat
- Conforms to IS 1580 for bituminous caulking compounds
- Low-cost, general-purpose solution that keeps well on the shelf for quick repairs
- Simple to apply by trowel, putty knife or hand with unskilled labour
- Can be over-coated with a compatible aluminium finish or paint for UV protection
Limitations & considerations
- Black bitumen finish is not decorative and needs an aluminium or paint over-coat where appearance or UV durability matters
- Suited to minor cracks, gaps and static-to-light-movement joints, not to wide structural or high-movement expansion joints
- Not a substitute for a full waterproofing membrane on large or ponding roof areas
- As a solvent-cutback bitumen it stays soft and can pick up dust or flow slightly at high roof-surface temperatures
- Requires a clean, dry, sound substrate; it will not bond reliably to wet, oily, loose or dusty surfaces
- Not chemical- or fuel-resistant, so it is unsuitable for trafficked joints exposed to oils and solvents
Selection & best practice
Start with the substrate: cut back loose or crumbling edges, wire-brush off scale and old paint, and make sure the gap is clean and dry before you begin, because a bituminous caulk bonds by wetting the surface and will lift off anything damp or dusty. Work the putty firmly into the full depth of the crack or hole rather than just skimming the top, so there is no void behind the seal for water to track through; on wider gaps, pack in stages. Tool the surface smooth with a wetted trowel or putty knife to shed water and improve appearance. On exposed roof areas that see direct sun, allow the seal to skin over and then apply STP TarPlastic AL or a compatible aluminium finish to reflect heat and protect the bitumen from UV, which extends the life of the repair considerably. Keep application to the cooler part of the day in peak summer so the compound is easier to tool and less prone to flowing.
Specifications
| Product type | Cold-applied bituminous caulking compound |
| Form | Putty / paste (thixotropic, non-sag) |
| Colour | Black |
| Composition | Bitumen blended with mineral fillers and reinforcing fibres |
| Standard | Conforms to IS 1580 |
| Application | Cold applied; no heating required |
| Coverage | Approx. 1.25 kg/m2 per 1 mm thickness; varies with joint depth |
| Pack size | 20 litre / 20 kg pack |
Industry use cases
Industrial & warehouse roofing
Sealing gaps around J-hooks and overlap joints on large AC and GI sheet roofs, where a fast cold-applied fix keeps sheds watertight through the monsoon without hot work near stored goods.
Residential & terraced housing
Stopping leaks at hairline cracks in RCC terraces and around roof tiles, a common low-budget remedial job that homeowners and small contractors handle without specialist equipment.
Facility & building maintenance
Keeping tins on hand for quick response to fresh leaks at parapets, pipe penetrations and old felt roofs, avoiding the delay and cost of mobilising a full waterproofing crew.
Institutional & older AC-sheet buildings
Schools, godowns and factory blocks with ageing asbestos-cement roofs use it to re-seal loosened fasteners and cracked sheets as a stop-gap ahead of planned re-roofing.
Frequently asked questions
Does STP TarPlastic need heating before use?
Will it stop leaks around J-hooks on AC and GI roofing sheets?
How much do I need to cover a given area?
Can I paint over it or make it a colour other than black?
Is it a replacement for a proper waterproofing membrane?
Can it handle a large expansion joint that moves a lot?
How we position STP TarPlastic
STP TarPlastic sits at the economical, general-purpose end of the sealing range: it is the product to reach for when you need to caulk cracks, gaps and sheet-fixing holes quickly and cheaply, not when you need an engineered movement joint. Within the STP family, pair it with STP TarPlastic AL where the repair is sun-exposed and needs an aluminium-finished, UV-reflective top coat, and step up to STP’s flashing-tape and APP-modified membrane systems (SuperThermoLay) when the job is a full roof rather than a spot repair. Where the joint is structural, trafficked, or exposed to fuels and chemicals, move to a purpose-built elastomeric sealant from our other brands, such as Fosroc’s polysulphide and polyurethane grades or Sika’s joint sealants, which are engineered for large cyclic movement and chemical resistance that a bituminous caulk is not intended to deliver.
Fosroc Nitoseal PS200 — Specify this black pitch-extended polysulphide when the joint is a real trafficked or high-movement expansion joint needing a durable elastic seal, rather than the minor crack-and-gap caulking that TarPlastic is built for.
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