Why Do They Put Rocks on Roofs? Unraveling the Mystery of Roof Ballast
The sight of rocks on a roof might seem unusual, even perplexing. Why would anyone place rocks—seemingly heavy and cumbersome—on a rooftop? The answer, however, isn't as simple as aesthetics. It's a practical solution with a long history, serving a variety of crucial purposes depending on the type of roof.
Let's delve into the reasons behind this seemingly unconventional practice:
1. What are the different types of roofs that use ballast?
Several roofing systems utilize ballast, predominantly flat roofs. These include:
- Built-up roofs (BUR): These traditional roofs consist of multiple layers of roofing felt and asphalt, with the ballast acting as a crucial weight to secure the layers and protect them from the elements.
- Green roofs (extensive and intensive): Green roofs use rocks as a growing medium for vegetation. The weight of the ballast is essential for holding the soil and plants in place, preventing wind damage. The type and size of rocks vary based on the type of green roof.
- Membrane roofs (TPO, EPDM, PVC): These modern roofing systems often utilize ballast to hold the membrane in place, especially in areas prone to high winds. The rocks also help protect the membrane from UV damage and punctures.
2. What is the primary function of roof ballast?
The primary function of roof ballast is weight. This weight provides several benefits:
- Securing the roofing system: The ballast's weight keeps the roof covering firmly in place, resisting strong winds, uplift from high winds and preventing damage. This is especially crucial for flat roofs, which are more susceptible to wind damage than sloped roofs.
- Protecting the membrane: The ballast protects the underlying roofing membrane from UV radiation, hail damage, and physical impact. The rocks act as a shield, extending the lifespan of the membrane.
- Providing drainage: In some cases, the ballast layer helps to improve drainage by allowing water to flow between the rocks. Properly designed ballast layers can help prevent ponding water, a significant concern for flat roofs.
3. What types of rocks are commonly used?
The choice of rocks depends on several factors, including the climate, the roof's design, and the weight requirements. However, some commonly used rocks include:
- River rock: Smooth, rounded stones that are aesthetically pleasing and readily available.
- Crushed stone: Angular fragments of various sizes, providing good drainage and stability.
- Slag: A byproduct of industrial processes, often used for its cost-effectiveness and weight.
The size and weight of the rocks are crucial. Larger, heavier rocks are typically used in areas with high wind speeds or for heavier roofing systems.
4. Are there any disadvantages to using rocks on roofs?
While roof ballast offers numerous advantages, there are some potential drawbacks:
- Weight: The significant weight of the ballast increases the overall load on the building's structure, requiring careful structural design.
- Maintenance: The ballast can shift over time, particularly during strong winds or heavy rainfall, requiring occasional maintenance and adjustments.
- Cost: The cost of purchasing and installing the ballast can be substantial, adding to the overall roofing project expense. This is especially true for extensive green roofs which require a significant depth of ballast.
5. What are the alternatives to using rocks on roofs?
Alternatives to ballast include mechanical fastening systems, which securely attach the roofing membrane to the roof deck. These systems eliminate the need for ballast but can be more expensive and complex to install. The choice between ballast and mechanical fastening depends on the specific roofing system, building design, and budget.
In conclusion, the use of rocks on roofs—ballast—is a practical and time-tested solution for securing and protecting roofing systems, particularly flat roofs. While there are some disadvantages, the benefits often outweigh the drawbacks, making it a popular choice for many roofing applications. The specific type and amount of ballast used depend on a variety of factors, requiring careful consideration by roofing professionals.
