Direct Answer
The type of asphalt used in a four-ply built-up roofing (BUR) system plays an important role in its adhesion, flexibility, temperature resistance, and long-term waterproofing performance. The asphalt bonds the roofing plies together, helps create a continuous membrane, and contributes to the system’s ability to resist water intrusion and weather exposure.
Different asphalt types have different softening points and application characteristics. Selecting the correct type helps ensure that the asphalt performs appropriately under the roof’s slope, expected temperatures, and installation conditions. Using asphalt with an unsuitable softening point or applying it outside the manufacturer’s specified temperature range can increase the risk of asphalt slippage, dripping, inadequate bonding, or membrane-related problems.
In a four-ply BUR system, performance depends on more than the number of plies. The asphalt type must be compatible with the reinforcing sheets, roof slope, surfacing, and overall system design. The contractor should follow the manufacturer’s specifications and applicable standards, including GAF’s Built-Up Roofing Systems specifications.
Who This Applies To
- Commercial property owners
- Facility and property managers
- Roofing contractors and consultants
- Commercial building developers
- Building owners evaluating BUR roof replacement
- Owners investigating recurring leaks or premature roof deterioration
Not for: This overview does not replace project-specific roofing specifications, manufacturer instructions, or professional evaluation of an existing BUR assembly.
1. Understanding a Four-Ply BUR System
A built-up roof consists of multiple reinforcing sheets bonded together with layers of bitumen, commonly roofing asphalt. The completed assembly may include a base sheet, reinforcing ply sheets, and protective surfacing such as aggregate or a mineral-surfaced cap sheet.
In a four-ply system, the reinforcing layers provide redundancy. If one area develops a localized defect, the other layers can provide additional resistance to water penetration, although they do not eliminate the need for proper installation and maintenance.
Asphalt is essential because it:
- Bonds adjacent roofing plies together.
- Helps form a continuous waterproofing membrane.
- Contributes to adhesion between system components.
- Helps the assembly resist weather exposure.
- Works with the reinforcing sheets to provide overall membrane durability.
According to GAF’s overview of built-up roofing, BUR systems combine reinforcing plies and asphalt, typically finished with a cap sheet or an asphalt flood coat and aggregate.
2. How Asphalt Type Affects Roof Performance
Roofing asphalt is classified according to properties such as softening point and penetration. These properties influence how the asphalt behaves during installation and under service conditions.
| Performance Factor | Why Asphalt Type Matters |
|---|---|
| Adhesion | The asphalt must bond the reinforcing sheets properly to form a continuous assembly. |
| Temperature resistance | The selected asphalt must suit the roof system’s design and expected service temperatures. |
| Slippage resistance | Asphalt with an unsuitable softening point can contribute to membrane slippage, particularly on slopes. |
| Application quality | Asphalt must be heated and applied within the manufacturer’s specified temperature range. |
| Waterproofing | Proper asphalt selection and application help the plies form a continuous water-resistant membrane. |
| Long-term durability | Compatible materials and correct installation help reduce the risk of premature system failure. |
The correct asphalt type is therefore part of the overall roof-system design, not simply a material choice based on price.
3. Understanding ASTM D312 Asphalt Types
ASTM D312 classifies roofing asphalt into different types with specified physical properties. In certain GAF BUR applications, Type III and Type IV asphalt are recommended based on roof slope and system requirements.
| Asphalt Type | Softening Point Listed in GAF Guidance | Maximum Slope in the Referenced Guidance |
|---|---|---|
| Type III | 185–205°F (85–96°C) | Up to 1/2:12 |
| Type IV | 210–225°F (99–107°C) | Up to 3:12 |
Important: These values come from the referenced GAF system guidance and should not be treated as universal limits for every BUR assembly. The permitted asphalt type and slope depend on the specific roofing system, manufacturer’s instructions, and project specifications.
For further technical details, consult the GAFGLAS Built-Up Roofing Systems manual.
4. Why the Asphalt Softening Point Matters
The softening point indicates the temperature range at which asphalt begins to soften under a specified test. It is an important consideration when selecting roofing asphalt for a particular application.
If asphalt with an unsuitable softening point is used, it may not perform as intended under the roof’s service conditions.
Potential concerns include:
- Asphalt movement on sloped roof areas.
- Bitumen dripping under certain conditions.
- Reduced reliability of the installed assembly.
- Problems associated with incorrect material selection.
However, a higher softening point does not automatically mean that an asphalt is better for every roof. The selected material must match the intended application and the complete roofing specification.
5. Asphalt Application Temperature Is Equally Important
Even the correct asphalt type can perform poorly if it is heated or applied incorrectly.
For hot-applied BUR systems, contractors must follow the specified application temperature and the asphalt manufacturer’s guidance. GAF uses the term equiviscous temperature (EVT) to describe the temperature at which asphalt reaches its optimum application viscosity.
The referenced GAF manual instructs contractors to apply asphalt within the specified EVT range and identifies a maximum heating temperature of 550°F (260°C) for ASTM D312 asphalt. The applicable product and project instructions must always be checked before installation.
Incorrect heating or application can contribute to:
- Insufficient asphalt coverage.
- Poor bonding between plies.
- Excessive asphalt movement or dripping.
- Installation inconsistencies.
- Reduced reliability of the completed roof assembly.
For these reasons, proper material selection must be combined with controlled installation practices.
6. How Asphalt Type Works With the Reinforcing Plies
In a four-ply BUR roof, asphalt and reinforcing sheets work together. The reinforcing sheets provide structural continuity within the membrane, while asphalt bonds the layers and contributes to waterproofing.
The reinforcing material also matters. Glass-fiber and other approved roofing felts have their own physical properties and product requirements.
For example, GAF’s Tri-Ply Ply 4 product data sheet identifies its reinforcing sheet as a component designed for built-up roofing and specifies its applicable product standard.
Contractors should verify that the asphalt and reinforcing sheets are compatible and approved for the intended assembly. Simply adding more plies does not compensate for incompatible materials or improper installation.
7. What Happens When the Wrong Asphalt Is Used?
Using an unsuitable asphalt type or failing to follow the specified application procedure can increase the risk of roof-system deficiencies.
Possible problems include:
- Asphalt slippage on roof slopes.
- Inconsistent bonding between reinforcing sheets.
- Premature deterioration in affected areas.
- Leaks associated with deficiencies in the completed assembly.
- Difficulty establishing whether the installation followed manufacturer requirements.
These outcomes are not inevitable whenever an asphalt selection error occurs. The actual impact depends on the materials, roof design, installation quality, and conditions experienced by the roof.
8. What Commercial Property Owners Should Verify
Owners replacing or evaluating a four-ply BUR roof should ask the roofing contractor to document the proposed asphalt type and confirm that it matches the specified roof system.
Useful records include:
- The roofing system specification.
- The asphalt manufacturer’s product information.
- The specified ASTM classification.
- The reinforcing sheet product data.
- Installation temperature requirements.
- Roof slope and surfacing requirements.
- Inspection and quality-control records.
- Applicable warranty documentation.
These records help facility managers assess whether the installed system matches the approved design and support future maintenance, repair, and warranty discussions.
9. Does the Asphalt Type Alone Determine BUR Roof Life?
No. Asphalt selection is important, but it is only one factor in roof performance.
The service life of a four-ply BUR roof also depends on:
- Quality of the reinforcing sheets.
- Correct installation and asphalt application.
- Roof slope and drainage.
- Protective surfacing.
- Exposure to weather and ultraviolet radiation.
- Foot traffic and mechanical damage.
- Routine inspection and maintenance.
- Quality of flashing and penetration details.
GAF’s overview of asphaltic roofing systems discusses BUR construction and the role of different asphaltic technologies in commercial roof assemblies.
Bottom Line
The asphalt type plays a critical role in the performance of a four-ply BUR roof because it affects bonding, temperature behavior, application quality, and resistance to membrane slippage. The correct asphalt must be selected for the specific roof design and installed according to the manufacturer’s instructions and applicable specifications.
For commercial property owners, the most important step is to verify that the asphalt, reinforcing sheets, roof slope, and installation procedure are compatible as a complete system. Asphalt type contributes to performance, but proper installation, drainage, surfacing, and maintenance are equally important to long-term roof reliability.
Related Questions
- What Does a Flat Roof Assessment Look for on TPO, PVC, Modified Bitumen, and Built-Up Roofs?
- When Is It Better to Replace a Commercial Roof Rather Than Repair It?
- What Is a Commercial Roof Condition Report?
Sources
- GAFGLAS Built-Up Roofing Systems Manual
- GAF: What Is Built-Up Roofing?
- GAF Tri-Ply Ply 4 Product Data Sheet
- GAF: Asphaltic Roofing Systems for Commercial Properties
- Asphalt Institute: Asphalt Roofing Research and Technical Resources
Last reviewed: October 2026
Related Resources
How Is Remaining Useful Life Estimated for a Commercial Roof?
Need Commercial Roofing Help?
ShieldLine Roofing can help commercial property owners evaluate BUR roof conditions, review installation concerns, identify deterioration, and plan appropriate inspection, maintenance, repair, or replacement work.
Disclaimer: This information is provided for general educational purposes and is not insurance, legal, engineering, or roofing-design advice. Product requirements vary by manufacturer and roofing assembly. Consult the applicable project specifications, manufacturer instructions, and qualified roofing professional before selecting or installing roofing materials.
Commercial Roofing Services in Florida
