17th February 2025
Hoists
Flat roofs are a common feature in buildings constructed in the latter half of the 20th century, particularly in schools, residential blocks, industrial or commercial properties. While flat roofs offer many benefits – including cost and time savings during installation, and space for HVAC equipment, rooftop gardens, or solar panel installations – they also can also be more prone to leaks than pitched roofs.
We explore the causes of leaks in flat roofs, and how proper design, material choice and maintenance can help prevent leaks and extend the life of your flat roofing project
Over the decades, advancements in roofing technology have increased the durability of flat roof systems. However, all roofing systems have a lifespan.
Older flat roofs – particularly those from the 1960s, ’70s, or ’80s – often used materials like bitumen-based ‘pour and roll’ systems, asphalt, or felt-based membranes which typically lasted 15–20 years. By contrast, modern membranes can last 30 years or more.
If your building has an older flat roof, it may simply be nearing the end of its natural life, evidenced by cracks, leaks, or previous repairs that are failing. Older flat roofs are more prone to failure due to several inherent design, material, and maintenance challenges.
Schedule regular inspections to assess your roof’s age and condition. If your roof was built before the 1990s, it may be time to consider a full replacement with modern materials designed for durability.
While commonly used in modern architecture for their sleek aesthetics and space efficiency, flat roofs require careful design to ensure durability, functionality, and performance. Flat roof designs that cut corners or fail to adhere to best practices are highly susceptible to leaks.
Ensure your flat roof is not overspanned. If the roof deck or structural supports (such as beams or joists) span a distance greater than they are designed or rated to support, your flat roof will be more susceptible to thermal movement, sagging and leaks.
Ensure your flat roof is designed with an adequate fall. Failing to do so will lead to pooling/ponding water and leaks.
The upstands in a flat roof – including those at roof edges, around penetrations, or around skylights, parapets, or vents – should be at least 150mm to provide a barrier against water ingress. As well as being best practice, the 150mm guideline ensures that the roof adheres to BS 6229 (Flat Roof Code of Practice) and is often the standard set by roof warranties and insurers.
Work with experienced roofing specialists with professional indemnity insurance to ensure proper design and detailing from the outset. Avoid “minimum standard” designs and aim for above-average specifications.
Flat roofs experience significant exposure to temperature changes. Whether the snow is falling, the sun is blazing, or the wind is blowing, the surface of your flat roof is fully exposed to the elements. Changes in temperature can cause the materials used in your roof to expand and contract, leading to cracks or membrane failures. Over time, this stress can cause cracks, blisters, and delamination.
This is especially true in older systems, like those relying on oxidized bitumen, which often struggle to cope with these thermal movements. Older roofing systems that were designed before expansion joints became widely used might also struggle to expand or contract with temperature changes.
Modern membranes, such as TPO, or elastomeric materials, are designed to withstand thermal movement, UV exposure, and standing water better than older systems. Ensure your roof is designed with structural movement in mind.
Even the best materials and designs can fail if improperly installed. Common errors include inadequate sealing, insufficient heating of bituminous membranes, and shortcuts during detailing. Poor workmanship is often a result of hiring unqualified or unregistered contractors.
Always work with certified, experienced roofing contractors to ensure quality installation. While it might cost more upfront, it saves money and provides assurance in the long run.
Blisters form when moisture becomes trapped within the roofing system. Heat can turn trapped water into vapor, which creates pressure that lifts the membrane. Delamination occurs when the layers of a bituminous membrane separate, typically due to improper installation or moisture infiltration. Interstitial condensation – a type of condensation that occurs when moist air penetrates a building’s structure and condenses into liquid – can cause further damage to the building structure.
Ensure all roof components are dry during installation, and repair blisters promptly to prevent membrane failure. Use proper installation techniques, including sufficient heating of bitumen layers, and apply vapour control layers to reduce condensation risks.
Rooftop HVAC, or other plant and equipment, units are an increasingly popular addition to many commercial buildings. One of the potential downsides to installing plant and equipment on the roof is the increased wear and tear that it can cause on the roof structure – more weight, more foot traffic and an increased risk of condensation causing damage to the roof structure. While many commercial flat roofs will have been built to a design specification that allows safe use of plant/equipment and footfall, it is important that roofing contractors and M&E contractors work together to ensure that any plant is safely installed whilst keeping the rooftop intact, avoiding call backs for roof repairs.
Ensure your roof is specified as fit for purpose at the design stage. If there is a change of use, it is important that the roof undergoes a structural assessment to make sure that it can carry the weight. Regular inspections of both the roof and the HVAC or plant equipment are also critical to prevent issues.
Flat roofs, like any other part of a building, require regular maintenance to perform well over time. As the most basic task, it is important to remove the leaves, dirt, and other debris that can collect on the roof over time. Left on the roof, this buildup will clog the roof drains. This can lead to problems such as:
Debris buildup, clogged drains, and minor damage left unrepaired can escalate into significant problems. Ensure you install leaf grates and guards to prevent vents and outlets becoming blocked by leaves and other debris and establish a proactive maintenance schedule that includes regular inspections, debris removal, and undertaking minor repairs before they become major issues.
To protect your flat roof from leaks and extend its lifespan:
Make sure your roof is fit for purpose, especially if it will be used as a walkway, for plant storage or as a roof garden. Avoid minimum standards for pitch, upstands, and drainage.
Upgrade to modern membranes that can withstand thermal stress and moisture. Ensure that the correct roofing outlets are used to ensure proper drainage.
Ensure proper installation and avoid the pitfalls of poor workmanship.
Early identification and repair of issues like cracks, blisters, or ponding water can help to ensure that the fabric of the roof isn’t damaged. Regular debris clearance and upkeep prevents small problems from turning into costly repairs.
Flat roof technology has come a long way in recent decades, and with proper care, even older roofs can serve their purpose for years to come.
At Lancastria we provide a complete range of high quality and cost effective waterproofing systems, ancillaries, equipment and tools for every flat roof refurbishment project.
If you need expert advice on the right solution for your flat roofing job, speak to our team on 0161 777 7554 or email us on sales@lancastria.com
Get in touch
Lancastria Group
(main office and hoist hire)
Unit J, Woodrow Way, Irlam,
Manchester, M44 6ZQ
Lancastria
(deliveries and collections)
Unit 8, Woodrow Way
Irlam, Manchester
M44 6ZQ
For deliveries and collections please go to Unit 8. On Woodrow Way, take the first right immediately around the corner from Howdens and Andrews Tiles. Our warehouse is the second unit. If you get to our main Lancastria office, you have come too far.
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