A Consultant’s View –  Waterproofing Membranes on Balconies with Lightweight Claddings

My job is to assist clients with waterproofing issues arising from leaks to their homes, apartments or places of business.

I am not going to dissect all of the challenges we face as an industry, but I will discuss in this issue one of the most common faults I see on an almost a daily basis, for which you as the waterproofing applicator are constantly blamed.

Balconies present a large part of the work I conduct when water ingress to the internal space of a property is to be investigated. These balconies are often built over living spaces. In this scenario the balcony is constructed of light weight materials, timber frame and fiber cement sheet. The walls encompassing the balcony are also of light weight construction consisting of a lightweight material such as a timber frame with fiber cement, polystyrene or timber weatherboards.

The waterproofing applicator has been contracted to install the waterproofing membrane to the balcony in preparation for installing tiles. Once on site the waterproofing applicator notes that the lightweight cladding has been installed (hard onto the balcony surface), the balcony flooring substrate is installed and ready for application of their preferred waterproofing system.

The waterproofing applicator prepares the substrate and starts applying his waterproofing membrane to the balcony substrate. He duly applies a primer to the balcony substrate and associated upstands, downturns and other details. Next, the internal corners are readied with a sealant fillet to the required dimensions and prepares to apply membrane as per the manufacturer’s instructions to all upstands, downturns and other details.

Great, so far everything is going well … but wait, those upstands around the balcony perimeter…The membrane has been applied to the cladding upstand, check, and the tiler will then apply a soldier tile around the outer perimeter, check.


The lightweight cladding has been installed and is placed onto the surface of the balcony. The waterproofing applicator has applied his membrane upstand to the outside of the lightweight cladding.

Application of a waterproofing membrane to the outside of lightweight cladding is a very poor practice and should never be carried out in this manner. Lightweight claddings are designed to drain/ vent water or moisture to the outside of the building envelope if a joint fails or window flashings fail over time.

Lightweight cladding designs require the waterproofing membranes to be applied to the balcony wall frame as an upstand typically of 150 mm before the cladding is installed. The external building wrap must then be installed with an overlap to the balcony waterproofing membrane upstand. Only then should the lightweight cladding be installed. The cladding is to be installed so that it is at least 20 – 30 mm above the finished floor level (i.e. top of the tiles) once the balcony is complete.

This detail allows any water that is able to penetrate the lightweight cladding to be shed to the external side of the construction.

Why is this the problem for the waterproofing applicator you ask. Because, inevitably it is the waterproofing applicator who is blamed. If it leaks it must be a waterproofing fault.

Strictly speaking this is a fault more likely attributable to the builder as the main contractor because it is the builder (or their sub-contractor) who determines the location of the cladding and fixes the cladding in place.

However, as a responsible waterproofing applicator you must also protect your business and reputation. The detail above is how your waterproofing should typically be integrated with the balcony and cladding junctions in this scenario. As the owner of a business, you have a duty of care to your business to assess your risks with every project you undertake. I would encourage all waterproofing contractors to take this detail on board. I realise that this detail is largely out of your control, however, it is one of the most misunderstood details in waterproofing in my view and causes very expensive litigation and repairs to be undertaken needlessly.

Karl Wootton