Flat roof waterproofing guide

Exterior view of Westmead hospital

Flat roofs are a distinct part of modern architecture design and act as an aesthetically appropriate frame for a modern or contemporary style building or home.

When planning a flat roof build or renovation, it is important to consider specific flat roof waterproofing provisions.

Here is our guide to flat roof waterproofing from the waterproofing experts.

Element analysis

When planning flat roof waterproofing, some factors we consider include:

Heavy rain on a black flat roof


  • What is the annual average rainfall? And how far is the property from the beach, as saltwater chloride corrosion (near the ocean) may need to be factored in from a concrete spalling or cancer perspective.


  • Will the roof simply act as a roof, or will it be used as an entertainment space or outdoor area?

Building type

In Australia, we tend to use cement or different types of metal for flat roofs. These need to sit at a 1 degree angle to allow for water runoff. The industry offers a variety of flat roof membrane brands, and each project is unique and requires detailed analysis.

Top view of IKEA store

About waterproof membrane

The waterproofing membrane needs to be durable, robust and flexible to withstand the climatic and usage conditions. When choosing flat roof waterproofing, our team would generally look for a membrane with UV and heat-reflective properties.

Waterproofing membrane can be water-based, solvent-based (elastomeric), torch-on bitumen or elastomeric sheet, which is either PVC, TPO, Butynol or rubber. Using waterproofing technology to analyse the area, the selection of materials used in the area will depend on the specifications of the building.

Types of flat waterproof membrane

When developing our comprehensive solution design, we would consider the following:

  • Elastomeric sheet (PVC, TPO, Butynol or rubber) is flexible and durable and offers a tear-resistant bridge to facilitate high traffic areas and construction or expansion joints. It needs to provide uniform thickness and to be root-resistant, UV stable and heat-reflective. The sheet can be laid over damp substrates and is compatible with PVC fittings and downpipes.
  • Torch-on modified bitumen comes as a rolled product. It comprises a robust waterproof membrane enveloped in a layer of blacktop or asphalt. This solution offers a guaranteed membrane thickness, is two layers thick for long-term durability, is UV stable and root resistant, plus offers vapour distribution base sheets.

Sheet membranes can be the most costly type of membrane but can have a lifespan of up to 50 years.

Each project is unique

There is no one-size-fits-all approach to flat roof waterproofing, and the installation requires expert analysis from industry professionals.

Contact us to discuss your flat roof waterproofing needs or for more about our waterproofing expertise, read more blogs.

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