Terry Wain is the Technical Secretary at the Liquid Roofing and Waterproofing Association (LRWA).

ot melt monolithic systems have delivered a viable waterproofing solution for flat, trafficked, inverted and bio-diverse roofs for more than 40 years. Yet, many do not realise they fall under the liquid waterproofing umbrella – albeit a niche sector. However, the use of this technology is on the rise and has provided an ideal waterproofing solution for many new-build and suitable refurbishment projects.

Despite its increasing popularity there are some challenges and, aligned with the recently-published LRWA ‘Hot Melt Code of Practice’, there are some key considerations to make during specification and application.

The case for hot melts

Hot melts offer a highly durable solution, providing excellent waterproofing performance with toughness, flexibility and adhesion over a wide range of temperatures. Systems are quick to apply and some hot melt monolithic membranes have a BBA-certified life expectancy of “the life of the structure it is applied to”, providing clients with reduced building maintenance, peace of mind and whole-life cost benefits.

Although the LRWA endorses the British Standards with regards to design falls in the flat roof sector, in some circumstances, such as the development of water features, certain green roofs and podium roof decks, zero falls, or perfectly flat roof surfaces, may be required. More often than not, hot melt systems appear to be favoured in these cases, and manufacturers’ instructions and advice should be taken.

Specification considerations

In the last decade, the industry has seen an influx of new manufacturers emerging to meet demand. With many ‘newcomers’ to the market, if a hot melt system is specified it’s important to seek advice from a trusted trade association such as the LRWA, which is the only recognised body in liquid-applied membranes to produce compliance guidance notes, a ‘Hot Melt Code of Practice’, plus stringent manufacturer membership criteria.

Most hot melt systems comprise two, 3mm layers of product with the reinforcement sheet enclosed between each layer. However, in the past few years, we have seen single-layer systems being introduced into the market. The specifier must consider the sustainability of this system, bearing in mind, in some cases, it will be buried under a substantial amount of surface ballast in the way of green roof materials, paving slabs or water – if a water feature is included in the project. To ensure a sound, long-term waterproofing solution, it’s important the correct decisions are made during specification.

A pre-inspection should be undertaken prior to installation, to assess its readiness for application. The performance of a system is dependent on the substrate construction and its condition, and documented bond and peel tests should be conducted to ensure correct bond strength to the substrate and suitable adhesion to the roof surface can be achieved.

All work also requires a post-inspection covering membrane thickness, reinforcement application, access sheet application, flashings and detailing, adherence to the design specification and electronic leak detection.

Contractors should also be aware of the adequate certification – these include ensuring products meet ISO 9001 standards and achieving independent product accreditations such as BBA and ETA certificates.

Application best practice

Site access can often present challenges if not planned properly before work begins. With hot melt materials, an agitating melter will be required on the roof at all times to prepare the product before application, a key consideration to have in mind during specification and before application. To ensure a project stays within budget and is delivered on time, adequate access to the roof is required, as well as space for equipment to be stored on site before a project begins.

During installation, contractors should also make sure they achieve the correct thickness or depth of the product and always adhere to manufacturer’s guidance to the correct temperature it should be when poured.

All materials must be stored carefully in accordance with the manufacturer’s instructions to prevent product damage or a health and safety risk to operatives or the general public.

The importance of training

In an ideal world, operatives should achieve a National Vocational Qualification (NVQ) for Hot Melt Monolithic Waterproofing Systems, and gain Construction Skills Certification Scheme (CSCS) cards – which new-build and local authority projects often insist contractors have before access to site. However, hot melt manufacturers should provide their own approved-contractor schemes, ensuring operatives are fully trained to work with the products properly. All LRWA manufacturer members offer this as part of meeting membership criteria.

To help address this, the LRWA launched its Basic Competency Programme (BCP) in 2015, recognised by the Construction Industry Training Board (CITB).

The BCP acknowledges the contribution of manufacturers towards providing a skills base through training in the use of their materials to satisfy the requirements of the red ‘Experienced Worker’ CSCS card.

Additionally, the LRWA has also developed and delivers a Specialist Applied-Skills Program (SAP) and Specialist Up-Skilling Program (SUP), aimed at improving the skill quality of all operatives and supporting the young installers of tomorrow, and provides a route to NVQ2.

Staying safe

Where operatives are working at height, fundamental working practices within the Health and Safety at Work Act should be adhered to. It is important to gather as much health and safety information about a project and the site before work begins. A site survey should be made by qualified personnel, also ensuring Safety Data Sheets are issued by the product manufacturer. These are essential reading prior to handling any materials, which detail the products ingredients, handling techniques, storage and disposal considerations.

Looking ahead

A key consideration for specifiers and contractors – and something the LRWA continues to assess – is ensuring correctly calculated U-values are in place for every project. In inverted roof systems, the design Lambda value should always be used, as this takes into account the rainwater cooling effect. Not using the declared Lambda value means the insulation will not meet the U-value requirements.

Looking ahead, the LRWA looks to work with the wider industry to develop an official guidance to explain how calculations are undertaken correctly.