Continuing Professional Development | Jan12: A watertight argument

Continuing Professional Development: Roof waterproofing systems

•  The different systems available
•  Why correct specification is essential
•  The common pitfalls to watch for

The CIOB requires members to assess and fulfil their own CPD needs. Members can therefore choose to study the CPD articles published in CM as a valid part of their personal record of CPD activity.

Click here to be taken to the online CPD test paper for the January 2012 issue

Stewart Bennett outlines the pros and cons of different roofing systems and examines the performance, environmental and aesthetic considerations that should play a part in their specification.Failure to use an efficient and effective waterproofing system that is appropriate for the roofing application can result in damage to the building and its contents, disruption to its occupants and costly, time-consuming repairs that can mean repeat visits to site and affect client relationships.

Ensuring that the waterproofing system is well specified and properly installed in the first place is much easier than repairing it once it has failed and the common causes for membrane failure are many and varied. Running water, standing water and damp are obvious culprits, but foot traffic, unprotected exposure to UV and damage from fixings or other penetrations can also compromise the integrity of the membrane. A successful roofing system must provide a complex mix of attributes: it must be strong while remaining flexible enough to allow for structural movement, it should resist puncturing and wear and tear, offer reliable waterproofing, durability and an appropriate level of aesthetic finish.

While maintenance of the roof is inevitable over the course of a building’s lifespan, a waterproofing membrane should be expected to provide a permanent seal against water ingress for a service life of at least 25 years.

For most types of commonly used roofing systems, this 25-year target is achievable, but only if the roof has been correctly specified and properly installed. Too often, faults in product formulation, inappropriate specification, mistakes in installation design or poor workmanship can contribute to system failure.

Whatever the root cause of the failure, the cost and upheaval of rectification can be significant and could even involve secondary damage to the building or its contents. It is therefore essential to understand the advantages and limitations of different types of waterproofing technology and consider the risk factors inherent in each.

Key considerations

There are pros and cons for all waterproof roofing systems and it is important to have a checklist of criteria as the most appropriate system for the application will depend on a wide range of factors. These include the complexity and size of the roof, whether it’s a new build or a refurbishment, the client’s environmental aspirations and the aesthetic requirements for the building.

Inevitably, cost and scheduling criteria must also be factored into any decisions and the specification should consider the cost of both the materials and the installation. Inherent in this are issues such as longer installation times or additional insurances for the use of heat equipment on site, which can have a significant impact on the cost.

It is important to note here, too, that cost calculations should be made on a whole-life cost basis rather than simply a purchase cost or even purchase plus installation cost basis.

Some of the less expensive options will need more maintenance and will not offer long-term durability — so often a single installation of a more expensive system can be more cost-effective than one that may need to be repaired several times over a comparable lifetime. It is essential that the specifier researches the reliability of the system, considering not only the required service life, but also the track record of different systems and what test certificates or warranties are offered to substantiate their durability.

While waterproofing technology has progressed significantly over the years, traditional systems are still commonly used, and in some applications they still have many advantages.

Torch-applied roofing systems consist of liquid bitumen which is poured onto the substrate and rolled to provide a flat finish. Relatively inexpensive, torch-applied roofing systems bond well to
most substrates and require no primer or membrane, so they are also fast to install. These systems can offer a strong and durable roofing surface, but mistakes and poor workmanship often lead to the failure of bituminous roofs and the added difficulty of installing them on complex roof detailing means this option is only really suitable for the most straightforward applications. 

The Guildhall in the City of London (above and above right) has benefited from a solvent free liquid membrane for repairs to its west wing roof

Health and safety risk

Finally, and probably most significantly in recent years, torch-applied systems carry an inherent health and safety risk as they require a naked flame on site. The move to a much more stringent health and safety culture and the cost of site insurance where there is an increased risk of fire often make torch-applied systems unviable.

Felt also provides an economical option that offers a good initial bond with most substrates. However, its advantages can be short-lived with debonding issues and degradation over time due to UV exposure. Felt systems can also prove problematic on roofs that feature complex detailing and fire risk issues also contribute to the negative points.

Single-ply waterproofing systems overcome the fire risk issues and provide a flexible, lightweight solution that is suitable for most substrates. However, difficulty in waterproofing roofing details, and the necessity for mechanical fixings and mastics can present a risk of failure.

Fibreglass systems are easy to install and, if applied correctly, provide a long-lasting, durable option, especially for smaller domestic applications. However, they are not suitable for use on larger roof areas due to expansion issues.

Nor do they lend themselves to use in refurbishment projects as they require the provision of a new, clean deck. What’s more, this system creates a very rigid waterproof layer that can be prone to cracking following building movement, especially at details and penetrations.

Developments in waterproofing technology have resulted in a rapid rise in the use of cold-applied liquid systems. These fall into two main groups: wet-on-dry and wet-on-wet systems.

Installation of wet-on-dry systems involves a three-step process starting with the application of a primer followed by a membrane and then a resin. This must then be left to dry before a second coat of resin can be applied to complete the system.

Wet-on-wet systems, meanwhile, can be applied in a single operation with the resin being applied directly onto the primer, followed by a layer of flexible fleece which is pressed into the resin to aid saturation. A further coat of resin is applied over the fleece to complete the system and this cures to create a single waterproof membrane that bonds securely to the substrate and cannot delaminate.

Liquid applied systems offer numerous advantages over traditional systems and provide a tough, durable waterproofing solution that is flexible enough to withstand movement while offering tensile strength, wear resistance and protection from risk of puncture.

The speed of wet-on-wet systems’ single operation installation gives these products a clear advantage, and the ease with which detailing can be securely waterproofed adds to the benefits. The viscous liquid resin can be applied over any horizontal or vertical feature and the fleece is simply cut to shape and size with scissors and laid onto the detail with a 5cm overlap ensuring a continuous impermeable membrane. Accordingly, correctly-applied wet-on-wet systems bond permanently with the substrate
and cure quickly to form a lightweight, seamless membrane.

Eco credentials

No CPD on the subject of waterproofing would be complete without a discussion of environmental specification. The proliferation of green and brown roofs makes an understanding of waterproofing systems compatible with such roofs essential. The in-built root resistance, light weight, tensile strength and permanent bond with the substrate offered by cold liquid-applied systems make them ideal for green roofs.

Heed should also be paid to the environmental impact of the waterproofing system throughout its lifetime, not only with regard to the materials from which it is made, but also in relation to how long it will last and whether it will need to be disposed of to landfill. Once again, the advantages of cold liquid applied systems become clear here as they can be applied on top of the incumbent system, removing the need to dispose of the existing roof surface to landfill, which has time and financial advantages as well as environmental benefits. Indeed, the Kemperol range of cold-liquid applied membranes from Kemper System can completely seal in asbestos, avoiding the health and safety and cost implications of disturbing and disposing of it.

Finally, there is also a new generation of eco-friendly waterproofing systems, including the Kemperol 2K PUR solvent-free wet-on-wet system, which uses castor oil from sustainable sources and a fleece reinforcement made from a high percentage of recycled plastic bottles to provide a credibly eco-friendly option that is extremely durable and reliable. The removal of solvents from the formulation allows roof refurbishment work to be carried out while the building remains occupied — just as it was during the recent refurbishment of Guildhall’s west wing for example — without any unpleasant fumes.

At a time when budgets are tight across the construction sector, specifying the right product first time is more vital than ever. The best value approach pioneered in the public sector is now prevalent in the private sector which, when it comes to roofing, means that more technically advanced roofing products that provide greater reliability and longevity are increasingly top of specifiers’ lists.

Stewart Bennett is executive director (specification liaison) at waterproofing system specialist Kemper System

Roofing solution proves to be Guildhall’s liquid asset

Solvent-free liquid membrane meets tough requirements at historic City building

At the Guildhall in the City of London, refurbishment of the west wing roof had to take into account a number of factors. The area of the roof in question sits above function rooms and high-ranking councillors’ accommodation and this section of the building had to remain operational throughout the roofing project.

In addition, the roof is complex with significant amounts of detailing and numerous service units that demanded a solution that could be fitted accurately to the exact contours of the structure without the risk of leaks. Finally, the specified system had to deliver best value, which meant using a waterproof membrane that could be laid quickly, with proven longevity and a reliable warranty.

Solvent-free Kemperol 2K-PUR, a cold applied liquid membrane, was specified because of its ability to meet all of these requirements.

Michael Coleman, from the City of London Corporation, says: “Choosing a solvent-free cold liquid applied system meant that we could continue on a business as usual footing without having to worry about fumes or the hazardous equipment used for hot-applied systems.”

The 1,000m² section of roof was refurbished by specialist contractor CRC Group in a 10-week programme beginning at the end of September. The Kemperol 2K-PUR membrane bonded directly on top of an existing foil-faced modified bitumen composite, which was cleaned and primed by CRC prior to the application of the membrane.

Steve Web from CRC says: “Being able to apply the new waterproofing membrane over the existing substrate saves time on site and means that very little, if anything, has to go to landfill. However, it’s important that the existing substrate is as clean and even as possible to ensure a secure bond and good finish, so we carry out thorough preparation works first.”

With the roof primed and ready, CRC applied the Kemperol 2K-PUR liquid resin to the substrate, spreading it out evenly before laying a durable, flexible non-woven fleece reinforcement directly onto the wet resin. More resin is then poured on top of the fleece and rollered to press it down to ensure complete saturation of the fleece with the resin in a single process, removing any folds, creases or bubbles.

Once cured, the fleece-reinforced liquid membrane becomes a single, seamless waterproof surface which forms a strong, but flexible, bond with the roof that will not delaminate. The fleece is made from 25% recycled materials and can be cut to size and shape quickly and easily on site. This enabled CRC to cut it to fit the many complex shapes and angles in the roof.

The roofing project formed part of a wider upgrade to Guildhall’s west wing, which also included cleaning of the external stonework.

CPD test paper: Roof waterproofing systems

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