About STP TarFelt FG

TarFelt FG sits at the traditional end of STP Limited’s waterproofing range — a glass-fibre reinforced bituminous felt built on the same IS 7193:2013 base specification that has governed Indian tar-felt roofing for decades, with STP itself (formerly Shalimar Tar Products Limited) having manufactured construction chemicals in the country for well over eighty years. What sets FG apart from the older hessian-based felts (STP’s own TarFelt BS and BH) is the 40 gsm glass-fibre core: unlike jute or hessian, glass fibre doesn’t rot, and it holds its dimensions through the heat cycles an Indian terrace sees between a May afternoon and a July downpour. The bitumen coating on both faces is finished with mica or sand purely so rolls don’t block together in a hot store — a small detail, but one that matters when a roll has sat in a site godown through a Delhi summer before anyone gets around to laying it.

What you’re actually buying, though, is two different products under one name. Grade 1 conforms to IS 1346:1991 and is meant for roofing — concrete decks, corrugated asbestos or GI sheet, timber, shell and folded-plate structures. Grade 2 conforms to IS 1609:1991 and is pitched at a different job entirely: damp-proofing courses for foundations, basements and retaining walls, plus lining duty in silos, grain godowns, warehouses and even the roofs of wagons, trams and buses. Specifying the wrong grade for the job — Grade 1 under a basement slab, say, or Grade 2 as primary roof waterproofing — is the kind of mismatch that shows up as a callback eighteen months later, so it’s worth confirming which grade a BOQ actually calls for before the rolls reach site.

The technical insight

The numbers on the TDS are unglamorous but tell you what the felt is actually for. Breaking strength is specified directionally — 50 kg minimum in the warp (roll) direction against 30 kg minimum in the weft — reflecting how a glass-fibre-reinforced felt is inherently anisotropic; it’s stronger along the length it’s rolled and unrolled in, which is also the direction that takes the most handling stress on site. Water absorption is capped at 2%, which keeps the felt itself from turning into a moisture sponge that defeats the point of laying it. The heat-resistance test (no flow at 68±2°C for 3 hours) and the storage-sticking test (no sticking at 55±2°C for 18 hours) are both really proxies for how the felt behaves in an Indian summer — on an exposed steel or GI roof, deck temperatures well past 60°C are routine, and a felt that softens or bonds to itself in the roll is a felt that fails before it’s even installed. The 300 mm pressure-head test, meanwhile, is a reasonable stand-in for the ponding water a flat terrace with a blocked outlet will inevitably see at some point in its life — passing it tells you the membrane itself isn’t the weak point; poor drainage detailing usually is.

Where it is used

• Pour-and-roll (built-up) waterproofing on RCC flat roofs and terraces, protected with screed, tiles or grit
• Roofing treatment over corrugated asbestos/GI sheet, timber, and shell or folded-plate roof structures (Grade 1)
• Damp-proofing courses for foundations, basements and retaining walls (Grade 2)
• Waterproof lining for silos, grain godowns and warehouses (Grade 2, IS 1609)
• Roof and floor lining for wagons, trams and buses in transport body-building (Grade 2)
• Chemical-resistant lining on walls of effluent treatment plants and similar corrosive environments
• Underlay/base felt beneath a protective PCC, tile or screed overburden on trafficable terraces
• Roofing for industrial sheds and stores where the atmosphere carries chemical vapour

Key advantages

• Rot-proof glass-fibre core, unlike hessian/jute-based felts that can degrade biologically over time
• Resistant to chemicals of a corrosive nature and to chemical vapour — a real fit for ETP walls and process areas
• A proven, decades-old built-up membrane system that most Indian waterproofing contractors already know how to lay correctly
• Dimensionally stable, with a mica/sand finish that stops rolls sticking together in hot on-site storage
• Passes the IS-specified 300 mm pressure-head test with no leakage and the 68°C/3-hour heat-resistance test with no flow
• Two IS-conforming grades (1346 for roofing, 1609 for damp-proofing/industrial lining) let you match the felt to the actual application
• Water absorption capped at 2%, so the membrane doesn’t become a moisture reservoir in itself
• Works with a straightforward, low-cost bonding system (hot STP Bitumen 85/25) using tools and skills already on most Indian sites

Limitations & considerations

• Needs hot bitumen heated and applied on site — that means LPG/heating arrangements, trained labour and fire-safety precautions, not a cold plug-and-play install
• A traditional built-up felt, not a modified (APP/SBS) membrane, so it generally offers lower elongation and flexibility than STP’s own TarFelt APP or a self-adhesive system like ShaliHDPE SA
• Must be protected — screed, tiles, PCC, grit or aluminium paint — since the felt alone isn’t meant to take direct UV or foot/vehicle traffic
• Final performance leans heavily on workmanship: correct primer dosage, bitumen application rate and properly sealed, staggered laps make or break the installation
• Not designed for exposed, unprotected long-term service; poor drainage detailing or repeated ponding will stress laps over time regardless of the membrane’s own test results
• This is a hot-bonded system, not a torch-applied one — sites where hot works are restricted need the cold ShaliBond BS/CS adhesive route or a different STP membrane altogether

Selection & best practice

Specifications

Industry use cases

How we position STP TarFelt FG

Within STP’s own bituminous-felt line-up, TarFelt FG is the rot-proof, glass-fibre step up from the hessian-cored TarFelt BS and TarFelt BH, while sitting a rung below the modified-bitumen TarFelt APP and the cold, self-adhesive ShaliHDPE SA and ShaliBond EPDM membranes in terms of flexibility and installed life — Space Arc typically points contractors toward FG when the job is a conventional hot-bonded built-up terrace, basement damp-course, or industrial/ETP lining where budget and familiar site practice matter more than a premium torch-on or peel-and-stick system. Outside the STP family, Fosroc’s Proofex range of bituminous and self-adhesive tanking membranes competes in a similar market segment for basement and terrace waterproofing; a specifier might lean toward Fosroc where a fully cold-applied, hot-works-free installation is a hard site requirement, whereas TarFelt FG remains the more economical, IS-1346/1609-certified choice for teams comfortable with a traditional hot-bitumen bonded felt.

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