Fosroc Nitoseal MB175
A firm, slump-resistant rubber-bitumen gun-grade sealant for sealing flashings, gutter laps, roof chases and bituminous waterproofing terminations.
What is it?
A single-component, gun-grade rubber-bitumen (rubberised bitumen) mastic joint sealant from Fosroc, formerly marketed as Plastiseal. It stiffens after application to a firm, flexible, weather-resistant seal with excellent slump resistance.
Where is it used?
Sealing and pointing in roofing and waterproofing work — flashings, gutter and drainage-channel laps, sheet metal sheet-laps, horizontal roof chases, and the terminations and overlaps of bitumen-based waterproofing membranes (Fosroc Proofex systems).
Why use it?
It is chemically compatible with asphalt and bituminous surfaces (where polyurethane and silicone sealants typically fail), holds its shape on slopes without sagging, bonds well even under immersion, and resists alkalis, sulphates and biodegradation.
How is it applied?
Gun-applied from a 600 ml sausage into a clean, dry joint 10-40 mm wide and at least 15 mm deep, tooled firm; porous substrates are primed with Fosroc Primer B2. It is left a minimum of 14 days to release solvent and stiffen before permanent water immersion.
About Fosroc Nitoseal MB175
Fosroc Nitoseal MB175 is a single-component, gun-grade rubber-bitumen mastic joint sealant, supplied ready to use and applied straight from the cartridge. It was sold for many years under the name Plastiseal, and many older specifications and as-built drawings in India still refer to it by that legacy name. Chemically it sits in a different family from the elastomeric Nitoseal sealants (the MS-polymer and polysulphide grades): rather than relying on a chemical cure to build elasticity, MB175 is a solvent-borne rubberised bitumen compound that stiffens by solvent release to leave a firm, flexible, mastic-like seal.
The defining characteristic of the product is its compatibility with bitumen. Most modern facade and movement-joint sealants — polyurethanes, silicones, MS-polymers — are not compatible with asphalt and bituminous membranes; bleeding, staining and adhesion loss are common where they meet bitumen. MB175 is formulated specifically to bond to and work alongside bituminous surfaces, which is exactly why it is positioned as the sealing and pointing companion to Fosroc’s Proofex bituminous waterproofing membranes. It is also notable for its slump resistance: as a low-slump semi-stiff mastic it can be applied to sloping and vertical roof details without slumping or running before it firms up.
It is important to be clear about what MB175 is not. It is not a high-movement structural movement-joint sealant, and Fosroc does not publish an ISO 11600 movement class or an ASTM C920 movement-accommodation factor (MAF) for it. It is a pointing and lap-sealing grade — the right tool for weather-sealing roofing details and terminating bituminous membranes, not for accommodating the large cyclic movement of a building expansion joint. For that role, an elastomeric Nitoseal, Thioflex or Colpor product is specified instead. Note also that in some markets Fosroc has rebranded this compound as Nitoseal MB180, a fibre-reinforced version of the same rubber-bitumen mastic; the application logic in this page applies to both.
The technical insight
The behaviour to design around is that MB175 is a solvent-release product, not a reactive-cure elastomer. After gunning it skins over in roughly 1-2 days and then continues to stiffen over time as solvent evaporates from the body of the bead — full firming through a deep bead can take up to two weeks at 20°C, and longer in cold weather or where the seal depth exceeds about 20 mm. This has two practical consequences. First, it must not be permanently immersed in water until it has been left for a minimum of 14 days, otherwise the seal is trapped in a soft, under-cured state and solvent cannot escape; this is the single most common cause of in-service failure for gutter and channel laps sealed in a hurry. Second, because curing depends on solvent leaving the bead, very deep or very wide beads cure slowly and unevenly, which is why the geometry is capped at 40 mm wide and detailed with a sensible depth rather than over-filled. Compared with the elastomeric Nitoseal grades, MB175 trades high movement capability and a fast chemical cure for bitumen compatibility, excellent immersed adhesion and resistance to alkalis, sulphates and biodegradation — the property set that matters for roof and membrane detailing rather than for moving structural joints.
Where it is used
- Weather-sealing and pointing of metal and bituminous flashings on roofs and parapets
- Sealing lap joints in gutters, valleys and drainage channels, including where water may pond
- Sealing sheet-metal sheet-laps and overlaps in cladding and roofing
- Sealing horizontal chases and joints in roofing and flashing details
- Terminating, lapping and sealing the edges of bitumen-based waterproofing membranes (Fosroc Proofex systems)
- Pointing around chimneys, rooflights, ventilation penetrations and pipe collars on bituminous roofs
- Sealing static, non-structural joints in below-ground retaining-wall and tanking details where a bitumen-compatible mastic is required
Key advantages
- Firm, flexible, weather-resistant seal with excellent slump resistance on slopes and vertical details
- Chemically compatible with asphalt and bitumen-based membranes, unlike PU, silicone or MS-polymer sealants
- Excellent adhesion even under permanent water immersion once fully dried
- Single-component and ready to use — gunned straight from the sausage with no mixing
- Good resistance to biodegradation, alkalis and sulphates, suiting roof, gutter and below-ground details
- Bonds to a wide range of common roofing substrates — metal, concrete, masonry and bituminous surfaces
- Backed by the wider Fosroc Proofex waterproofing system as the matched sealing/pointing grade
Limitations & considerations
- Not a high-movement sealant — no published ISO 11600 class or ASTM C920 movement-accommodation factor; unsuitable for structural expansion or movement joints
- Solvent-release cure is slow: up to ~14 days drying at 20°C, longer in cold weather or for beads deeper than ~20 mm
- Must be left a minimum of 14 days before permanent water immersion, which constrains fast-track gutter and channel programmes
- Available only in black, so it is a functional roofing/membrane sealant, not an architectural finish for visible facades
- Joint width is limited to 10-40 mm; very narrow joints are hard to seal effectively and over-wide joints cure poorly
- Contains solvent — requires ventilation during application and care over fire and health precautions per the SDS
- Being a bituminous mastic it can stain porous light-coloured substrates such as natural stone and fair-faced concrete
Selection & best practice
Design joints within the 10-40 mm width band with a minimum seal depth of 15 mm; treat 10 mm as the practical minimum width because very narrow joints cannot be filled and tooled reliably. Substrates must be clean, sound and dry — remove laitance, rust, oil and old sealant, and prime porous surfaces (concrete, masonry) with Fosroc Primer B2 before gunning; clean metal and bituminous surfaces generally take the sealant directly. For deeper or wider joints, use a closed-cell polyethylene backer rod or bond-breaker tape to control bead depth, prevent three-sided adhesion and let the bead cure from both faces. Gun the sealant firmly to avoid voids, tool it to make full contact with both joint faces, and respect the application temperature window of 5-50°C. Critically, programme the work so the seal gets its full drying time: tack-free in 1-2 days but a minimum of 14 days at 20°C (longer when cold or where beads are deeper than ~20 mm) before any permanent water immersion in gutters, channels or tanking. Because MB175 is a non-movement pointing grade, never substitute it where the design calls for an elastomeric movement-joint sealant — specify a Nitoseal, Thioflex or Colpor grade for those joints instead.
Specifications
| Product type | Single-component, gun-grade rubber-bitumen (rubberised bitumen) mastic joint sealant |
| Former name | Plastiseal |
| Grade / consistency | Non-sag, low-slump semi-stiff mastic (gun-applied, excellent slump resistance) |
| Cure mechanism | Solvent release (dries and stiffens as solvent evaporates) |
| Colour | Black |
| Specific gravity | approx. 1.4 |
| Minimum joint width | 10 mm |
| Maximum joint width | 40 mm |
| Minimum seal depth | 15 mm |
| Surface / tack-free time | 1-2 days depending on exposure |
| Full drying before immersion | Minimum 14 days at 20°C (longer if colder or depth >20 mm) |
| Application temperature | 5°C to 50°C |
| In-service temperature | -20°C to +80°C |
| Primer | Fosroc Primer B2 for porous substrates |
| Suitable substrates | Metal, concrete, masonry, asphalt and bitumen-based membranes |
| Movement accommodation | Not classified (non-movement pointing/lap-sealing grade; no ISO 11600 class / ASTM C920 MAF published) |
| Pack size | 600 ml sausages, cartons of 10 |
| Resistance | Alkalis, sulphates and biodegradation; excellent immersed adhesion |
Industry use cases
Industrial & warehouse roofing
Sealing sheet-metal sheet-laps, gutter laps and flashing details on large-span roofs where bitumen compatibility and slump resistance on slopes are needed.
Waterproofing & tanking contractors
Terminating and lapping Fosroc Proofex bituminous membranes on basements, podiums and retaining walls, where the sealant must bond to bitumen and survive immersion.
Drainage & water channels
Sealing lap joints in gutters, valleys and concrete drainage channels subject to ponded water, exploiting MB175’s excellent immersed adhesion after full drying.
Building repair & maintenance
Re-pointing around chimneys, rooflights, parapets and pipe penetrations on existing bituminous and felt roofs during refurbishment.
Infrastructure & precast
Sealing static, non-structural joints in below-ground precast and retaining-wall details where an alkali- and sulphate-resistant bitumen-compatible mastic is specified.
Frequently asked questions
Is Fosroc Nitoseal MB175 the same as Plastiseal?
Can I use MB175 on an expansion or movement joint?
How long before it can be in contact with water?
Do I need a primer with MB175?
What joint width and depth should I design for?
Does Space Arc supply Nitoseal MB175 in India?
How we position Fosroc Nitoseal MB175
Within the Fosroc range, Nitoseal MB175 is the specialist bitumen-compatible member of the joint-sealing family, and it should be chosen for what it does well rather than as a general-purpose sealant. Its natural partners are the Proofex bituminous membranes, which it seals, laps and terminates. Where a joint actually moves, step up to an elastomeric grade: a Nitoseal MS-polymer sealant for fast-cure facade and floor joints, Thioflex 600 (two-part polysulphide) for traditional construction and movement joints, or Colpor 200PF for heavy-duty trafficked highway and pavement joints. MB175 is the wrong choice for any of those moving-joint roles, just as those elastomers are the wrong choice against bitumen. Among credible alternatives a specifier in India might weigh, Sika’s Sikaflux/Sikaflex bituminous and PU range, BASF/MBCC MasterSeal sealants, and STP Limited’s bituminous jointing products occupy comparable niches — but for projects already detailed around Fosroc Proofex waterproofing, MB175 (or its current MB180 form) is the matched, system-compatible sealant. Space Arc Engineering can help consultants and contractors confirm whether MB175 or an elastomeric Nitoseal/Thioflex/Colpor grade is correct for a given joint before it is specified.
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