1. Introduction
Liquid applied waterproofing is a widely used method of protecting flat and low-pitched roofs from water ingress. It is particularly suitable for commercial, industrial and residential buildings where an existing roof requires refurbishment, renewal or additional waterproofing without necessarily removing the existing roof covering.
Liquid waterproofing systems are applied in liquid form directly to a prepared roof substrate. Once cured, the material forms a continuous, fully bonded waterproof membrane. Depending on the system selected, reinforcement, primers, detailing materials and protective or decorative finishes may also be incorporated.
Liquid applied membranes are a leading solution for both new-build and refurbishment applications, with systems available for a wide range of roof types and substrates.
2. What Is Liquid Waterproofing?
Liquid waterproofing is a roof waterproofing system manufactured from specially formulated liquid resins, polymers or modified bituminous materials. The liquid is applied by brush, roller, squeegee or spray to the prepared roof surface and subsequently cures to form a seamless waterproof membrane.
There are several different liquid waterproofing chemistries available, including polyurethane, polyurea, PMMA, polyester, polymer-modified bitumen, water-dispersible polymers and silane-modified polymers. The appropriate system depends upon the existing roof construction, substrate, exposure conditions, expected movement, traffic requirements and the intended service life.
A typical liquid waterproofing system may consist of:
- Substrate preparation and cleaning.
- Primer or substrate stabiliser.
- Liquid waterproofing membrane.
- Reinforcement fleece or mat where required.
- Additional detailing materials.
- Topcoat or protective finish where specified.
The resulting membrane can provide a continuous waterproof layer with substantially fewer joints than traditional sheet-based systems.
3. Why Is Liquid Waterproofing Necessary?
The primary purpose of roof waterproofing is to prevent rainwater and moisture from entering the building fabric. Failure of a roof waterproofing system can result in water entering insulation, roof decks, ceilings, internal finishes and building services.
Water ingress can also cause deterioration of the roof structure. On reinforced concrete roofs, repeated exposure to moisture can contribute to carbonation, chloride contamination and reinforcement corrosion. On timber or metal roof constructions, prolonged moisture exposure can contribute to decay or corrosion.
Roof defects commonly occur at joints, laps, penetrations, outlets, upstands, parapets, flashings and areas where previous repairs have failed. Liquid systems can be particularly useful at these locations because the material can be formed around complex details and can provide a continuous bonded waterproofing layer.
Waterproofing is therefore an important part of protecting the building envelope and maintaining the long-term performance of the roof.
4. Liquid Waterproofing for Existing Roofs
One of the principal advantages of liquid waterproofing is its suitability for refurbishment.
Subject to a satisfactory condition survey and compatibility assessment, some existing roof coverings can remain in place and be used as the substrate for a new liquid waterproofing system. This can reduce the extent of strip-out and minimise disruption to building occupants.
Before installation, the existing roof must be inspected to determine whether the substrate is suitable. Defective areas, blisters, loose materials, contamination, standing water and damaged components should be identified and addressed.
5. Surface Preparation
Correct surface preparation is essential to the performance of a liquid waterproofing system.
The roof should be thoroughly inspected and cleaned, with dust, dirt, vegetation, oils, loose materials and other contaminants removed. Existing defects should be repaired and unstable areas removed or stabilised.
The substrate must be suitable for receiving the waterproofing system. Depending on the substrate, preparation may involve mechanical cleaning, grinding, abrading, pressure washing or other approved methods.
Primers are often used to improve adhesion between the existing substrate and the waterproofing membrane. However, primer does not replace the requirement for proper substrate preparation.
6. Reinforcement and Detailing
Reinforcement may be incorporated into the liquid membrane where additional strength, crack-bridging capability or durability is required.
Particular attention should be given to roof details, including:
- Roof outlets and drainage points.
- Upstands and parapets.
- Roof penetrations.
- Plant supports.
- Rooflights.
- Flashings.
- Expansion and movement joints.
- Changes in roof level.
- Internal and external corners.
- Existing cracks and repaired areas.
These locations are often more vulnerable to water ingress than the main roof field. Proper detailing ensures continuity of the waterproofing system and reduces the likelihood of premature failure.
7. How Liquid Waterproofing Helps the Roof
Once installed and cured, the membrane forms a continuous waterproof barrier over the prepared substrate. This helps prevent rainwater from penetrating the roof construction.
Many systems are flexible and capable of accommodating a degree of thermal and structural movement. This can be particularly beneficial on roofs exposed to significant temperature variations.
The seamless nature of liquid membranes also reduces the number of conventional joints and laps that would otherwise represent potential leakage points. Seamless construction, flexibility, ease of detailing and ease of maintenance are among the principal benefits of liquid applied systems.
Liquid membranes can also be used as overlay systems over suitable existing waterproofing. This can extend the serviceable life of an existing roof without requiring complete removal of the existing covering.
8. Benefits of Liquid Waterproofing
Liquid waterproofing provides a number of technical and practical benefits.
Seamless waterproofing: The cured coating forms a continuous membrane, reducing reliance on numerous laps and joints.
Complex detailing: Liquid materials can be applied around penetrations, outlets, upstands and other difficult roof details.
Refurbishment suitability: In appropriate circumstances, systems can be applied over existing roof coverings, reducing demolition and waste.
Reduced disruption: Many systems are cold applied, avoiding the use of open-flame torch application and potentially reducing disruption to building occupants.
Flexibility: Elastomeric systems can accommodate thermal movement and, where appropriately designed, bridge certain cracks and joints.
Durability: Correctly specified and installed systems can provide long-term protection against weathering and water ingress.
UV resistance: Certain liquid chemistries provide strong resistance to ultraviolet exposure and weathering.
Aesthetic finishes: Liquid systems can be supplied in different colours and finishes, including decorative and solar-reflective options.
Ease of maintenance: A continuous membrane can be inspected and locally maintained where necessary.
Reduced waste: Refurbishment overlay systems can reduce the amount of existing roofing material requiring removal and disposal.
9. Quality Control and Installation
The performance of liquid waterproofing depends heavily on correct installation.
The manufacturer’s specification should be followed for substrate preparation, primer selection, application rates, curing times, reinforcement and environmental conditions. Application should only be undertaken by suitably trained and competent personnel.
Quality control should include inspection of the substrate, confirmation of primer application, monitoring of coverage rates, checking reinforcement installation and inspection of completed detailing.
Where appropriate, electronic integrity testing can also be considered to identify breaches or discontinuities in the completed waterproofing membrane.
Selecting systems with appropriate third-party accreditation, such as BBA, BDA or ETA certification, where applicable are recommended.
Relevant UK roofing guidance should also be considered, including BS 6229 for flat roofs, together with other applicable standards, Building Regulations and manufacturer requirements.
10. Maintenance
Although liquid waterproofing can provide a durable solution, regular inspection and maintenance remain necessary.
Roofs should be periodically inspected for cracking, damage, ponding water, blocked outlets, deterioration around penetrations, mechanical damage and defects to perimeter details.
Drainage systems should remain clear so that water can discharge effectively. Any defects identified during inspections should be repaired before they develop into larger failures.
Early intervention is particularly important because minor defects are generally easier and less costly to repair than widespread membrane failure and associated internal water damage.
11. Conclusion
Liquid applied waterproofing provides an effective method of protecting roofs against water ingress and weather-related deterioration. Its ability to form a seamless, fully bonded membrane makes it particularly useful for complex roof layouts, refurbishment projects and situations where numerous joints, penetrations and existing details need to be waterproofed.
The system can provide significant benefits including durability, flexibility, ease of detailing, reduced disruption, refurbishment capability and long-term protection of the underlying roof structure.
However, successful performance depends upon correct system selection, detailed inspection, suitable substrate preparation, competent installation and ongoing maintenance. A roof should therefore be assessed before specification to establish the condition of the existing construction and determine whether liquid waterproofing is appropriate.
When correctly designed, specified, installed and maintained, liquid waterproofing can significantly extend the serviceable life of a roof, reduce the risk of water ingress and provide a cost-effective method of maintaining and protecting the building envelope.