Flat Roof Waterproofing Systems
There are six different systems for waterproofing a flat roof. Each has a specific appropriate application. Choosing the wrong system for the substrate, size or use case is the most common cause of failure. This page is a decision framework — not a product page.
What Is Flat Roof Waterproofing?
Flat roof waterproofing is the application of a membrane or coating system to a flat or low-pitch roof to prevent water ingress, protect the building structure and extend the roof's service life.
Unlike pitched roofs where water runs off by gravity, flat roofs rely entirely on the integrity of the waterproof membrane. When that membrane fails — through age, UV degradation, physical damage or installation error — water enters the building.
The technology available for flat roof waterproofing has changed significantly. Modern systems — EPDM rubber membranes, liquid-applied coatings, spray-applied rubber, polyurea membranes — are substantially more reliable and durable than older felt-based approaches. Understanding which system is appropriate for a specific roof, substrate and use case is the critical decision.
This page covers all six systems Forti Nova supplies for flat roof waterproofing, their appropriate applications, and the scenarios where each system is not the right choice.
Types of Flat Roof
Flat roofs are not all the same. The substrate, use, size and condition all influence which waterproofing system is appropriate.
Flat Roofs on Garden Rooms & Extensions
Typically timber deck construction. EPDM is the dominant system — single sheet, cold-applied, no seams. Liquid rubber is used for complex detailing and areas where the sheet cannot conform. Both are suitable for DIY and self-build.
Residential Flat Roofs
Houses with flat or low-pitch roofs. EPDM and liquid rubber are the standard approaches. Substrate preparation and drainage falls must be checked before any system is applied. Existing felt roofs typically require removal before a new system is installed.
Commercial Flat Roofs
Warehouses, offices, retail buildings and industrial units. Larger areas suit spray-applied systems (SprayGrade, HotSpray) for output efficiency. Complex detailing is completed with liquid rubber. System selection depends on roof condition, substrate and programme.
Balconies & Trafficable Decks
Require a waterproofing system designed to withstand foot traffic. Prokol MonoSeal (hand-applied polyurea) is the appropriate system. Not all flat roof waterproofing systems are suitable for trafficable applications — check product specification.
Concrete Flat Roofs
Common on commercial and multi-storey buildings. Accept liquid waterproofing and spray-applied systems. Substrate preparation — crack treatment, priming and moisture assessment — is critical before any liquid system is applied.
Flat Roofs with Active Leaks
Immediate containment of water ingress is the first priority. ProFix can be applied to wet surfaces as an emergency measure. A full survey and specification should follow once the active leak is controlled.
Two Questions Before Choosing a System
Before any system can be correctly selected, two questions must be answered. Getting either wrong leads to the wrong system.
What is the roof being used for?
- A non-trafficable flat roof (garden room, extension, warehouse) — most systems apply
- A balcony or terrace that people walk on — requires a trafficable polyurea system (MonoSeal)
- An emergency repair to an active leak — ProFix
What is the scale and context of the project?
- Small domestic roof or garden room — EPDM or liquid rubber
- Complex geometry, repairs or detailing — liquid rubber HBS200
- Large commercial or industrial roof — SprayGrade or HotSpray
Flat Roof Waterproofing Systems
Each system is appropriate for specific applications. Each system has clear situations where it is not the right choice. No system is universally suitable for all flat roofs.
EPDM Roofing
EPDM is a pre-formed synthetic rubber sheet membrane installed as a single piece over the roof deck. It is cold-applied, requires no hot works and creates a seamless waterproof layer without joints.
Suitable for
- Residential flat roofs, garden rooms and extensions
- New flat roof construction
- Full roof replacement on smaller domestic roofs
- Projects where a sheet membrane can be installed without major detailing constraints
- Self-build and DIY-accessible applications
Not appropriate when
- Roofs with highly complex penetrations and detailing where a sheet cannot conform
- Very large commercial roofs where seaming multiple sheets is undesirable
- Refurbishment overlay where EPDM is not the substrate system
Liquid Rubber HBS200
HBS200 is a brush or roller-applied liquid waterproofing membrane that cures to a seamless, flexible finish. It conforms to complex shapes, angles and penetrations that sheet membranes cannot accommodate.
Suitable for
- Complex detailing — upstands, penetrations, gutters, parapet walls
- Localised waterproofing repairs and patch work
- Refurbishment of smaller roof areas and balconies
- Overlay applications onto stable prepared substrates
- Preparation and detailing alongside spray-applied systems on larger projects
Not appropriate when
- Very large commercial roof areas where spray output is more efficient
- Substrates with structural defects, retained moisture or failed drainage
- Projects with very short programme requirements
SprayGrade
SprayGrade is a spray-applied liquid rubber coating for large-area flat roof waterproofing. It creates a seamless high-build membrane across the entire roof surface and is suited to commercial and industrial buildings where output speed is a requirement.
Suitable for
- Large commercial and industrial flat roofs
- Warehouses, distribution centres and factory buildings
- Flat roof refurbishment at scale
- Asbestos cement roof encapsulation (subject to survey and compliance)
- Projects where a seamless membrane across a large area is required
Not appropriate when
- Residential flat roofs where mobilisation cost is disproportionate
- Metal roofs requiring a polyurethane coating system
- Balconies and trafficable decks requiring a polyurea surface
- Sites where spray equipment access is not feasible
Prokol MonoSeal
Prokol MonoSeal is a hand-applied pure polyurea waterproofing membrane suited to balconies, terraces, walkways and podium decks. It provides a trafficable, seamless and chemically resistant surface.
Suitable for
- Balconies, terraces, walkways and podium decks
- Trafficable flat surfaces requiring a waterproof membrane
- Complex flat roof detailing requiring a polyurea membrane
- Gutters and drainage channel waterproofing
- Refurbishment of accessible decks and balconies
Not appropriate when
- Large roof areas where spray application is more appropriate
- Metal roofs requiring a polyurethane topcoat system
- Substrates that have not been correctly prepared and primed to system specification
Rocathaan HotSpray
Rocathaan HotSpray is a spray-applied hot polyurea system for large commercial flat roofs. It cures faster than spray liquid rubber and delivers a high-build seamless membrane suited to demanding commercial and industrial waterproofing specifications.
Suitable for
- Large commercial flat roofs requiring spray-applied polyurea
- High-specification commercial and industrial waterproofing
- Projects where faster cure than liquid rubber spray is a requirement
- Extensive roof areas where a high-build seamless membrane is specified
Not appropriate when
- Residential or small domestic projects where mobilisation cost is disproportionate
- Metal roofs where a polyurethane coating system is specified
- Projects without access to trained polyurea spray operatives and specialist equipment
ProFix
ProFix is an MS hybrid polymer sealant for emergency flat roof leak repair. It can be applied to wet surfaces and is suited to situations where immediate action is required to stop active water ingress while a full specification is developed.
Suitable for
- Emergency leak repair on flat roofs
- Wet surface application where drying is not possible
- Temporary sealing of joints, laps, cracks and penetrations
- Immediate action to stop active water ingress
Not appropriate when
- Permanent waterproofing of new or refurbished roofs
- Large area waterproofing applications
- Replacement for a full waterproofing system specification
New Roof, Refurbishment or Repair?
System selection also depends on the project type. These are three distinct scenarios with different implications.
New flat roof
The deck is new or has been stripped to a sound substrate. The full range of systems is available subject to substrate compatibility. EPDM is the standard choice for domestic projects. Liquid rubber and spray systems suit commercial new-build where seamless coverage is required.
→ Full system selection — no substrate compatibility constraint from an existing membrane.
Flat roof refurbishment
The existing deck is sound but the membrane has failed, aged or is approaching end of life. A new system is applied over or in place of the existing membrane without full deck replacement. Compatibility between the new system and the existing substrate must be assessed.
→ Substrate assessment required. Liquid applied and spray systems are commonly used for refurbishment. Sheet membranes may require removal of the existing layer first.
Flat roof repair
The roof has localised failure — a specific leak point, failed joint, split flashing or damaged penetration. The majority of the membrane is intact. Targeted repair using liquid rubber or emergency sealant addresses the failure point without a full system programme.
→ Targeted repair. ProFix for emergency action. HBS200 for planned localised repair. If repairs are repeatedly failing, a full survey and re-waterproofing programme should be considered.
Related Applications
If your project is more specific, these pages go into greater depth on particular applications.
Commercial Roof Waterproofing
System selection and decision framework for commercial and industrial flat roofs
Read more →Commercial Roof Refurbishment
Refurbishment vs. replacement decision framework for commercial and industrial roofs
Read more →Metal Roof Refurbishment
Polyurethane coating systems for metal roof structures
Read more →Waterproofing Systems Hub
Overview of all waterproofing systems and families supplied by Forti Nova
Read more →Flat Roof Waterproofing Systems Guide
In-depth comparison guide across all flat roof systems
Read more →Prokol MonoSeal — Balconies
Hand-applied polyurea for trafficable balcony and terrace waterproofing
Read more →Support Pathways
Depending on your role and requirement, these routes provide access to the appropriate support.
Approved Contractor Route
Project scoping and approved-contractor introductions across the North West
View Contractor Route →Training Courses
Professional installer training for EPDM, Liquid Rubber, SprayGrade and Prokol systems
View Courses →Approved Applicator Programme
Find or become an approved contractor for Prokol systems
Learn More →Frequently Asked Questions
Not Sure Which System Is Right?
Tell us about your roof — the substrate, area, condition and what you're trying to achieve — and we'll identify the correct system for the job.
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