Flat concrete roofs are a common feature in Bali, but they often come with their own unique set of challenges, particularly when it comes to waterproofing. Understanding these challenges and implementing the right solutions is crucial for maintaining the integrity of your property and avoiding the headaches of leaks and water damage.

The Unique Challenges of Flat Concrete Roofs in Bali

• Cultural Practices: In Bali, there’s a unique cultural perspective on leaking roofs, where they are sometimes seen as something to be expected. While this may be part of the local tradition, it doesn’t negate the importance of proper waterproofing for modern homes.
• Improper Finishing: A major issue is the way flat concrete roofs are finished. Unlike in many Western countries, where the concrete is shaped to drain while still wet, in Bali, a thin layer of concrete called a screed is often added later. This screed can crack due to expansion and contraction from the sun, allowing water to seep underneath.
• Drainage Issues: Flat concrete roofs often have a low parapet around the edge and drains to carry water away. However, these drains are frequently too small, or are built into the structural columns, making them hard to access for unblocking. The common practice of developers removing trees, which often leads to leaves blocking the drains, exacerbates these problems.
• Structural Vulnerabilities: Incorrect placement of drains within structural columns weakens the building’s structure. Additionally, any cracks in the concrete slab, joints between slabs, or holes drilled through the concrete can all create entry points for water.

Understanding the Construction Process

To address waterproofing issues effectively, it’s important to understand how concrete roofs are typically built in Bali.

• Reinforced Columns: The process starts with strong reinforced columns that support the roof.
• Wooden Formwork: A wooden formwork acts as a mold where the flat roof slab is cast.
• Concrete Pouring: The concrete should be poured in one piece, and the mixture needs the correct ratio of sand, cement, and small stones. The water content and mixing during the pour are critical for proper curing.
• Importance of Thickness: The thickness of the concrete and steel reinforcing bars depends on the roof’s span. Longer spans require thicker concrete and stronger supporting structures.

The Importance of Proper Design and Construction

• Structural Engineering: Always consult a structural engineer for the design of concrete roofs. These roofs are heavy and can be dangerous during earthquakes.
• Adequate Drainage: Ensure that your roof has enough drainage for heavy rainfall and that there is a backup in case one drain is blocked. Drains should be large (100mm is good) and straight enough to allow for easy unblocking.
• Shaping the Slab: The concrete roof slab should be shaped to allow water to drain properly, rather than relying on a screed.

Identifying and Addressing Leaks

• Check for Cracks: If your roof is leaking, start by checking for cracks in the surface. These are likely cracks in the screed.
• Remove the Screed: To find the point of leakage, you may need to remove the screed.
• Parapet Issues: Pay special attention to the parapet around the roof’s edge, which is often made of lightweight concrete and can crack away from the main roof slab.
• Professional Help: Waterproofing is a specialized job, and you should hire someone with expertise in this field.

Waterproofing Solutions

• Waterproofing Membranes: Once the source of the leak is found, consider applying a waterproofing membrane. This could be a thick coat of polyester paint or a thicker bituminous layer. Bitumen, or tar, is a long-trusted material for sealing roofs.
• Asphalt: For shaping the roof to improve drainage, thick asphalt is an effective waterproofing membrane.
• Screed Replacement: If the screed needs replacing, keep it cool until it cures properly and protect it from sunlight by painting it white or covering it with a layer of stones.

Safety First

• Electrical Hazards: If water is coming through your light fittings, turn off the power immediately and seek help.

By understanding the common pitfalls of flat concrete roofs in Bali and taking proactive steps in design, construction, and maintenance, you can protect your home from water damage and ensure your peace of mind.