The New Zealand construction industry is becoming more aware that the building envelope needs careful attention.

Until recently, most wall cladding was a single line of direct-fix. Now, we have adopted the standard practice of cavity construction — an extra line of defence. Not long ago we used single glazed windows and reserved double glazing as an exclusive or premium option. Once the advantages were experienced and clear, the extra benefits of cavity construction and double glazing become essential, or in other words, the new norm.

Another term to explain the security of the belts-and-braces approach is built-in redundancy. This a simple principle where any installation error or unforeseeable impact in one layer does not cause a problem because the additional layer is there as a backup.

The quality and type of membrane systems that are available commercially in New Zealand is broad. There is still some perception that multi-layer products are exclusively for premium projects and that when a project budget is restrictive, a reduced system can be used. That is not necessarily the case when single layer options require specialised accessories and extra labour hours to ensure a perfect surface finish can be achieved. Checking and comparing the actual costs of a robust multi-layer waterproofing system with the costs of a single layer system might show little difference between them.

The single layer membrane options are suited to certain projects, particularly where the designer wants either fewer or no visible seams. Most single layer membranes are much wider than double layer torch-on systems — wide enough to cover a balcony.

Damage to membranes by sub-trades on site is an enduring problem, and contractors need care and diligence to protect those membranes on the construction site. On larger jobs a double layer system allows flexibility in construction sequencing. The first layer may be installed at the start of the build and any damage repaired before installing the final layer toward the end of the project.

Clients must always be reminded of both the importance of their roof and also the risks of specifying inappropriate systems in the roofing of a building. At the design stage, the peace of mind and long-term savings are gained from selecting a reliable membrane system with good product assurance documentation. Clients also gain peace of mind by insisting on well-trained and approved membrane installers.

Is single or double layered membrane best? It depends on the requirements of the project.