Rooftop water leaks are always a tough job to diagnose. Solving and rectifying them permanently can be even more difficult. 

Understanding the root cause of roof water leaks and finding a solution is no easy task.

By their nature, rooftops suffer from lack of or insufficient maintenance. This is a case of being out of sight and out of mind.

I would like to share my personal insights about water leaks and ‘flat concrete roofs’ – which predominate in multi-storey buildings such as apartments and office towers.

As both a Registered Building Practitioner and a waterproofing specialist, I consider building and structural elements when evaluating the complex problem of water ingress.

I have seen cases where leaking roofs have been incorrectly diagnosed as a waterproofing matter when in fact there were major plumbing issues and structural design elements at play. Where this happens, the underlying causes have not been understood and simply installing a new membrane does not prevent water ingress. At some stage, the roof will invariably leak again.

It is important to diagnose the underlying reasons for the leaks. Logically, this will lead to the correct approach for permanent rectification.

However, where structural issues are not understood, rectification decisions may not solve the problem.

In many instances, there may be several options for remediation.  And of course, cost is a significant consideration. A leaking roof does not necessarily require a new roof membrane – there are options.  It comes to investigation.

This is where my so-called ‘roof forensics, assessment and investigation’ comes into play. Understanding the underlying cause of water leaks helps to determine the best course of action.

Mostly, when we are called in to rectify a leaking roof, we are dealing with older buildings with a membrane at the end of its life cycle. Often, these membranes have been patched up over many years to stem intermittent water leaks.

Sometimes, it is possible to carry out temporary work and to delay the permanent solution until later. This can be ideal where factors such as a lack of funding or inclement weather and conditions are at play.

Eventually, however, there comes a time when money must be spent and spent well. This means waterproofing rectification works that provide a solution to meet performance criterion in terms of lifetime service, permanent leak resistance, weather resilience.

Though product marketing may say otherwise, waterproofing membranes do not always offer lifelong protection against water damage.

Over time, a membrane’s effectiveness will diminish. This is especially the case for membranes which are exposed to the rigours of the Australian climate.

Knowing when to re-apply or install a new membrane is crucial to maintaining the long-term structural integrity of a building.

Sheet type of roof membranes (strips of bitumen backed material laid out and joined together with flame heated bitumen or sheet rubber like material that is glued at the seams and glued to the roof, etc) tend to leak at the joins. Membrane joins can be areas of weakness and though supposed to bond to the concrete roof slab, sheet membranes do “de-bond” (mostly due to poor install methods). Consequences are a “waterbed” effect when water migrates via the open joints.

A frequently asked question regarding rooftop waterproofing is: ‘can the existing surface be re-coated, or does the damaged membrane need the costly and time-consuming option of total removal’?

This is particularly relevant with “busy” roofs, such as those with mechanical installations and/or rooftop plant deck equipment.

Over topping an aging roof with a membrane is less expensive than a “remove-and-replace” roofing upgrade.

When restoring a rooftop with a new coating system, it is important to understand what the existing roof system is and to ensure that it is indeed suitable for restoration. Performing adhesion tests can help determine if restoration is a good fit. Correct and detailed surface preparation is key and the discovery of a litany of roof coverings make this a challenging exercise.

Selection of the correct membrane is difficult as there are many different products available. There are a range of solutions to meet the current performance demands.

I would not be so bold as to suggest there is any one product for roof restorations. In general, they are all reasonably good. If not, they would not or should not be on the market.

Whatever product is chosen, meticulous application and installation is critical.

In my opinion, roof waterproofing membranes should have at the minimum the following properties:

  • Flexibility – membrane needs to move with normal climatic fluctuations of roof surface i.e. expansion and contraction.
  • UV Stable – roof membranes need to be not just UV resistance but UV stable for longevity.
  • Robust Adherence – membrane requires strong bond to the existing roof surface. If an “overlay system” that means adherence to any historic surface coatings that remain.
  • Ability to Contour to Shape – rooftops often involve utilities, air conditioning facilities, penetrations etc. Roof membranes must seal and make these watertight.

The integrity of a roof is critical. Slow leaks can cause devastating structural damage. Most of this will be unseen as water migrates into the structure itself.

If concrete remains wetted (by roof leaks or other), rust on the reinforcing steel will be accelerated. This will lead to swelling of rusted sections and cracking of the concrete. At some point, the outcome will be structural failure.

Wetting of concrete greatly speeds up the process of deterioration – “concrete cancer”. The Building Code requires buildings to survive for several decades. If structures are not maintained, however, deterioration can begin much sooner. When concrete started to be used in modern construction, the “specification” was for approximately for 50-100 years.

This timeline for many buildings in our cities has come and gone. Water ingress via rooftops is part of a slow creeping disease in aging buildings.

The answer is to address the integrity of aging roofs for watertightness. If remediation is required, this should happen sooner rather than later.

By Paul Evans

Managing Director: Findlay-Evans Waterproofing

President: Australian Institute of Waterproofing (AIW)

Registered Building Practitioner – Vic (Commercial & Domestic – Unlimited)