Three Common Flat Roof Waterproofing Problems – and Remedies

Relax corner on condominium rooftop garden with chairs

With longstanding experience across the strata, commercial and government sectors, Danrae Group is an expert in holistic waterproofing solutions, including diagnostic, solution design, remediation and preventative maintenance.

We’ve seen every type of waterproofing issue – from concrete spalling to flooded basements – and we have a strong understanding of problems inherent in particular property types and areas.

Given their shape, it’s clear why flat roofs, for instance, are prone to pooling water and leaks; however, these aren’t the only potential problems. Here’s our guide to the common flat roof waterproofing issues – and our recommended remedies.

Ponding water on flat roof covered with tree debris

Pooling water and leaks


Traditional sloped roofs have an obvious advantage when it comes to waterproofing. Water won’t slide off a flat roof, which is why a comprehensive solution design that includes adequate drainage is crucial. Without it, pools of water are likely to linger after rain or storms, or if you have clogged drains.

Pools of water on a flat roof have nowhere to go but down and given that natural building movement may open up tiny cracks, water is likely to enter the ceiling cavity – and eventually the apartments or offices below.

Results can include leaks, excessive moisture and mould, concrete spalling and efflorescence. And without treatment, it will eventually impact the building structure.

Pools of water are also likely to affect the condition of a rooftop waterproofing membrane, hastening wear and tear and the need for remediation.

Asphalt crack

Crocodiling, buckling and blistering


Asphalt rooftops in particular are prone to a condition called ‘crocodiling’ (or sometimes ‘alligatoring’). A flat roof surface is susceptible to climate conditions and over time UV rays will dry out the roof surface.

This leads to small cracks, resembling crocodile skin, that generally worsen with storms or extreme temperatures.

During construction, builders allow for natural material expansion and contraction in heat or cold; however, this may sometimes affect a waterproofing membrane if waterproofers have not laid it correctly. Movement can lead to minor buckles in the membrane, which damage the membrane and allow water to penetrate.

Blistering occurs when air or moisture is trapped between the layers of a flat roof membrane or between the membrane and ceiling. It will only further damage the membrane if left untreated and is most likely to degrade faster if there are also signs of cracking, if near a membrane seam, or if in an area with regular foot traffic, such as a rooftop garden.

Roof repair and replacement work in progress

Flashing failures


Waterproofing membranes can be most susceptible to water ingress at termination points, walls and penetrations, and flashing failures on flat roofs are relatively common with shoddy workmanship.

If not installed correctly, flashing may shift or split, providing a direct path for water from the roof to the membrane or ceiling cavity.

This may occur when metal flashing has been installed directly over a waterproofing membrane, when masonry has not been primed correctly prior to flashing application, or when flashing is not properly secured to the substrate.

Like many other waterproofing issues, flashing failures will eventually require roof remediation, a new membrane and/or structural repairs, depending on the degree of damage.

Mat at the roof using torch

Flat roof waterproofing remedies


Many flat roof issues can be avoided with a comprehensive and tailored waterproofing solution consisting of the most suitable waterproofing membrane, expert installation and full remediation.

Our recommendations for a flat roof membrane may vary based on property type – from hospital roof to residential rooftop garden, for instance – but our team will likely opt for a membrane with UV and heat-reflective properties.

We may recommend an elastomeric sheet membrane that is flexible, durable, has a uniform thickness, is root-resistant, compatible with PVC fittings and downpipes and which we can lay over damp substrates. Or we could suggest torch-on modified bitumen, which has a guaranteed membrane thickness, double layers, long-term durability, vapour distribution base sheets and is root-resistant. It also requires specialist skill.

Danrae Group waterproofers are trained and licensed to work with every brand – and in every technique including torch-on. With tradies at hand, we also look after full project management for seamless waterproofing and roof remediation.

Two men standing in front of a pool on a roof and smiling at the camera while holding a book

Preventative maintenance for flat roofs


As part of our holistic waterproofing approach, Danrae Group will recommend a preventative maintenance plan on the completion of every job.

It’s a very effective way to ensure the waterproofing for your strata or commercial property will last. With regular inspections, we can clear drains and check any damage inflicted by wild weather or other factors.

Preventative maintenance will help preserve your membrane – which protects your product warranty – and it will help ensure your flat roof waterproofing is working effectively to protect your property and your tenants.

At Danrae Group, we’re dedicated to quality waterproofing solutions – and we guarantee our work. Please contact us for your expert flat roof waterproofing.

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