Daniel Bosworth, Design & Technical Manager at SIG Design & Technology discusses best practice for the design, installation and maintenance of hot melt roofing with Architecture Today’s Technical Editor John Ramshaw

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With a successful track record dating back more than 50 years, hot melt waterproofing has established itself as one of the go-to flat roofing systems for performance, durability, life-expectancy and cost-effectiveness. So when should architects specify hot melt roofs? And how should they approach the design, specification and installation processes? Daniel Bosworth, Design & Technical Manager at SIG Design & Technology, discusses these questions and more with Architecture Today’s Technical Editor John Ramshaw.


Hot melt installation process

What is hot melt roofing?
Most commonly used as part of an inverted roof system, hot melt is a liquid-applied membrane that adheres to the structural deck, creating a seamless waterproofing layer. SIG Design & Technology recommends SIG’s Permatec Hot Melt, a hot-applied membrane that is manufactured by IKO and comprises a combination of refined bitumen, synthetic rubbers, fillers and other additives. In use, it is melted in a purpose-built boiler and applied to the prepared structural deck in two nominal 3mm coats with a reinforcement layer in between them. Rigid insulation is then placed over the top of the membrane and weighted down with ballast, such as gravel, paving slabs or a green roof system.


Hot melt build-up for podium deck and inverted roof

What are the benefits of hot melt?
Permatec Hot Melt is a BBA-certified zero-falls rated product, which means that the roof fall can be designed down to 1:80 (approximately one degree). A ‘flat’ roof can be beneficial where there is no space for falls, or where planning considerations restrict building heights. While ponding should not exist on well-designed flat roofs, it will nevertheless not affect the hot melt lifetime performance or guarantee period.

Other benefits include an oven-free formulation, excellent low-temperature flexibility, speed of application, and the ability to be installed in temperatures as low as minus 18 degrees Celsius (providing the substrate surface is clean, dry and frost-free). Furthermore, the product can be walked on immediately after installation of the system, helping to facilitate fast-track flat roofing programmes.

SIG’s Permatec also has a number of important environmental benefits – the most obvious being that it is manufactured in the UK and therefore provides the most efficient carbon footprint in terms of delivery mileage. Added to this are zero wrapper waste and low-melt technology, which reduces energy consumption and allows the material to remain workable for longer.

Perhaps most importantly of all, hot melt roofing is highly dependable and designed to last the lifetime of the building. It forms an ultra durable monolithic waterproof barrier that eschews seams, lap joints and water tracking if damaged.


Hot melt build-up for intensive and extensive green roofs

What types of project is hot melt roofing suited to?
Hot melt is particularly suited to new-build construction, including tower blocks, offices, residential developments and podiums, as well as green and blue roofing installations. It is generally used in conjunction with concrete substrates, although it can be applied to timber decks. In both cases the substrate must be level and suitably prepared.

What projects should be avoided when using hot melt?
Specifiers should not use hot melt on projects where the deck cannot support the weight of the inverted roof system. The minimum ballast weight needed to hold down the insulation is 80kg per square metre. Hot melt is not particularly suited to metal substrates, as these must first be overlaid with timber prior to receiving the liquid membrane. Finally, the roofing system may not be suitable for projects where there are health and safety or management reservations about having a hot boiler on site and/or a product that is heated to 180 degrees Celsius prior to application.

How should architects approach the design and installation of hot melt roofs?
First and foremost, specifiers should take into account the weight of the roofing system that is required, and design the deck and supporting structure accordingly. Specifiers should also engage with hot melt manufacturers or distributors at the earliest opportunity. In the case of SIG, the company can assist with a range of technical issues, such as wind uplift ballast calculations, drainage calculations and solutions, U-value calculations, the NBS roof specification, including for green and blue roof systems, as well as general design advice.

It is worth considering that standard XPS and EPS inverted roof insulation is not as thermally efficient as PIR insulation, when used as part of a standard warm roof design. However, there are other products on the market, such as Enertherm XPS Plus, which outperform traditional XPS boards. Alternatively, there are vacuum insulation panels which can provide good levels of thermal performance on projects where upstands and thresholds are of limited height.


Pitch pocket installation

Sand and cement screeds are not recommended for use with hot melt waterproofing as they can take a long time to dry out prior to application of the membrane. The favoured product for SIG’s Permatec Hot Melt applications is Permascreed, a fully compatible, high-performance, mastic-asphalt levelling screed. One of the main advantages is that there is no need to prime a Permascreed-to-falls application before applying Permatec. Other benefits include zero water content and quick curing times. The main disadvantages of Permascreed are that it can only be applied by a mastic asphalt installer, and it is relatively expensive.

In terms of installation, hot melt roofs should always be applied by an experienced and certified contractor. The waterproofing system must be electronically tested to ensure that it is fully sealed prior to installation of the insulation and ballast or green roof system. If this is not done and a fault is detected, the roof build-up above the membrane may have to be removed in order to rectify the problem.

If for any reason the membrane becomes compromised, the area of water ingress will correspond precisely to the damaged portion of waterproofing. This is because the hot melt is intimately bonded to the substrate. On other types of flat roof, such as a mechanically fixed single-ply, water ingress from damage or poor installation tends to track across the substructure, only revealing itself at the lowest point of the roof. This can make tracing the source of the failure highly problematic.


Electronic leak testing

What guarantees are available for hot melt roofs?
The guarantee period for SIG’s Permatec Hot Melt roofs is generally 20 or 25 years. There are two types of guarantee available: a dual signature guarantee whereby the manufacturer covers the product and the roofing contractor covers the installation; or a single-point guarantee whereby the manufacturer covers both the product and the onsite workmanship.

How should hot melt roofs be maintained after installation?
In common with other types of flat roof, hot melt installations should be inspected at least twice a year, with gutters and outlets (if used) cleared, and any debris swept away.

Contact Details
For more information on hot melt roofing visit https://bit.ly/SIGHotMelt.