Flat roofs are often associated with modern architecture, but they’ve been used for centuries—especially on industrial, institutional, and commercial buildings. In Ontario and across North America, many historic properties from the late 1800s through the mid-20th century feature flat roofing systems that were engineered with the materials and technologies of their time.
Restoring or repairing these aging roofs requires a blend of historical knowledge and modern roofing expertise. Let’s explore how flat roofs were originally built on historic structures—and how skilled contractors preserve their integrity today.
The Origins: Flat Roofs in Early 20th-Century Architecture
Flat roofs gained popularity in the early 1900s due to:
- Urban density and commercial growth: Flat roofs allowed full use of space below and provided room for mechanical systems above.
- Industrial-scale construction: Factories, warehouses, and government buildings favored simple, efficient designs.
Common Flat Roofing Materials Then:
- Built-Up Roofing (BUR) — Layers of tar and felt, alternated and topped with gravel
- Coal Tar Pitch — A thick, durable waterproofing material made from coal byproducts
- Metal Flashing and Drainage — Hand-soldered and often custom-made for each building
- Concrete Roof Decks — Used in many public and institutional buildings for fire resistance
These systems were effective for their time but are now well past their intended service life.
Unique Challenges of Historic Flat Roofs
Restoring or repairing a historic flat roof involves more than just fixing leaks. These roofs present distinct challenges, including:
1. Fragile Substrates
Old wood decking or concrete slabs can be weakened by decades of water damage, freeze-thaw cycles, and structural shifts.
2. Outdated Drainage Systems
Historic internal drains and scuppers may no longer meet today’s building codes—or may be clogged with decades of debris.
3. Preservation Rules
Heritage-designated buildings often require that visual elements (like parapets, cornices, or metal details) be preserved or replicated.
4. Hazardous Materials
Older roofing systems may contain asbestos, lead, or coal tar, requiring specialized removal and disposal procedures.
How We Repair or Restore Historic Flat Roofs Today
Modern flat roof repair specialists take a conservation-first approach—balancing building integrity, code compliance, and visual authenticity.
Step 1: Detailed Roof Assessment
- Infrared scanning to locate moisture within the system
- Core samples to assess layers and deck condition
- Structural review to determine load-bearing capacity
Step 2: Material Compatibility
- In many cases, modern materials like EPDM or Modified Bitumen are used in a way that mimics the original look or function.
- For visible or historic details (e.g., copper flashings), custom-fabricated pieces are installed to match the original aesthetics.
Step 3: Upgrading Insulation & Drainage
- Insulation is brought up to modern R-value standards without disturbing the building’s interior or roofline
- New internal drains, overflow protection, and scuppers are added or upgraded to code
Step 4: Preservation-Approved Workmanship
- All work must meet heritage standards if the building is protected under municipal or provincial preservation laws
- Contractors often collaborate with architects or conservation consultants
Real-World Examples in Ontario
- Post offices, schools, and churches from the early 20th century often had coal tar or gravel BUR systems. Many have since been replaced with multi-ply SBS modified bitumen.
- Downtown Toronto warehouses built in the 1920s have been retrofitted with white reflective membranes that preserve their industrial appearance while meeting energy standards.
- Heritage buildings in Kingston and Ottawa now incorporate green roof elements where structurally feasible, blending old architecture with modern sustainability.
Final Thoughts
Historic flat roofs are part of Ontario’s architectural legacy—but they can’t be left to deteriorate. When treated with respect and technical skill, these roofs can be restored with modern materials while preserving their original character.
If you own or manage a historic building with a flat roof, working with an experienced contractor who understands both roofing technology and architectural conservation is key to a lasting, compliant, and visually authentic solution.