By Steven Garrigan, Director of Exterior Envelope Restoration
Rooftop amenity spaces—think gardens, lounges, and outdoor recreation areas—are a great way to elevate tenant experience and increase property value. But if your building dates back to the early or mid-20th century, there’s an important factor to consider before moving forward: the roof’s original structure.
Many older buildings were built using cinder concrete slab roofs—a construction method that comes with limitations when it comes to supporting new amenities. If you’re a property manager considering rooftop upgrades, understanding this roof type is key to making informed, cost-effective decisions.
What is Cinder Concrete?
Cinder concrete is a lightweight mix of cement, gravel, and cinders (a byproduct of coal). It was widely used in buildings from the early 1900s through the 1950s—and occasionally into the 1960s—because it was lighter and more affordable than solid concrete.
These slabs typically consist of a thin structural layer (about 3 to 4 inches thick), topped with additional cinder fill and a concrete cap to create slope for drainage. In some areas, this sloping layer can be over 24 inches deep. Reinforcement comes from a lightweight wire system, which unfortunately is highly vulnerable to corrosion over time—especially around the roof’s perimeter.
How Does This Compare to Modern Roof Construction?
Modern roofs are built to handle significantly more weight and stress. They often include:
- Steel decking underneath
- Thicker concrete slabs (4–6 inches)
- Reinforcing steel bars (rebar)
- Strength ratings of 3000–4000 psi (pounds per square inch)
By contrast, cinder concrete slabs have a compressive strength of only 400–800 psi. That’s a major difference, especially when you’re thinking about placing furniture, decking, landscaping, or even groups of people on top of the roof.
What Are the Risks and Limitations?
Because of their age and materials, historic cinder concrete roofs present several challenges:
- Structural limitations: They simply weren’t designed to support the extra loads associated with rooftop amenities.
- Water damage: Leaks over the years may have caused the cinder fill to weaken or even crumble, especially in freeze-thaw cycles.
- Steel mesh deterioration: Water infiltration can damage the embedded mesh that connects the concrete to the building’s structure.
- Expensive remediation: In some cases, the sloping cinder fill may need to be removed and replaced—a costly process if not anticipated early.
So, Is It Still Possible to Add a Rooftop Amenity?
Yes, with the right planning and structural assessment, there may be options to enhance your rooftop. At Diaz Architect & Associates, we typically recommend:
- Roof probes and structural evaluations: This gives us a clear picture of the roof’s actual condition.
- Strategic reinforcement: In some cases, we can strengthen the structure from below.
- Lighter-weight materials: Using modular, secured site furnishings can help you create a welcoming space without overloading the structure.
- Creative design solutions: A modest rooftop lounge or garden may still be possible, even if larger-scale amenities aren’t.
Bottom Line
While the idea of transforming your building’s rooftop is exciting, cinder concrete roofs demand extra care. Understanding their limitations is the first step toward a safe, smart investment.
If your building falls into this category, our team at Diaz Architect & Associates can guide you through the feasibility assessment, planning, and design process. We’ll help you determine what’s possible—and what’s practical—so you can make the best decision for your property, your tenants, and your budget.
Ready to explore the potential of your rooftop?
Reach out to our team for a consultation.
