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Chris Page, product manager for Premium Liquids & Felts at SIG Roofing, on best practice liquid waterproofing

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When to use
Liquid waterproofing can be appropriate for both refurbishment and new-build projects. Since these products can often withstand long-term ponding, they can provide a more stable and durable waterproofing solution than sheeted products with joints that might over time become eroded. Suitable applications include:

Complex roofs
Liquid is particularly suitable for roofs with a lot of detail such as changes of direction, rooflights, pipes and edge details, which may be hard for sheet systems to accommodate. As a continuous surface without joints, it can also be advantageous in locations where aesthetics are particularly important.

Refurbishment and repair
Since liquid waterproofing is seamless and fully bonds to the substrate, it is ideal for use over existing roofing. The building is not exposed to water ingress during application, and there are no disposal costs for discarded roofing.  Another benefit, especially for occupied buildings, is that liquid waterproofing can be a lot quieter to install than single ply systems. Properly installed and maintained, liquid systems can last more than 25 years, and provide a cost-effective alternative to a roof replacement.

Pitched roofs
Cold applied liquids can be applied above pitches of 15º, making it a useful option for roofs combining both flat and pitched areas.

Inverted roofs
Liquid waterproofing is suitable for warm and cold roof systems and for inverted roofs where the membrane is laid under the insulation and ballast, including roof gardens.

Different options
Liquid applied waterproofing systems typically incorporate base coats, reinforcement and topcoats, and systems vary according to the number of coats and stabilisers required. There is a strong argument that the quicker and simpler an installation the better, since the more operations there are, the more chances of errors. The true financial cost of the product should take installation into account; systems that can be applied in a single pass give time and logistical advantages. Health and safety is another important consideration, with the products types listed below varying in terms of odour, VOCs and other hazards.

Moisture-triggered, cold-applied process that typically requires a primer.

This product type involves mixing a catalyst into a liquid and allowing it to cure for up to 40 minutes before typically applying UV stabilised top coat, to give a very tough surface.

Glass fibre reinforced polyester resin applied cold on site. Typically it can only be used over new plywood or OSB decking. It is the only option without early rain resistance or resistance to ponding.

These use moisture in the atmosphere to trigger the chemical changes from liquid to solid. Can be applied in a single sweep, with one installer applying a base coat, embedding the polyester reinforcing, then a further coat to saturate the fabric and then repeating as they progress across the roof.

Hot melt
Delivered in solid form, this polymer modified bitumen is heated in a mixer to 220ºC before being applied, reinforced with a felt layer, and topped with another coat of hot melt. Suitable for buried rather than exposed installations, such as beneath a green roof.

Further information on generic types of liquid waterproofing is available from the Liquid Roofing and Waterproofing Association (

Common pitfalls
A liquid applied system is only as good as the substrate it’s applied to, so preparation is vital. If the roof hasn’t been properly cleaned and primed it is likely to fail sooner rather than later. Most problems such as ‘pin-holing’ are down to incorrect installation, in particular insufficient liquid. All installers should be trained by the manufacturers to ensure they are fully conversant with the product they’re installing.  A good installer will monitor the absorption of the liquid, which varies according to the contours, as they work across the roof and ensure the correct amount is applied accordingly.

SIG Zinc & Copper is part of SIG Design & Technology and offers a complete and impartial design and supply service, which covers all eight steps to help create the perfect roof. It designs flat roofs, green roofs, and zinc, copper and stainless steel roofing and cladding.
Find out more at or call 0845 869 4887

SIG’s Hydrostop EU AH-25 product was chosen to refurbish 450m2 of roof at the Sacred Heart Primary School in the Bridgeton area of Glasgow. The project was specified by contractor City Building Glasgow, formerly the building service department of the local authority, as part of a wider refurbishment of the school premises. The school buildings date back to the 1950s. Areas of the roof had been leaking, leading to considerable water damage and disruption to some classrooms.
According to City Building Glasgow assistant investment manager Charlie McLean, the priority for the roof works was to procure a waterproofing system with minimal hot works that would cause as little disruption to the school’s everyday activities as possible. The chosen system would also need to be suitable for installation on a distinctively pyramidal part of the school roof. Another important factor was health and safety, both for those installing the product and the school community.
City Building Glasgow chose the Hydrostop EU AH-25 system because it met these criteria and offered additional safety, health and time benefits. A key issue was odour. After using an odorous liquid applied product on another school project, City Building Glasgow was encouraged by health and safety colleagues to find a product that was less pungent, according to McLean.
‘Hydrostop could be installed with minimum personal protective equipment as it has no obvious smells or safety constraints,’ he says, adding that installation process was quick since it could be laid on a damp substrate by roller in a single application.

  • Before
  • After

This meant the Hydrostop EU AH-25 system could be installed in just two weeks, with the whole waterproofing project, including the scaffolding, taking four to six weeks.
The Sacred Heart School project was the first time that City Building Glasgow has specified the Hydrostop EU AH-25 system. In preparation for the installation, SIG set up a training facility at the City Building Skills Centre in Glasgow and ran a two-day training course for the contractor and subcontractor in the correct use of its product. City Building Glasgow expects to use the waterproofing system in further school refurbishment projects in the future.
Hydrostop EU AH-25 is a wet-on-wet, cold applied liquid waterproofing product that is reinforced with a polyester fabric. It is free of solvents and hazardous materials, and has a very low VOC content, making it fume-free and virtually odourless.

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