Tar Roof: Complete Guide to Tar and Gravel Roofing (2025)

The Basics of Built-Up Roofing (BUR)

A tar roof consists of multiple layers of bitumen (asphalt-based waterproofing material) alternated with reinforcing fabric, topped with a protective layer of gravel. This multi-layer construction method gives the system its technical name: built-up roofing, commonly abbreviated as BUR.

The term "built-up" refers to the layer-by-layer construction process where each successive layer adds strength and waterproofing protection. A typical BUR system contains 3-5 plies (layers) of reinforcing felt or fabric, with hot asphalt bitumen applied between each layer. The more plies a system has, the longer its expected lifespan and the higher its cost.

This roofing method is primarily used on flat or low-slope roofs where traditional pitched roofing materials like shingles wouldn't function properly. Built-up roofing has been protecting buildings since the early 1900s, making it one of the most time-tested flat roofing systems available today.

Why It's Called a "Tar" Roof

The colloquial term "tar roof" comes from the appearance and historical composition of these systems. Early built-up roofs used coal tar as the waterproofing agent, which gave the roofing its name. Modern BUR systems use asphalt bitumen rather than coal tar, but the "tar roof" name persists.

During installation, the asphalt bitumen is heated in a specialized kettle to temperatures of 400-500°F, creating a hot liquid that's mopped or sprayed onto the roof deck. This hot application process is sometimes called "hot mopping" or "hot tar roofing," further reinforcing the tar terminology.

The gravel surface layer distinguishes tar and gravel roofs from other flat roofing systems. This ballast of pea gravel or crushed stone not only protects the underlying waterproofing layers from UV damage but also provides fire resistance and helps hold the roofing system in place.

Common Applications Today

Despite the availability of modern single-ply membranes like TPO and EPDM, tar and gravel roofing remains widely used for specific applications. Flat commercial buildings, industrial warehouses, and manufacturing facilities commonly choose BUR systems for their proven reliability and cost-effectiveness.

Many older apartment buildings and multi-family residential properties feature tar and gravel roofs, particularly in urban areas. These buildings benefit from BUR's durability and ability to handle foot traffic from maintenance personnel accessing rooftop HVAC equipment.

When existing BUR systems need replacement, building owners often choose to install another built-up roof rather than switching to a different system. The roof structure already supports the weight, contractors are familiar with the building, and proven performance provides confidence in the replacement system.

The Layer-by-Layer Build

Built-up roofing creates waterproofing through redundancy. Unlike single-ply membrane systems that rely on one continuous sheet, BUR uses multiple overlapping layers that each provide independent waterproofing protection. If one layer develops a problem, the other layers continue protecting the building.

A typical BUR system includes 3, 4, or 5 plies of reinforcing fabric, creating what's called a 3-ply, 4-ply, or 5-ply system. Each ply consists of a layer of reinforcing felt embedded in hot asphalt bitumen, with additional bitumen applied on top. More plies increase the system's durability, lifespan, and cost, while also adding weight to the structure.

This layered approach provides exceptional waterproofing because water must penetrate multiple barriers before reaching the roof deck. The overlapping construction creates a thick, monolithic waterproofing membrane that can withstand decades of weather exposure.

Key Components of a Tar Roof

Every built-up roofing system contains several essential components working together:

Roof deck: The structural base, typically wood, concrete, or metal decking, provides the foundation for all other layers. The deck must be strong enough to support the weight of the entire roofing system, which can be substantial.

Base sheet: The first roofing layer is mechanically fastened to the deck with screws or nails, providing a secure attachment point for the built-up system.

Bitumen: Hot asphalt serves as the primary waterproofing agent, applied at 400-500°F between each layer. The bitumen bonds the layers together and creates the waterproof seal.

Reinforcing felt: Fiberglass or organic mat fabric provides tensile strength and dimensional stability. Each felt layer is embedded in hot bitumen, creating a composite waterproofing membrane.

Flood coat: A thick final layer of bitumen covers the top felt layer, providing additional waterproofing and a surface for the gravel to embed into.

Gravel ballast: The surface layer consists of 3/8-inch to 1/2-inch pea gravel or crushed stone, typically applied at 400-600 pounds per square (100 square feet). This gravel protects the bitumen from UV degradation, provides fire resistance, and helps hold the roofing system in place.

How the Layers Protect Your Building

The multi-layer design creates redundant waterproofing barriers that work synergistically. If age or weather causes a crack in one layer, the bitumen's self-sealing properties often close small openings, while the layers beneath continue providing protection.

Gravel serves multiple protective functions beyond aesthetics. Ultraviolet radiation from sunlight degrades bitumen over time, but the gravel layer shields the asphalt from UV exposure, significantly extending the roof's lifespan. The gravel also provides a Class A fire rating, the highest fire resistance classification available for roofing systems.

The weight of a completed BUR system—typically 10-25 pounds per square foot—creates a ballasted roof that resists wind uplift without relying solely on adhesives or mechanical fasteners. This mass helps the roof remain stable during high winds. The multiple layers also provide some insulation value and thermal mass, helping moderate temperature fluctuations in the building below.

Advantages of Tar and Gravel Roofs

Built-up roofing systems offer numerous benefits that explain their continued popularity:

• Long lifespan: Properly installed and maintained tar roofs last 20-30 years, with some well-maintained systems reaching 40+ years through periodic recoating.
• Excellent waterproofing: Multiple redundant layers provide superior leak protection compared to single-layer systems. Water must penetrate several barriers before reaching the interior.
• Fire resistant: Gravel surfacing provides Class A fire rating, the best available, making BUR ideal for buildings where fire safety is paramount.
• Cost-effective: At $3-7 per square foot installed, built-up roofing offers competitive pricing for flat roof systems, particularly for larger projects.
• Proven technology: With over 100 years of real-world performance data, BUR has a track record that newer roofing systems can't match.
• Foot traffic friendly: The gravel surface creates a stable walking area, making BUR excellent for roofs requiring regular access for HVAC maintenance or other rooftop equipment.
• UV protection: Gravel shields the waterproofing layers from damaging ultraviolet radiation, preventing premature deterioration.
• Repairable and recoatable: Damaged areas can be patched, and entire roofs can be recoated to extend lifespan, offering flexibility not available with all roofing types.
• Energy mass: The thickness and mass of BUR systems provide thermal buffering, helping moderate temperature swings in the building.
• Weather resistant: Properly designed systems handle temperature extremes, from intense summer heat to freezing winter conditions.

Disadvantages and Limitations to Consider

Despite its advantages, built-up roofing has notable limitations:

• Heavy weight: BUR systems weigh 10-25 pounds per square foot, requiring buildings with strong structural support. Older or weak structures may need reinforcement.
• Professional installation required: The hot asphalt application and specialized equipment make DIY installation impossible. Only experienced commercial roofers should install BUR.
• Hot asphalt safety concerns: Installation involves 400-500°F asphalt that produces toxic fumes and poses severe burn risks, requiring strict safety protocols.
• Installation odor: Hot asphalt creates strong odors that can affect building occupants and neighbors during installation, sometimes for several days.
• Not suitable for pitched roofs: BUR only works on flat or very low-slope roofs. Pitched roofs require different roofing materials.
• Ponding water vulnerability: Poor drainage allows water to pool on the surface, which accelerates deterioration and can lead to premature failure.
• Weather-dependent installation: Rain, cold temperatures, or high winds prevent installation, potentially causing project delays.
• Messy repair process: Repairs require removing gravel, cutting into the existing layers, and applying hot bitumen—a more involved process than patching membrane roofs.
• Gravel maintenance: Wind and foot traffic displace gravel over time, requiring periodic redistribution. Gravel can also clog drains and gutters.
• Dated appearance: Some property owners prefer the cleaner look of modern single-ply membrane roofs over traditional gravel surfaces.

When Tar Roofing Makes Sense

Built-up roofing is the best choice in several situations:

• Flat or low-slope commercial roofs where proven performance matters more than cutting-edge technology
• Buildings with strong structural support capable of handling the weight without reinforcement
• Rooftop access needs for HVAC servicing, maintenance, or other equipment requiring regular foot traffic
• Budget-conscious projects seeking long-term value rather than premium pricing
• Replacement of existing BUR systems where the structure already supports the weight and contractors know the building
• Fire safety priorities where Class A fire rating is required or preferred
• Conservative building owners who prefer proven 100-year-old technology over newer systems with shorter track records

Surface Preparation

Successful tar roof installation begins with thorough preparation. Contractors inspect the roof deck for structural integrity, checking for rot, damage, or weakness that could compromise the new roofing system. Any deck problems must be repaired before roofing begins.

The surface is cleaned to remove debris, dirt, and any loose material. Proper drainage is essential—the roof should slope at least 1/4 inch per foot toward drains to prevent water ponding. Drains, scuppers, and roof penetrations are inspected and prepared. If needed, a primer is applied to the deck to improve adhesion.

Layer-by-Layer Application: The BUR Process

Built-up roof installation follows a systematic process:

1. Step 1: The base sheet is mechanically fastened to the roof deck using screws or specialized fasteners, providing a secure foundation for the built-up layers.
2. Step 2: Asphalt bitumen is heated in a specialized kettle to 400-500°F, creating a workable liquid consistency.
3. Step 3: Hot asphalt is applied to the base sheet using mops, squeegees, or spray equipment, creating an even coating across the surface.
4. Step 4: Reinforcing felt or fabric is rolled out onto the hot asphalt while it's still liquid, embedding the fabric in the bitumen.
5. Step 5: Additional hot asphalt is applied on top of the felt layer, and another felt layer is rolled out. This process repeats for each ply—3-ply systems get three felt layers, 4-ply get four, and so on.
6. Step 6: Once all felt layers are installed, a heavy flood coat of asphalt is applied as the final waterproofing layer.
7. Step 7: While the flood coat is still hot, 400-600 pounds per square of pea gravel is spread evenly across the surface, embedding into the asphalt.
8. Step 8: The roof is allowed to cure for 24-48 hours before foot traffic or other disturbances.

Safety Considerations and Professional Installation

Tar roof installation poses serious safety hazards that make professional installation essential. The hot asphalt reaches temperatures exceeding 400°F, hot enough to cause severe third-degree burns instantly. Workers must wear protective equipment and follow strict safety protocols.

Heating and applying asphalt produces toxic fumes containing hydrogen sulfide and other harmful compounds. Proper ventilation is critical, and building occupants should be kept away from installation areas. OSHA requires specific safety measures for hot asphalt work.

The specialized equipment needed—asphalt kettles, pumps, mops, and safety gear—represents a significant investment that only professional roofing contractors possess. Attempting DIY installation risks serious injury and roof failure. Installation typically takes 2-5 days depending on roof size and complexity.

Installation Cost per Square Foot

Tar and gravel roof installation typically costs $3-7 per square foot, with most projects averaging $4-5 per square foot. The number of plies significantly affects pricing:

• 3-ply system: $3-4 per square foot (15-20 year expected lifespan)
• 4-ply system: $4-5 per square foot (20-25 year expected lifespan)
• 5-ply system: $6-7 per square foot (25-30 year expected lifespan)

For example, a 1,500 square foot flat roof would cost $6,000-$10,500 installed for a typical 4-ply system. Larger projects benefit from economies of scale, with per-square-foot costs decreasing as roof size increases.

Factors Affecting Tar Roof Pricing

Several variables influence the final cost of a built-up roofing project:

Roof size: Larger roofs cost less per square foot due to efficiency gains, while small roofs carry higher unit costs.

Accessibility: Roofs on tall buildings requiring cranes or hoist equipment cost more. Difficult access increases labor time and equipment needs.

Labor rates: Geographic location significantly impacts costs, with urban areas and high-cost regions commanding premium pricing.

Old roof removal: If existing roofing must be removed (tear-off), add $1-3 per square foot to the project cost. Some BUR systems can be installed over existing roofs, saving this expense.

Deck repairs: Rotted or damaged decking must be replaced before roofing, adding material and labor costs not included in basic installation pricing.

Complexity: Numerous penetrations (vents, pipes, skylights), complex shapes, or multiple roof levels increase installation time and cost.

Insulation: Adding or upgrading roof insulation adds $2-4 per square foot but improves energy efficiency and may be required by code.

Maintenance and Repair Costs

Ongoing maintenance represents a small fraction of installation costs but significantly extends roof lifespan:

• Annual inspection: $200-500 for professional twice-yearly inspections
• Minor repairs: $300-800 for patching small leaks, blisters, or cracks
• Major repairs: $1,500-5,000 for extensive damage, multiple leak points, or large area repairs
• Recoating: $2-4 per square foot to apply new flood coat and gravel, extending lifespan 10-15 years
• Full replacement: $3-7 per square foot when the roof reaches end of life

Cost Comparison: Tar Roof vs Alternatives

Built-up roofing offers competitive pricing with proven longevity, making it cost-effective over the system's 20-30 year lifespan.

Regular Inspection Schedule

Proactive maintenance dramatically extends tar roof lifespan. Conduct thorough inspections twice yearly, typically in spring and fall, to catch problems before they escalate. After major storms with high winds, heavy rain, or hail, perform additional inspections to identify weather-related damage.

During inspections, check for: ponding water that remains 48+ hours after rain, blisters or bubbling in the surface, cracks in the bitumen, areas with displaced or missing gravel, clogged drains or scuppers, and damage to flashings around penetrations and roof edges.

For accessible roofs, monthly quick walkthroughs help spot obvious problems. However, comprehensive professional inspections every 2-3 years provide expert evaluation that catches subtle deterioration.

Common Tar Roof Problems and Solutions

Blistering: Blisters form when trapped moisture or air heats up and creates bubbles in the roofing layers. Caused by poor installation, moisture in the roof deck, or inadequate ventilation. Solution: Cut out the blister, dry the area, and patch with fabric and hot bitumen.

Cracking: The bitumen becomes brittle with age and thermal cycling, developing cracks. Temperature extremes and UV exposure accelerate cracking. Solution: Fill small cracks with roof cement; extensive cracking may require recoating the entire roof.

Ponding water: Water that remains on the roof surface 48+ hours after rain indicates drainage problems from roof sag, clogged drains, or inadequate slope. Solution: Improve drainage by clearing drains, installing additional drains, or adding tapered insulation to create proper slope.

Gravel loss: Wind and foot traffic gradually displace gravel, exposing the underlying bitumen to UV damage. Solution: Redistribute existing gravel from thick areas to bare spots, or add new gravel to maintain proper coverage.

Seam separation: The overlapping felt layers can separate due to inadequate bitumen application or thermal movement. Solution: Professional repair involving reopening the seam, cleaning it, and resealing with hot bitumen.

Flashing failures: Metal flashings around penetrations, edges, and transitions commonly develop problems before the main roof surface. Solution: Replace failed flashings and reseal connections with bitumen and fabric reinforcement.

DIY Maintenance vs Professional Service

Property owners can handle some basic maintenance tasks:

• Clearing leaves, branches, and debris from the roof surface and drains
• Redistributing gravel to cover bare spots
• Removing standing water with brooms or squeegees
• Visual inspections from safe access points
• Documenting roof condition with photos

However, leave these tasks to professional roofing contractors:

• Leak detection and repair requiring hot bitumen application
• Blister repair involving cutting and patching
• Recoating the entire roof surface
• Structural repairs to the deck
• Seam resealing and major flashing work

Professional contractors have the specialized equipment, materials, and expertise to perform repairs correctly and safely. They also carry liability insurance protecting property owners from accidents.

When to Repair vs Replace

Deciding between repair and replacement depends on several factors:

Repair makes sense when:

• Less than 25% of the roof shows damage
• Problems are isolated to specific areas
• The roof is under 15-20 years old
• No structural deck issues exist
• Budget constraints limit options

Replacement is recommended when:

• More than 25% of the roof surface is compromised
• Multiple widespread leaks occur throughout the roof
• The system is 20+ years old
• Roof deck shows rot, sagging, or structural damage
• Multiple previous repairs have failed
• Energy efficiency upgrades are desired
• Frequent ongoing repairs exceed replacement cost over time

A professional roofing contractor can assess your specific situation and provide recommendations based on roof condition, building needs, and budget considerations.

Modern Single-Ply Membranes

TPO (Thermoplastic Polyolefin): White or light-gray single-ply membrane that's heat-welded at the seams. The reflective surface significantly reduces cooling costs in hot climates. TPO costs $4-8 per square foot installed, lasts 15-25 years, and weighs much less than BUR. Best for buildings prioritizing energy efficiency and reduced structural load.

EPDM (Rubber Membrane): Black rubber membrane available in large sheets that minimize seams. Seams are glued or taped rather than heat-welded. EPDM costs $4-8 per square foot, lasts 20-25 years, and offers excellent durability with minimal maintenance. The black color absorbs heat, making it less energy-efficient than white membranes. Best for buildings wanting proven reliability with straightforward repairs.

PVC (Polyvinyl Chloride): Premium single-ply membrane with excellent chemical resistance and the longest manufacturer warranties. Heat-welded seams create strong waterproof connections. PVC costs $6-10 per square foot, lasts 20-30 years, and resists chemicals, grease, and bacterial growth. Best for restaurants, laboratories, or other buildings with rooftop chemical exposure.

Modified Bitumen Roofing

Modified bitumen represents an evolution of traditional built-up roofing, using asphalt-based materials with polymer modifiers for improved performance. These systems typically feature 2-3 layers rather than the 3-5 plies of traditional BUR.

Installation uses torch application (heating the back of rolled material) or self-adhered sheets, eliminating the hot asphalt kettle. Modified bitumen costs $3-6 per square foot, lasts 15-20 years, and provides BUR's benefits with easier installation. Best for building owners wanting built-up roofing advantages with modern installation methods.

Quick Comparison Table

Which System Is Right for Your Building?

Choose tar/BUR when: You want proven century-old technology, the building structure supports the weight, foot traffic access is important, you prefer traditional roofing, budget is a primary concern, or you're replacing an existing BUR system.

Choose TPO when: Energy efficiency is a priority, you want a lightweight system, a reflective white surface is desired, the building is in a hot climate, or you need lower structural loading.

Choose EPDM when: You want straightforward maintenance, proven rubber durability matters, ease of repair is important, or you prefer a fully-adhered or mechanically-attached system.

Choose modified bitumen when: You want BUR performance with modern installation, torch-applied or self-adhered systems are preferred, or you need a middle ground between traditional and modern roofing.

Choose PVC when: Chemical resistance is required, you want premium warranties, long-term value justifies higher upfront cost, or you need the highest-performing single-ply membrane.

Consult a licensed roofing contractor familiar with your local climate, building codes, and specific building requirements for a professional assessment and recommendations.

Tar and gravel roofs represent a time-tested solution for flat and low-slope buildings, with over 100 years of proven performance backing their continued use. While modern single-ply membranes offer advantages in certain applications, built-up roofing remains an excellent choice for many commercial and residential flat roof projects.

The multi-layer construction provides exceptional waterproofing through redundancy, with each ply offering independent protection. When properly installed and maintained, BUR systems deliver 20-30 years of reliable service, protecting buildings through decades of weather extremes. The gravel surface handles foot traffic better than most alternatives, making BUR ideal for roofs requiring regular equipment access.

Key considerations when evaluating tar roofing include: ensuring your building structure can support the weight, accepting that professional installation is mandatory due to safety concerns, committing to regular twice-yearly inspections and maintenance, ensuring proper drainage to prevent ponding water, and understanding the 20-30 year lifecycle with eventual replacement or recoating needs.

For flat commercial roofs, buildings with strong structural support, property owners preferring proven technology over newer alternatives, and situations where foot traffic access is important, built-up roofing continues to offer excellent long-term value.

When considering a tar roof for your building—whether new installation or replacement—obtain quotes from at least three licensed commercial roofing contractors with specific BUR experience. Check references, verify insurance coverage, and compare warranties carefully. Proper installation and regular maintenance are critical to achieving the full 20-30 year lifespan that makes built-up roofing a cost-effective choice.