Sealing Flat Roofs – How to Achieve Reliable Waterproofing

A watertight roof build-up is the foundation of any flat roof waterproofing. Only when all layers are properly coordinated will the roof remain watertight in the long term.

A typical flat roof build-up consists of the following components:

1. Load-bearing structure – usually concrete, timber or trapezoidal sheet metal
2. Vapour barrier – prevents indoor moisture and condensation from penetrating
3. Thermal insulation – ensures energy efficiency and temperature stability
4. Waterproofing layer – acts as the water-bearing layer protecting against moisture
5. Protective or gravel layer – provides UV and weather protection

Particularly important is the roof fall: according to flat roof guidelines, it must be at least 2% to ensure reliable drainage. Incorrect or uneven falls often result in standing water, which places long-term stress on the waterproofing and may cause damage.

So, what is the best way to waterproof a flat roof? Every flat roof is different – and the same applies to waterproofing. The choice of material depends on roof use, weather exposure, roof build-up and budget. Below is an overview of the most common flat roof waterproofing materials.

Bitumen membranes have been the standard material for flat roofs for decades. Made from polymer-modified bitumen, they are usually installed in two layers and bonded by hot welding or cold adhesion. The result is a particularly robust, elastic and durable waterproofing layer.

When it comes to complex connections, penetrations or rooflights, liquid waterproofing is often the material of choice. It is applied in liquid form, cures seamlessly and forms an elastic, joint-free waterproofing layer.

The material adheres to almost all substrates – from concrete and bitumen to metal – and is suitable for both new builds and refurbishments.
Advantages:

• Seamless, joint-free and fully bonded
• Permanently elastic and UV-resistant
• Ideal for complex detailing

A major benefit is its versatility: liquid waterproofing can be used as a full waterproofing system or as an emergency or refurbishment solution for leaking roofs. Upstands and rooflights can also be sealed particularly well using this method.

Professional tip: Ensure system-compatible reinforcement fleece is used – only then can the waterproofing permanently bridge cracks.
 

PVC roof membranes (polyvinyl chloride) have been a proven flat roof waterproofing material for many years. They are usually installed in a single layer, hot-air welded and offer low weight combined with fast installation – ideal for large roof areas or refurbishments with limited load-bearing capacity.

Note: PVC is not compatible with bitumen. When refurbishing, a separation layer must always be installed between existing bitumen membranes and new PVC waterproofing.

Another flat roof waterproofing option is EPDM membranes. Made from synthetic rubber, they are exceptionally durable and often last over 50 years, remaining elastic even under extreme temperatures. As membranes are typically installed in large sheets, there are very few seams – a clear advantage over bituminous systems.

Liquid-applied roof coatings based on PU, PMMA or silicone resin are particularly popular for refurbishment. They can be applied directly to existing waterproofing layers, forming a new watertight surface. These systems are a good option when repairs are required or when a complete renewal would be too complex or costly.

Most leaks do not occur on the main roof surface, but at detail areas. Roof penetrations, rooflights, drainage outlets or upstands can become typical weak points if waterproofing is not carried out professionally. Precise workmanship and suitable materials are therefore essential.

Professional tip:

Installer-friendly upstands for skylights – such as those from LAMILUX – provide practical support for professional waterproofing. Whether GRP surfaces suitable for liquid waterproofing or special PVC connection rails and clamping profiles, LAMILUX offers the right solution for every flat roof.

Before waterproofing work begins, a thorough inspection of the roof surface is essential.

Even the best waterproofing system will fail if the substrate is not properly prepared. Dust, moss, leaves and loose particles must be thoroughly removed – ideally using brooms, vacuum cleaners or hot-air equipment. Damp areas must be completely dried before primers or new membranes are applied. Depending on the system, a bonding primer or separation layer may be required (e.g. PVC over bitumen).

The selected waterproofing system is then installed:

• Bitumen membranes are hot-welded or cold-adhered with 8–10 cm overlaps.
• EPDM membranes are cold-bonded or mechanically fixed, with joints sealed using special adhesive or tape.
• Liquid waterproofing is reinforced with fleece, applied in liquid form and cures seamlessly.
• PVC membranes are hot-air welded and mechanically fixed.

All systems must be installed stress-free, wrinkle-free and with cleanly formed connections. Corners, penetrations and upstands are the most common weak points and require particular care.
 

After completion, a leak-tightness test is mandatory. Depending on roof size and system, suitable methods include:

• Flood testing (classic for flat roofs)
• Electronic leak detection (low-voltage method)
• Thermography for moisture detection

In addition, seam overlaps, connections and edge details should be visually inspected. Only after successful inspection is the waterproofing considered complete. Finally, the surface may be finished with a protective layer, gravel or paving slabs, depending on the system design.

Practical tip:
Detailed photographic documentation of all work steps improves traceability – especially in the event of warranty claims.
 

The cost of waterproofing a flat roof depends largely on the material used, roof size and the level of work involved. On average, prices range between €30 and €70 per m².

A professionally waterproofed flat roof can last several decades with regular maintenance. Especially for industrial buildings, garages and roof terraces, investing in high-quality materials and professional installation pays off in the long term.

A watertight flat roof requires regular inspection, which is why a roof inspection at least once a year is recommended. During this process, drainage outlets should be cleaned, leaves, moss and standing water removed, and waterproofing layers checked for cracks or detachment. Emergency repair materials should also be kept readily available. This allows minor damage to be temporarily sealed quickly until a permanent repair can be carried out.