Built-Up Roofing Code in Georgia

Georgia Adopts IRC R905.9 for Built-Up Roofing

Georgia regulates built-up roofing (BUR) through the International Residential Code, adopted with state-specific amendments by the Georgia Department of Community Affairs (DCA). Section R905.9 of the IRC controls every aspect of built-up roof coverings: material standards, ply counts, surfacing methods, slope thresholds, and fire classification requirements. Every built-up roof installed in the state must satisfy these provisions.

Built-up roofing has served flat and low-slope structures for over a century. The system layers multiple plies of reinforcing felt or fiberglass with hot asphalt, cold adhesive, or torch-applied modified bitumen to create a monolithic waterproof membrane. Georgia's code treats BUR as a distinct roof covering category with its own material references and installation rules separate from asphalt shingles, metal panels, or single-ply membranes.

The 2024 IRC cycle maintains the core BUR provisions while updating referenced ASTM standards and clarifying fire classification requirements. These are incremental refinements rather than structural changes to the code section. Contractors who installed BUR under previous code cycles will recognize the framework, but the specific ASTM edition years and testing protocols carry updates that affect material procurement and quality documentation.

Local jurisdictions enforce R905.9 through their building departments. When you pull a roofing permit for a flat roof project in Alpharetta, the building inspector verifies compliance with this section during the final inspection. The same applies in Marietta, Sandy Springs, Buckhead, and every municipality across the metro area. Some jurisdictions add local amendments that exceed the state baseline, and your contractor must verify local requirements before starting work.

For homeowners, BUR matters when your property includes flat or low-slope sections. Pool houses, covered patios, garage roofs, modern architectural additions, and mixed-use structures with flat sections all fall under R905.9 rather than the shingle provisions of R905.2. Luxury homes in Buckhead and Sandy Springs often combine steep-slope shingle roofs with flat BUR sections over living spaces, garages, or outdoor kitchens.

For a broad overview of how Georgia adopts and enforces roofing codes, including the permit process and inspection requirements, see our Georgia residential roofing code guide. This page focuses on the specific provisions of R905.9 that govern built-up roofing installations.

Minimum Slope and Drainage Requirements

IRC R905.9 sets the minimum slope for built-up roofing at 0.25:12. That equals one-quarter inch of rise per twelve inches of horizontal run. This is a near-flat configuration that still provides enough pitch for water to move toward drains, scuppers, or roof edges without ponding.

Ponding water is the primary enemy of any flat roof system. Water that sits on a BUR membrane for more than 48 hours after rainfall accelerates degradation of the surfacing material, weakens interply adhesion, and increases the structural load on the roof deck. Georgia code addresses this through the slope minimum rather than through explicit ponding standards, but the practical effect is the same: the roof must drain.

Achieving 0.25:12 slope on a structurally flat deck requires tapered insulation, cricket framing, or structural slope built into the deck framing. Tapered insulation is the most common solution for existing structures. Rigid polyisocyanurate insulation boards cut in graduated thicknesses create positive slope across the roof surface toward designated drainage points. The insulation serves double duty: it provides the R-value required by Georgia's energy code and creates the slope required by R905.9.

Slope Range	BUR Permitted?	Drainage Method	Common Application
Less than 0.25:12	No	Must reframe or add tapered insulation	Structurally flat decks without modification
0.25:12 to 0.5:12	Yes	Internal drains or scuppers required	Commercial flat roofs, large residential flat sections
0.5:12 to 1:12	Yes	Scuppers, drains, or edge drainage	Pool houses, garage roofs, additions
1:12 to 3:12	Yes	Standard edge drainage	Low-slope residential sections, covered patios

Internal roof drains connect to the building's storm drainage system and require their own code compliance under the plumbing provisions. Scuppers penetrate parapet walls and direct water to exterior downspouts. Edge drainage on non-parapet flat roofs uses gravel stops or metal edge flashing to control water discharge. Your contractor must select the drainage method that matches the roof geometry and connect it to a system that moves water away from the building foundation.

For flat sections on luxury homes in Roswell, Johns Creek, and Alpharetta, the slope requirement intersects with aesthetic design. Architects often specify minimal visible slope on flat-roof additions and modern home designs. Tapered insulation systems can achieve the required 0.25:12 while maintaining the visual lines the design requires. The code does not set a maximum slope for BUR, so steeper configurations are permitted where the design calls for them.

Georgia's rainfall patterns make proper drainage critical. Metro Atlanta receives an average of 50 inches of rain per year, with intense summer thunderstorms that can deliver two or more inches in a single event. A flat roof that drains at 0.25:12 slope handles these events if the drainage system is sized for the tributary area. A flat roof that ponds water creates a structural and waterproofing liability that no amount of quality membrane work can overcome.

Ply Requirements and System Assembly

The "built-up" in built-up roofing refers to the layered construction method. Each ply consists of a reinforcing sheet (felt or fiberglass) bonded with bitumen (hot asphalt, cold adhesive, or torch-applied modified bitumen). The number of plies determines the membrane thickness, puncture resistance, fire rating, and service life of the assembly.

IRC R905.9 does not prescribe a specific minimum ply count by number. Instead, it references the manufacturer's assembly specification and the fire classification requirements of the jurisdiction. In practice, Georgia contractors install three-ply and four-ply systems as the standard for code-compliant BUR assemblies.

Three-Ply Systems

A three-ply BUR assembly uses three layers of reinforcing felt bonded with bitumen. This is the minimum configuration that most manufacturers approve for warranty coverage. Three-ply systems provide approximately 150 mils (0.15 inches) of finished membrane thickness. They carry adequate puncture resistance for foot traffic during maintenance and meet the fire classification requirements for most residential applications.

Three-ply assemblies work well for pool houses, covered patios, and detached structures where foot traffic is minimal and the roof serves a weather protection function without heavy mechanical equipment loads. For luxury homes in Sandy Springs and Buckhead, three-ply systems cover flat-roof additions, sunrooms, and architectural flat sections that complement the main steep-slope roof.

Four-Ply Systems

A four-ply assembly adds a fourth reinforcing layer and interply bitumen application. The additional ply increases membrane thickness to approximately 200 mils, improves puncture resistance, and provides greater redundancy against interply delamination. Four-ply systems are the standard for commercial applications and high-value residential installations where the roof covers conditioned living space.

The fourth ply also upgrades the fire rating of the assembly. UL-classified four-ply BUR assemblies with gravel surfacing achieve Class A fire ratings, the highest classification under ASTM E108. Jurisdictions in the wildland-urban interface may require Class A assemblies, making four-ply the baseline rather than an upgrade.

"A 3-ply BUR membrane measures approximately 150 mils thick. A 4-ply membrane reaches 200 mils and earns UL Class A fire rating with aggregate surfacing. The structure's use, fire zone designation, and warranty targets dictate which you need."

System Components Beyond Plies

A complete BUR assembly includes more than the membrane plies. The system starts at the roof deck and builds upward through these layers:

• Vapor barrier: A sheet membrane or coating applied to the roof deck to prevent moisture migration from conditioned space into the insulation and membrane layers. Georgia code references vapor barriers under IRC R905.9 when the building's interior conditions create vapor drive toward the roof assembly.
• Insulation: Rigid board insulation (polyisocyanurate, expanded polystyrene, or extruded polystyrene) provides thermal resistance. Georgia's energy code under IECC Chapter 4 sets minimum R-values for roof assemblies by climate zone. Metro Atlanta sits in Climate Zone 3A, requiring R-25 for roof insulation in new construction.
• Base sheet: A mechanically fastened or adhered base sheet anchors the membrane system to the insulation or deck. ASTM D2178 Type VI felt or ASTM D4601 fiberglass base sheets are standard options.
• Interply bitumen: Hot asphalt (ASTM D312) or cold adhesive bonds each ply to the one below it. The bitumen type must match the felt type and the ambient temperature conditions during installation.
• Surfacing: Aggregate (gravel or slag), mineral cap sheet, or reflective coating protects the top ply from UV degradation and provides fire resistance.

ASTM Material Standards for Built-Up Roofing

IRC R905.9 references multiple ASTM standards that govern the materials used in built-up roofing assemblies. Each component in the system must comply with its designated standard. Your contractor should verify ASTM compliance markings on all materials before installation begins.

ASTM Standard	Material	Key Requirements	Application
ASTM D2178	Asphalt-saturated organic felt	Type IV (No. 15) or Type VI (No. 30) weight classes	Interply reinforcing sheets
ASTM D312	Asphalt for BUR	Type I (dead level), II (flat), III (steep), IV (special steep)	Interply and flood-coat bonding
ASTM D6162	SBS modified bitumen sheets	Reinforcement type, minimum thickness, elongation	Modified bitumen cap sheets and base sheets
ASTM D4601	Asphalt-coated fiberglass base sheet	Tensile strength, tear resistance	Mechanically fastened base layer
ASTM E108	Fire test standard	Class A, B, or C fire rating by assembly	Complete roof assembly fire classification

ASTM D312: Asphalt Types and Slope Matching

The asphalt used to bond BUR plies comes in four types, each formulated for a specific slope range. Type I asphalt has the lowest softening point and flows at lower temperatures, making it suitable for dead-level and near-flat applications. Type IV asphalt has the highest softening point and resists flow on steeper slopes.

Georgia's climate adds a variable that the code addresses indirectly. Summer roof surface temperatures in metro Atlanta can exceed 170 degrees Fahrenheit on a dark flat roof. The asphalt type must maintain its bond strength at these temperatures without softening to the point where it flows between plies. Type III asphalt (softening point 185-205 degrees F) is the standard selection for most metro Atlanta BUR installations. Type II (softening point 165-180 degrees F) works on dead-level roofs with reflective surfacing that reduces surface temperatures.

ASTM D2178: Felt Reinforcement

Organic felt sheets provide the reinforcing matrix within each ply. ASTM D2178 classifies felt by weight: Type IV corresponds to No. 15 felt (approximately 15 pounds per 100 square feet), and Type VI corresponds to No. 30 felt (approximately 30 pounds per 100 square feet). Heavier felt provides greater tear resistance and dimensional stability but costs more per ply.

Four-ply systems often use Type VI (No. 30) felt for the base and intermediate plies, then switch to a mineral-surfaced cap sheet for the top layer. Three-ply systems may use Type VI throughout to maximize the membrane's strength within a lower ply count. The manufacturer's assembly specification dictates the felt type for each position in the system.

ASTM D6162: Modified Bitumen Sheets

Many modern BUR assemblies incorporate SBS (styrene-butadiene-styrene) modified bitumen sheets as cap sheets or replacement for traditional felt plies. ASTM D6162 governs these products. SBS modification gives the bitumen greater elongation (the ability to stretch without cracking), improved low-temperature flexibility, and enhanced fatigue resistance from thermal cycling.

In metro Atlanta, where roof surfaces cycle between freezing winter nights and 170-degree summer afternoons, the elongation properties of SBS-modified products provide a measurable performance advantage over straight asphalt systems. The code permits SBS-modified assemblies under R905.9 when the products meet ASTM D6162 and the manufacturer's listed assembly meets the required fire classification.

Proper material documentation protects you during inspections and future insurance claims. Every ASTM-compliant product carries identification markings on its packaging. Your contractor should retain material documentation that shows ASTM compliance for every component in the assembly. This documentation supports permit closeout, warranty registration, and insurance coverage verification.

Aggregate Surfacing and Fire Rating Requirements

The surfacing layer on a built-up roof serves two code functions: UV protection and fire resistance. Exposed asphalt degrades under ultraviolet radiation within months. The surfacing layer shields the membrane from UV exposure and contributes to the fire classification of the complete assembly.

Aggregate Surfacing

Aggregate (gravel or slag) embedded in a flood coat of hot asphalt is the traditional surfacing method for BUR. The aggregate provides a physical barrier against UV radiation, ballast weight that resists wind uplift, and a fire-resistant top layer that contributes to Class A fire classification. Aggregate surfacing uses rounded gravel (3/8-inch to 5/8-inch diameter) or crushed slag applied at 400 to 600 pounds per 100 square feet.

Aggregate surfacing adds significant weight to the roof assembly. At 400 pounds per square (100 square feet), the aggregate alone contributes approximately 4 pounds per square foot of dead load. The structural framing must support this weight in addition to the membrane, insulation, and code-required live loads. Your structural engineer or contractor must verify load capacity before specifying aggregate surfacing.

Smooth-Surface and Coated BUR

Mineral-surfaced cap sheets provide an alternative to aggregate that reduces weight and simplifies maintenance. Cap sheets incorporate factory-applied granules similar to asphalt shingles, providing UV protection without the additional weight of loose aggregate. Smooth-surface BUR with reflective coatings (aluminum-pigmented, white elastomeric, or silicone) offers the lowest weight and highest solar reflectivity.

Georgia's energy code creates an incentive for reflective surfacing on flat roofs. Cool roof coatings that meet Energy Star requirements reduce cooling loads in metro Atlanta's hot summers. A white or aluminum-coated BUR surface can reduce roof surface temperature by 50 to 60 degrees compared to a dark aggregate surface. This temperature reduction extends membrane life and reduces air conditioning costs for the conditioned space below.

Fire Classification by Assembly

Fire ratings under ASTM E108 apply to the complete roof assembly, not to individual components. A BUR system earns its Class A, B, or C rating based on testing the specific combination of deck, insulation, membrane plies, and surfacing as an integrated system. Changing any component can change the fire classification.

UL publishes Roofing Materials and Systems Directory listings for BUR assemblies that have passed ASTM E108 testing. Your contractor must verify that the specific assembly specified for your project carries a current UL listing at the required fire classification. Class A is the highest rating. Jurisdictions with wildland-urban interface designations. Most four-ply BUR assemblies with aggregate surfacing achieve Class A without difficulty.

Vapor Barriers and Insulation Requirements

Moisture migration through the roof assembly causes more BUR failures than any other single factor. Warm, moist interior air migrates upward through the roof deck into the insulation and membrane layers. When that moisture condenses within the assembly, it degrades insulation R-value, corrodes metal deck components, and delaminates interply bonds. Georgia code addresses this through vapor barrier requirements and insulation standards.

IRC R905.9 references vapor barriers when the building's interior conditions create vapor drive potential. In metro Atlanta's climate (hot and humid summers, mild winters), the vapor drive direction reverses seasonally. Summer vapor drive pushes exterior humidity inward. Winter vapor drive pushes interior moisture outward through the roof. A correctly positioned vapor barrier must account for the dominant drive direction.

Vapor Barrier Placement

Standard practice places the vapor barrier on the warm side of the insulation. In Georgia's mixed-humid climate zone, this means below the insulation layer, directly on the roof deck. The vapor barrier prevents interior moisture from reaching the insulation and membrane layers above. Common vapor barrier materials include:

• Polyethylene sheet: 6-mil or 10-mil polyethylene film, lapped and sealed at joints
• Modified bitumen sheet: Self-adhering SBS-modified bitumen applied directly to the primed deck surface
• Spray-applied vapor retarder: Two-component urethane or acrylic coating sprayed on the deck to a specified mil thickness

The vapor barrier must be continuous across the entire roof area. Penetrations for mechanical equipment, drains, and structural connections require careful sealing to maintain the vapor barrier's integrity. A single unsealed penetration can allow enough moisture migration to saturate insulation boards over a single heating season.

Insulation Code Requirements

Georgia's energy code under IECC Chapter 4 requires minimum R-25 for roof insulation in Climate Zone 3A (metro Atlanta) for new construction and additions. Existing building re-roofing projects may qualify for lower thresholds under the existing building provisions, but most contractors recommend bringing insulation to current code when replacing a BUR membrane.

Polyisocyanurate (polyiso) rigid board insulation is the standard choice for BUR assemblies. Polyiso delivers the highest R-value per inch (approximately R-5.7 per inch at mean temperature) and provides a stable, fire-resistant substrate for the membrane. Two layers of polyiso board installed with staggered joints eliminate thermal bridging at board edges.

"A single unsealed penetration through the vapor barrier can saturate insulation boards within one heating season. The IECC requires continuous vapor barrier coverage across the entire roof area before insulation placement."

For luxury homes in Johns Creek and Roswell with flat-roof sections over conditioned living space, the insulation and vapor barrier quality directly affects comfort and energy costs. A flat roof over a kitchen addition or master suite requires the same thermal performance as the rest of the building envelope. Cutting corners on insulation thickness or skipping the vapor barrier creates condensation problems that manifest as ceiling stains, mold growth, and premature membrane failure.

How 1 Source Installs Built-Up Roofing in Metro Atlanta

Every flat roof and BUR project that 1 Source Roofing performs meets or exceeds IRC R905.9 requirements. We also follow manufacturer assembly specifications that trigger full system warranty coverage from GAF and CertainTeed.

Pre-Installation Assessment

Before any materials arrive at your property, our project manager inspects the existing roof structure. We verify deck condition, measure slope, evaluate drainage pathways, check structural load capacity for the specified assembly, and identify any local code amendments that apply to your jurisdiction. For re-roofing projects, we assess whether the existing assembly requires full tear-off under Georgia's re-roofing provisions or qualifies for recover installation.

We pull permits for every project that requires one. In Alpharetta, Sandy Springs, Johns Creek, Roswell, Marietta, and throughout the 30-mile radius we serve, we maintain active business licenses and know each jurisdiction's permit process.

Assembly Execution

Our crews install each component of the BUR assembly according to the manufacturer's published specifications and IRC R905.9 requirements. We verify asphalt temperature at the point of application, maintain proper interply bitumen thickness, position felt sheets with the correct side laps and end laps, and apply surfacing at the specified coverage rate. Every step follows a documented sequence that produces a code-compliant, warranty-eligible assembly.

On projects that combine flat BUR sections with steep-slope shingle roofing, we coordinate the transition details between the two systems. The flashing code at the intersection of flat and steep sections requires careful detailing to prevent water infiltration. We install step flashing, counter flashing, and cant strips according to both R905.9 (BUR) and R905.2 (shingles) at these transitions.

Post-Installation Inspection

After installation, we schedule the final building inspection with the local jurisdiction. Our project manager meets the inspector on site. A closed permit with a passed inspection documents that the roof meets Georgia building code, protects your insurance coverage, and supports your property's resale position.

For homeowners in Buckhead, Johns Creek, and other premium neighborhoods, a code-compliant BUR installation protects both the structure and the investment. A flat roof on a $2 million home needs more than waterproofing. It needs a passed inspection, an active manufacturer warranty, and documentation that supports future insurance claims and property transactions.

Related Building Code and Technical Guides

These pages cover related code requirements and technical installation standards for Georgia roofing:

• Georgia Residential Roofing Code Guide -- Complete overview of IRC Chapter 9, permits, and inspections
• TPO and PVC Roofing Code -- Single-ply thermoplastic membrane requirements under IRC R905.13
• EPDM Rubber Roofing Code -- Thermoset single-ply membrane requirements under IRC R905.12
• Spray Foam Roofing Code -- SPF roofing requirements under IRC R905.14
• Roof Insulation and Energy Code -- R-value requirements and insulation standards for Georgia
• Roof Flashing Code -- Flashing requirements at transitions, penetrations, and edges
• Reroofing and Tear-Off Requirements -- When tear-off is required vs. recover installation
• Roofing Permit Requirements -- Permit process and inspection requirements by jurisdiction
• Roof Flashing Installation Guide -- Technical installation methods for all flashing types

Questions about built-up roofing code requirements for your property? Call 1 Source Roofing at (404) 277-1377 for a free roof inspection and code compliance evaluation.