What Is A Ballasted Roof?

Choosing the right roofing system is a critical decision, especially when it comes to commercial structures. While ballasted roofing systems aren't as common as they used to be, their ease of installation, cost-effectiveness, and aesthetic appeal still make them an option worth considering, particularly if you're building a green roof or a recreational rooftop space. In this article, we'll provide you with an in-depth look into what a ballasted roof is, its pros and cons, and answers to frequently asked questions. By the end, you'll have a clearer understanding of whether a ballasted roofing system is a good choice for your building.

Table of Contents

• What is a Ballasted Roof?
• The Pros and Cons of Ballasted Roofs
• Different Types of Ballast Used in Ballasted Roofs
• Ballasted Roof Systems Compared to Other Adhesion Methods

What is a Ballasted Roof?

A ballasted roof is a system in which the waterproofing membrane is loose laid over the roof deck and held in place by a layer of ballast, usually stones, gravel (sometimes called aggregate), or concrete pavers.

The waterproofing layer is usually a single-ply roofing membrane like EPDM, PVC, or TPO. A contractor will strategically lay the membrane over the insulation and decking, and then distribute the ballast uniformly over it.

Because there are no adhesive or mechanical fasteners, the ballast method is only suitable for  your classic flat roof, i.e., a roof with a pitch of 3/12 or less.

The Pros and Cons of Ballasted Roofs

You may have some misgivings about ballasted roofing. With no adhesives or fasteners, will the roof really stay in place during major storms?

Fortunately, the answer is yes. A properly designed ballasted roof can achieve excellent wind ratings. This is because the stones are at least 1.5” in diameter, and are deposited on the roof until the weight distribution reaches 1,000 - 1,300 pounds per 100 square feet.

Pros of Ballasted Roofs

• Ease of Installation: With no adhesives and just a few plates and fasteners (to prevent excess movement), installation can be quick and easy.
• Lower Pricepoint: The stones used on a ballasted roof can be relatively inexpensive, while the simplicity and speed of installation reduce labor costs.
• UV Protection: The ballast isn't just holding the membrane down. It's also shielding the membrane against the sun's UV rays, extending its useful life.
• Fire Resistance: The gravel or pavers used in ballasted systems are virtually fireproof, making the system more resistant to some fire threats.
• Aesthetic Flexibility: While functional, a ballasted roof’s stones or pavers also provide a pleasant, natural looking surface, and can even be combined with plants and other landscaping materials. 
• Energy Savings: The ballast layer can provide additional insulation, potentially helping to regulate the building's internal temperature and leading to energy savings.
• Protection Against Impact: The stones or pavers protect the underlying roof membrane from damage from both hail and foot traffic.

Cons of Ballasted Roofs

• Weight Concerns: Ballasted roof systems are heavy. Buildings need to be structurally sound to support the added weight. It's important to consult a structural engineer before going this route.
• Repair & Maintenance Difficulties: For a ballasted roof using stone ballast rather than concrete pavers, detecting and repairing damage may be difficult.
• Wind Uplift: In areas prone to high winds, ballasted roofing can be at risk. Though the weight holds the membrane down, there's a limit to how much wind force it can resist, and this is particularly true of ballasted systems using stones rather than pavers.
• Water Retention: Stones can sometimes trap water, leading to potential issues if there's a lack of proper drainage. Over time, standing water can compromise the roofing system's integrity.
• Migration of Ballast: Over time, especially in areas with frequent strong winds or heavy rainfall, the ballast can shift, leading to uneven distribution and potential exposed spots on the membrane.

Different Types of Ballast Used in Ballasted Roofs

In the world of ballasted roofing, the term "ballast" doesn't just refer to one specific type of material. Instead, there's a range of materials that can be employed.

River Rock or Smooth Stones

As the name suggests, these are naturally rounded stones.

Benefits: They are excellent for ensuring the roofing membrane remains undamaged due to their smooth surfaces. They also have an aesthetic appeal, presenting a natural and organic look.

Considerations: It's essential to ensure the stones are free from sharp edges or debris that could potentially damage the membrane.

Concrete Pavers

These are rectangular or square blocks of concrete, sometimes available in various colors or finishes.

Benefits: These provide a uniform and neat appearance. They're especially suitable for roofs that might have foot traffic since they offer a walkable surface. They also provide excellent UV protection for the membrane beneath.

Considerations: Due to their weight and solid structure, it's crucial to ensure that the building structure can support them. Furthermore, they can be more expensive than other ballast types.

Crushed Stone or Gravel:

This type comprises smaller, irregularly shaped pieces of stone, not as smooth as river rocks.

Benefits: Crushed stone is often cheaper than other types of ballast and can provide good coverage over the membrane.

Considerations: The sharper edges of crushed stone can pose a risk to the membrane, so using a protection mat or layer is essential when opting for this ballast type.

Slag Ballast:

Slag is a by-product of metal smelting, and it's sometimes used as a ballast material.

Benefits: Slag is often a cost-effective option and can be recycled from other industries, making it an eco-friendly choice.

Considerations: Like crushed stone, slag can have sharper edges, so using an additional protective layer is advisable.

Ballasted Roof Systems Compared to Other Adhesion Methods

Mechanically Fastened System:

This system uses mechanical fasteners, often screws and plates, to secure the roofing membrane to the roof deck.

Pros:

• Lightweight, adding minimal weight to the building structure.
• Can be faster to install than fully adhered systems.
• Suitable for buildings in windy regions when appropriately installed.

Cons:

• Fastener locations can become potential leak points if not properly sealed.
• Penetration of the roofing membrane might reduce its lifespan.
• Appearance of fastener "dimples" might be less aesthetically pleasing to some.

Fully Adhered System:

This method involves using special adhesives to bond the roofing membrane directly to the insulation or roof deck.

Pros:

• Provides a smooth appearance without the "dimples" of mechanical fasteners.
• Resists wind uplift effectively, making it suitable for high-wind regions.
• Minimizes potential leak points since there's no penetration of the membrane.

Cons:

• Adhesives can be sensitive to temperature and moisture during installation, which may limit when you can install.
• Typically takes longer to install than mechanically fastened systems.
• Often more expensive due to the cost of adhesives and labor-intensive process.

The Upshot