What is Tar and Gravel Roofing?
Tar and gravel roofing, officially called built-up roofing (BUR), is one of the oldest and most proven flat roofing systems in the industry. Dating back to the mid-1800s, this technology has been protecting commercial, industrial, and institutional buildings throughout the United States for generations.
The term "built-up" refers to the construction method: multiple layers of waterproofing materials are literally built up on the roof deck to create a redundant, highly durable membrane. Each layer provides an additional barrier against water infiltration, giving BUR systems their legendary reliability.
Components of a Tar and Gravel Roof
Base Sheet
The foundation layer, typically fiberglass or organic felt, mechanically fastened or adhered directly to the roof deck
Bitumen Layers
Hot-applied asphalt or coal tar that waterproofs and bonds the layers together into a monolithic membrane
Ply Sheets (Felts)
Reinforcing fabric layers (2-4 plies) embedded in bitumen for tensile strength and redundancy
Surfacing (Gravel)
Aggregate stones (river rock or slag) embedded in the top flood coat to protect against UV and physical damage
The number of plies determines the roof's classification and durability. A "3-ply" roof has three reinforcing layers plus the base sheet, while a "4-ply" system provides even greater redundancy. More plies generally mean longer lifespan and better puncture resistance.
Why It's Called "Tar and Gravel"
While the term "tar and gravel" is commonly used, most modern BUR systems actually use asphalt rather than coal tar. True coal tar systems are still available and offer superior waterproofing, but they're less common due to environmental concerns and handling requirements. Asphalt-based BUR provides excellent performance with easier installation and is the standard in most markets, including the Myrtle Beach area.
How Tar and Gravel Roofs Are Installed
Installing a built-up roof is a labor-intensive process that requires specialized equipment and experienced crews. The hot-applied asphalt must be kept at precise temperatures, and each layer must be properly bonded before the next is applied.
BUR Installation Process
Deck Preparation
The roof deck (typically metal, concrete, or wood) is cleaned, dried, and inspected for damage. Any deteriorated decking is replaced. Insulation boards are installed to provide thermal protection and create proper drainage slope if needed.
Base Sheet Installation
A heavy-weight base sheet is mechanically fastened or adhered to the insulation. This layer serves as the foundation for the built-up system and provides initial waterproofing protection during construction.
Hot Asphalt Application
Roofing asphalt is heated in a kettle to approximately 400-450 degrees Fahrenheit. The hot bitumen is carried to the roof (by pump or bucket) and applied in a continuous layer using mops or mechanical applicators.
Ply Sheet Embedding
While the asphalt is still hot, reinforcing felt sheets are rolled into the bitumen. This process is repeated for each ply layer, creating a laminated membrane. Seams are staggered to prevent water paths.
Flood Coat Application
A heavy "flood coat" of hot asphalt is poured over the finished plies. This coat is typically 60-75 pounds per 100 square feet and provides the adhesion surface for the gravel surfacing.
Gravel Surfacing
Aggregate (river gravel, crusite, or slag) is spread into the hot flood coat at approximately 400-500 pounds per 100 square feet. The gravel must be embedded while the asphalt is still fluid to ensure proper adhesion and complete coverage.
Flashing Installation
Metal and membrane flashings are installed at all penetrations, walls, curbs, and edges. These critical details require careful attention as most BUR failures occur at flashing points rather than in the field.
The installation of a tar and gravel roof typically takes longer than single-ply systems due to the multi-layer construction and hot asphalt requirements. Weather conditions are critical - installation should not occur during rain or when temperatures are too cold for proper asphalt application.
Benefits of Tar and Gravel Roofing
Despite the emergence of single-ply membranes, built-up roofing continues to offer unique advantages that make it the preferred choice for many commercial applications. Here's why property owners and facility managers still choose BUR.
Proven Track Record
With over 100 years of documented performance, BUR is the most time-tested flat roofing system available. Many existing roofs have exceeded 30-40 years with proper maintenance, demonstrating reliability that newer systems have yet to prove.
Redundant Waterproofing
Multiple layers mean multiple barriers against water. If one layer is compromised, the underlying layers continue to protect the building. This built-in redundancy is unmatched by single-ply membranes that rely on a single sheet for waterproofing.
Superior Puncture Resistance
The combination of multiple felt layers and protective gravel makes BUR extremely resistant to punctures, foot traffic, and mechanical damage. This makes it ideal for roofs with heavy equipment, frequent maintenance access, or industrial operations.
Excellent UV Protection
The gravel surfacing completely shields the bitumen membrane from ultraviolet radiation, the primary cause of roofing material degradation. This protection is particularly valuable in sunny climates like the Myrtle Beach area.
Fire Resistance
Gravel-surfaced BUR systems routinely achieve Class A fire ratings, the highest available. The aggregate and mineral surfacing provide excellent protection against external fire exposure, a critical consideration for commercial properties.
Low Maintenance Requirements
Once properly installed, tar and gravel roofs require minimal maintenance beyond regular inspections and drain clearing. The robust construction tolerates minor neglect better than many modern systems.
Seamless Waterproofing
Unlike single-ply membranes with heat-welded or adhesive seams, hot-mopped BUR creates a truly monolithic membrane. The hot asphalt flows together, eliminating discrete seams that can fail over time. This seamless quality is why many older BUR roofs continue to perform decades beyond their expected lifespan.
Drawbacks of Tar and Gravel Roofs
While built-up roofing offers many advantages, it's important to understand the limitations before choosing this system for your property. Here are the main considerations that make property owners explore alternatives.
Significant Weight
A fully installed BUR system with gravel weighs 300-600 pounds per 100 square feet, significantly heavier than TPO (50-80 lbs) or EPDM (40-70 lbs). This requires a stronger roof structure and limits installation options on older buildings not designed for this load.
Complex Installation
BUR installation requires specialized equipment (asphalt kettles, hot carriers), skilled crews experienced with hot work, and careful coordination. Installation generates smoke, fumes, and odors that can affect building occupants. Weather windows for installation are more limited than with single-ply systems.
Difficult Leak Detection
When leaks do occur in tar and gravel roofs, locating the source can be challenging. Water can travel between layers, appearing far from the actual entry point. The gravel must be displaced to inspect the membrane, making leak detection more labor-intensive than with exposed membranes.
Less Energy Efficient
Traditional gravel-surfaced BUR is darker than white reflective membranes like TPO, absorbing more heat. While light-colored gravel or reflective coatings can improve this, BUR typically doesn't qualify for cool roof incentives without additional treatments.
Removal Challenges
When replacement is needed, removing tar and gravel is labor-intensive and costly. The heavy gravel must be collected and disposed of, multiple layers of adhered material must be separated, and disposal costs are higher than for single-ply removal.
Tar and Gravel vs Modern Alternatives
Single-ply membranes have captured significant market share from traditional BUR. Understanding how these systems compare helps you make an informed decision for your specific application.
| Feature | Tar & Gravel (BUR) | TPO | EPDM |
|---|---|---|---|
| Lifespan | 15-30 years | 20-30 years | 25-30 years |
| Puncture Resistance | Excellent | Good | Very Good |
| Energy Efficiency | Fair (with coating) | Excellent (white) | Fair (black) |
| Installation Speed | Slow (multi-day) | Fast | Fast |
| Weight | Heavy (300-600 lbs/sq) | Light (50-80 lbs/sq) | Light (40-70 lbs/sq) |
| Leak Detection | Difficult | Easy | Easy |
| Fire Resistance | Excellent (Class A) | Good | Good |
| Best For | Heavy traffic, industrial | Energy savings, commercial | Budget-conscious, residential |
When to Choose BUR Over Single-Ply
- +Heavy rooftop equipment or frequent access
- +Industrial environments with debris risk
- +Maximum fire resistance required
- +Proven long-term performance is priority
- +Matching existing BUR on multi-section roof
When to Choose Single-Ply Over BUR
- +Building structure has load limitations
- +Energy efficiency is primary goal
- +Faster installation needed
- +Building is occupied during installation
- +Easier maintenance and inspection preferred
For more information on single-ply alternatives, see our TPO roofing guide or flat roofing services page.
Tar and Gravel Roof Maintenance and Repair
Proper maintenance extends the life of any roof, and BUR systems are no exception. While tar and gravel roofs are notably forgiving, regular attention prevents small issues from becoming major problems.
Recommended Maintenance Schedule
Twice Yearly (Spring & Fall)
- +Visual inspection of entire roof surface
- +Check and clear all drains and scuppers
- +Remove debris, leaves, and vegetation
- +Inspect all flashing and penetration seals
- +Look for ponding water areas after rain
After Major Storms
- +Inspect for displaced gravel or bare spots
- +Check for impact damage from debris
- +Verify drain function and water flow
- +Document any damage for insurance
- +Check interior for signs of leaks
Common BUR Repairs
Blister Repair
Cut open blistered area, dry thoroughly, apply new hot asphalt and patch material, re-gravel
Bare Spot Coverage
Apply flood coat of asphalt to exposed membrane areas and embed fresh gravel for UV protection
Flashing Restoration
Strip and replace deteriorated base flashings, install new metal cap flashings as needed
When to Call a Professional
While minor maintenance can be performed by building staff, most BUR repairs require professional roofing contractors with hot asphalt equipment. Never attempt repairs involving heat or bitumen without proper training and equipment. If you notice persistent leaks, extensive bare areas, or widespread blistering, contact a qualified commercial roofing contractor for assessment.
Is Tar and Gravel Right for Your Building?
Choosing between tar and gravel and alternative systems depends on your building type, budget, performance priorities, and long-term plans. Here's how to evaluate whether BUR is the right choice.
Tar and Gravel Is Ideal For:
- +Warehouse and industrial buildings with heavy equipment
- +Properties requiring maximum fire resistance
- +Buildings designed for heavy roof loads
- +Long-term ownership with proven reliability priority
- +Matching existing BUR on phased replacement projects
Consider Alternatives If:
- -Building has structural load limitations
- -Energy efficiency is a primary objective
- -Installation disruption must be minimized
- -Future roof maintenance access is limited
- -Budget requires most economical option
Myrtle Beach Climate Considerations
The South Carolina coastal climate presents specific challenges for flat roofing. High UV exposure, humidity, salt air, and occasional tropical storms all affect roofing system performance. Tar and gravel roofs perform well in this environment due to excellent UV protection (gravel surfacing) and wind resistance (heavy weight). However, proper drainage is critical to prevent ponding, and regular inspection after storms is essential.
For buildings prioritizing energy efficiency in our hot, humid climate, white TPO membranes may offer better cooling cost savings. Our team can evaluate your specific building and recommend the optimal system.
Need Expert Flat Roofing Advice?
Weather Shield Roofing has installed and maintained tar and gravel roofs across the Grand Strand for years. Whether you're evaluating BUR for a new installation or need maintenance on an existing system, our commercial roofing experts provide honest assessments and professional service.