Fosroc Proofex Torchseal 4S 250 (-20)
A 4mm SBS-modified bitumen torch-on membrane with a non-woven polyester core and verified flexibility to -20°C, engineered for basements, roofs, podiums and tunnels.
What is it?
A 4mm-thick, torch-applied waterproofing membrane made from SBS (styrene-butadiene-styrene) modified bitumen, reinforced with a non-woven polyester fabric core weighing around 180 g/m². The ‘4S’ denotes the 4mm SBS-type sheet and the ‘(-20)’ denotes its certified low-temperature flexibility down to minus 20 degrees Celsius.
Where is it used?
Below-grade and buried structures across India — basement rafts and retaining walls, podium and car-park decks, concrete roof slabs, water-retaining structures, tunnels and subways — wherever a robust, monolithically bonded bituminous tanking system is specified.
Why use it?
The SBS elastomer modification gives the membrane elastic recovery and crack-bridging that plain oxidised bitumen lacks, and the -20°C rating means it stays flexible and resists cold cracking far beyond the -5°C of standard plain-core felts. The 180 g/m² polyester core adds high puncture and tensile resistance.
How is it applied?
The primed concrete substrate is coated with Fosroc Proofex Torchseal Primer and allowed to go tack-free. The roll is then unrolled, re-rolled, and bonded by heating its underside with a propane gas torch so the bitumen melts and fuses to the deck, with 10 cm side and 15 cm end overlaps fully sealed.
About Fosroc Proofex Torchseal 4S 250 (-20)
Fosroc Proofex Torchseal 4S 250 (-20) belongs to Fosroc’s premium “S” series of torch-applied bituminous membranes, the range engineered around SBS (styrene-butadiene-styrene) polymer modification rather than the older APP or oxidised-bitumen chemistries. In Fosroc’s coding, the leading number is the nominal sheet thickness in millimetres, so 4S is the 4mm SBS-type membrane; the suffix figure identifies the specific grade within the family, and the bracketed “(-20)” is the headline performance claim — certified flexibility with no cold cracking at minus 20 degrees Celsius. That single figure is what separates this product from a commodity waterproofing felt. The membrane is built from premium-grade, heavily modified bitumen carried on a rot-proof, non-woven polyester fabric core weighing approximately 180 g/m². The polyester reinforcement is the structural heart of the sheet: it delivers high resistance to puncture and tearing, allows useful elongation so the membrane can accommodate substrate movement and shrinkage, and keeps the sheet dimensionally stable during torching and over its service life. Because the bitumen is SBS-elastomer-modified, the finished membrane behaves more like a rubberised sheet than a rigid bituminous board — it stretches under stress and recovers, which is exactly the property a buried waterproofing layer needs when the structure it protects flexes, shrinks or settles. For Space Arc Engineering’s clients across Delhi NCR and Pan-India, the practical appeal is a proven, single-source tanking solution that fuses monolithically to the concrete, handles aggressive ground-water conditions, and carries the warranty backing of a Fosroc-authorised supply chain. It is applied as part of a defined Fosroc system — Proofex Torchseal Primer first, then the torch-bonded membrane — so the specification, the primer and the membrane all come from one technically supported source. This page covers the verified composition, the recommended applications, honest limitations and the best-practice points an applicator or specifier in India needs before ordering.
The technical insight
The defining technical distinction of this product is the SBS modification combined with the -20°C cold-flexibility rating. Plain-core Proofex Torchseal variants (the 3P/4P plain-felt grades) are published with a low-temperature flexibility of only about -5°C; the SBS “S” range is engineered to stay crack-free at -20°C. SBS is a thermoplastic elastomer that disperses through the bitumen as an interconnected rubbery phase, raising the softening point (typically around 150°C for this family) while dramatically lowering the temperature at which the bitumen turns brittle. The result is a wide service window — the membrane resists flow and slumping in Indian rooftop summer heat yet does not crack when the same structure is in a cold basement, a high-altitude site, or a cold store. The 180 g/m² non-woven polyester carrier is deliberately chosen over glass-fibre: polyester elongates rather than snapping, so the composite can bridge minor cracks and survive settlement. During torching, the underside compound melts and the molten bitumen wets the primed concrete, forming a continuous chemical-mechanical bond with no air gap — this monolithic adhesion is what prevents the lateral water tracking that plagues loose-laid or partially-bonded systems. For specifiers, the takeaway is that “4S 250 (-20)” is not just a felt thickness — it is a defined elastomeric performance class, and substituting an ordinary 4mm APP felt is not an equivalent.
Where it is used
- Basement tanking — rafts, retaining walls and substructures subject to hydrostatic ground-water pressure
- Podium decks, terraces and car-park deck slabs over occupied or buried spaces
- Concrete roof and floor slabs requiring a durable buried waterproofing layer beneath screed or protection
- Water-retaining and water-excluding reinforced concrete structures
- Tunnels, subways and underground transport infrastructure
- Foundations and buried civil works in aggressive or saline ground-water conditions
- Bridge decks and approach structures (under wearing course / protection)
Key advantages
- SBS-modified elastomeric bitumen gives elastic recovery and crack-bridging beyond plain oxidised felts
- Verified low-temperature flexibility to -20°C — far better cold-crack resistance than standard -5°C grades
- High-strength 180 g/m² non-woven polyester core delivers excellent puncture, tear and tensile resistance
- Torch-bonding fuses the membrane monolithically to the substrate, eliminating lateral water tracking
- Rot-proof polyester carrier suited to permanently buried, damp service
- Malleable sheet readily dressed into upstands, fillets and contours
- Resists aggressive ground-water, making it suitable for chemically loaded soils
- Backed by a complete Fosroc system (Proofex Torchseal Primer + membrane) and authorised-distributor warranty
Limitations & considerations
- Requires skilled application with an open propane flame — a fire-watch, fire extinguisher and trained operatives are mandatory, and torch-on work is unsuitable near combustible insulation or sensitive finishes
- Substrate must be float-finished, dry, frost-free and free of laitance and cavities; surfaces must be re-primed if left more than 24 hours before membrane application
- As a buried/protected system it is not intended as a final exposed, foot-trafficked or UV-stable wearing surface — it needs screed, protection board or a wearing course over it
- Open-flame application is not permitted on confined gas-rich substrates, over live waterproofing of incompatible chemistry, or where hot works are prohibited
- Performance depends on correct overlaps (10 cm side, 15 cm end minimum) and fully sealed laps — poor torching technique is the most common cause of failure
- 12-month shelf life when stored cool, dry and horizontal (max one pallet high); rolls degrade if stored upright or near heat
Selection & best practice
Always apply the matched Fosroc Proofex Torchseal Primer to a float-finished, dry, laitance-free concrete deck and let it dry tack-free before torching; re-prime any area left longer than 24 hours. Stagger and fully bond all laps — minimum 10 cm at side overlaps and 15 cm at end joints — and check each seam visually for a continuous bitumen bleed-out bead, which confirms fusion. Heat the membrane underside evenly so the polymer film just melts; over-torching burns off the SBS modifier and under-torching leaves voids. Dress the membrane into upstands and form fillets at all internal angles, and protect the finished membrane promptly with screed or protection board before following trades begin. Keep a fire extinguisher and fire-watch on station during and for at least an hour after hot works, and store rolls horizontally in a cool dry place, used within the 12-month shelf life.
Specifications
| Product type | Torch-applied SBS-modified bitumen waterproofing membrane |
| Polymer modification | SBS (styrene-butadiene-styrene) elastomer |
| Nominal thickness | 4 mm (the ‘4’ in 4S) |
| Reinforcement | Non-woven polyester fabric core, approx. 180 g/m² |
| Low-temperature flexibility | To -20°C, no cold cracking (the ‘(-20)’ designation) |
| Softening point (S-range typical) | approx. 150°C (ASTM D36 method) |
| Roll size | 10 m long x 1 m wide |
| Application method | Propane gas torch, fully bonded to primed substrate |
| Primer | Fosroc Proofex Torchseal Primer (applied tack-free first) |
| Side / end overlaps | 10 cm minimum side, 15 cm minimum end |
| Shelf life / storage | 12 months, stored cool, dry, horizontal, max one pallet high |
| Manufacturer | Fosroc International |
Industry use cases
Commercial real estate (Delhi NCR)
Tanking of deep basement car parks and substructures in high-water-table NCR sites, where the -20°C-rated elastomeric core resists cracking from settlement and the polyester carrier survives backfill and rebar puncture.
Infrastructure & transport
Waterproofing of metro tunnels, subways and underpasses where a monolithically bonded, rot-proof membrane is essential to exclude aggressive ground water over decades.
Institutional & high-rise
Podium decks and terrace slabs over occupied spaces in residential towers and campuses, bonded beneath protection screed to give a durable buried waterproofing plane.
Water & civil works
Water-retaining and water-excluding RC structures, treatment tanks and retaining walls, where elastomeric crack-bridging guards against hairline shrinkage cracks.
Cold-climate & hill projects
Sites in northern hill regions and cold stores where the -20°C cold-flexibility rating prevents the brittle cracking that defeats standard -5°C bituminous felts.
Frequently asked questions
What does ‘4S 250 (-20)’ actually mean?
How is it different from Proofex Torchseal 4P or an ordinary APP felt?
Do I need a primer, and which one?
What roll size and overlaps should I plan for?
Can it be left exposed as a finished roof surface?
Is Space Arc Engineering an authorised source, and do you supply Pan-India?
How we position Fosroc Proofex Torchseal 4S 250 (-20)
Within Fosroc’s own catalogue, Proofex Torchseal 4S 250 (-20) sits at the premium, cold-flexible end of the torch-on bituminous range. It is a step up from the plain-core Proofex Torchseal 3P/4P felts (rated to roughly -5°C) and is closely related to the mineral-finished 4SI and aluminium-faced variants; for fully bonded sheet-membrane tanking it pairs with Proofex Torchseal Primer, while Fosroc’s self-adhesive Proofex Engage / Proofex 3000 and liquid Brushbond / Nitoproof systems are the alternatives where open-flame torching is not permitted. Against the wider Indian market, it competes directly with Sika’s SBS torch-on membranes (e.g. the Sika BituSeal / Sikaplan torch range), STP Limited’s APP/SBS torch-applied membranes, MBS (My Building Shop) and Dr. Fixit’s torch-on and liquid-membrane systems. Honest positioning: where the project demands cold-temperature flexibility, high mechanical strength and a warranty-backed Fosroc system, the 4S 250 (-20) is a strong specification; where open-flame hot works are prohibited, a budget commodity APP felt is acceptable, or a seamless liquid-applied finish is preferred, a self-adhesive or liquid alternative may suit better. Space Arc supplies the genuine Fosroc product and will advise the correct system for the substrate and site constraints.
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