Flat concrete roofs have become increasingly popular in modern construction, but they often present unique challenges, particularly in tropical climates like Bali. While they offer a sleek and contemporary aesthetic, it’s crucial to understand the potential pitfalls and how to address them effectively.

The Design and Construction of Flat Concrete Roofs

Reinforced concrete flat roofs require careful design and proper construction to avoid issues like poor drainage, cracking, and leaks. A typical flat concrete roof consists of a 10 to 15 cm thick reinforced concrete slab topped with a thin 2 to 5 cm layer of cement, known as a “screed”. It’s important to note that the screed’s purpose is not waterproofing. Instead, it provides a smooth, sloping surface to ensure proper drainage.

Common Construction Problems with Flat Roofs

Several factors can contribute to problems with flat concrete roofs:

• The reinforced concrete slab, not the screed, is responsible for keeping water out. It must be correctly designed and constructed.
• The supporting walls must be strong enough for the slab.
• Ideally, the slab should be cast as a single piece of concrete without joints to prevent leaks.
• The reinforcing steel needs to be the correct size and correctly placed.
• The concrete mix must be correct, avoiding excessive water, and a vibrator should be used during pouring to eliminate trapped air.
• The cement screed can also cause issues. It expands more than the concrete beneath when exposed to intense sunshine, leading to cracks.
• These cracks can cause the screed to separate from the concrete, creating pathways for water to travel across the roof and find any leaks.

Repairing Leaking Flat Concrete Roofs

When dealing with a leaking roof, the primary issue is that the reinforced concrete is leaking. Here’s how to address it:

• Remove the screed to expose the reinforced concrete to find and seal leaks. This should be done carefully, avoiding jackhammers that may further crack the concrete.
• Specialized techniques, such as crack injection, can seal the concrete and glue it back together without removing the screed.
• Use correct methods and properly selected materials for effective repairs.
• Leaks often occur through cracks in the concrete or along joints from different pours.
• Cracks and joints can result from earthquakes, ground subsidence, building settlement or flexing of the roof.
• Water penetrating the concrete can rust steel reinforcing bars, causing them to expand and split the concrete.
• To identify the source of the leak, you may need to expose the roof slab from underneath by removing part of the ceiling.
• Sealing should be done from above to prevent water from entering, not from below.
• Applying a cement-based sealant to the screed’s surface is unlikely to be effective due to its brittle nature and potential for cracking and spalling.

Waterproofing Solutions for Flat Roofs

Consider these waterproofing methods:

• Avoid rubber or plastic sheeting as they are usually ineffective.
• Hot tar (bitumen) based systems can be effective.
• Other systems can work with correct application and attention to detail.
• The contractor’s knowledge and skill are key to finding and sealing leaks effectively.

Alternative Solutions

If sealing a flat roof is not possible, consider these options:

• Install a pitched roof over the flat roof, using options like tiled roofs, zinc alume, or Onduline sheeting.
• Traditional roofs are often cheaper, more effective, and easier for local builders to construct.

Choosing the Right Contractor

In Bali, there’s a gap between large, professional construction companies and small, private builders. Most smaller builders lack the expertise to build reliable flat concrete roofs. Selecting a contractor with the necessary knowledge and experience is crucial for a durable and leak-free flat roof.

Conclusion

While flat concrete roofs may be aesthetically appealing, they are not without their challenges. Understanding these issues and their solutions is crucial for ensuring a long-lasting, leak-free roof. By addressing the design, construction, and repair aspects of flat concrete roofs, homeowners and builders can minimize problems and maintain structural integrity.