What Is a Built Up Roof

A built up roof (BUR) is a durable, multi-layer roofing system that combines asphalt, bitumen, and reinforcing fabrics topped with a protective outer layer. This traditional method has evolved into a modern, resilient solution for commercial and some residential structures. BUR systems are known for their seam integrity, excellent waterproofing, and broad versatility across climate zones. Understanding its composition, installation, and maintenance helps facility managers and builders make informed decisions about long-term roof performance.

How A Built Up Roof Is Constructed

A built up roof consists of several distinct components arranged in layers. The typical sequence includes a solid or prepared substrate, a base sheet or batt insulation, and multiple plies of reinforcing felt saturated with hot or cold asphalt or coal tar pitch. The final surface layer is a reflective or granule-covered cap sheet. In hot-m asphalt systems, the asphalt is melted and applied in successive coats, while cold-adhesion BURs use asphalt-emulsion adhesives. The outermost layer often includes ballast such as gravel or rubber blisters to shield the system from UV exposure and weathering.

Key Materials And Variations

Common BUR configurations use polyester or fiberglass felts for strength and dimensional stability. The waterproofing is primarily achieved by the bituminous plies and the cap sheet. Variations include:

• Hot Asphalt BUR: Traditional method applying hot asphalt between plies for robust waterproofing.
• Cold Adhered BUR: Adhesives secure plies without heating, reducing fire risk.
• Saturated Felt BUR: Felts saturated with asphalt prior to installation for enhanced waterproofing.
• Cap Sheet Options: Glass-fiber or mineral granulated cap sheets provide UV protection and reflectivity.
• Ballasted BUR: Gravel or pavers protect and add ballast weight, aiding wind resistance.

Performance And Benefits

Built up roofs deliver long-term waterproofing, excellent puncture resistance, and extended service life when properly installed and maintained. The multiple plies create redundancy: if one layer is compromised, others continue to protect the structure. The ballast in ballasted BUR helps resist wind uplift and can contribute to thermal mass, aiding energy performance in certain climates. When designed with reflective or cool-cap components, BUR systems can reduce cooling loads by lowering solar heat gain.

Challenges And Limitations

BUR systems can be heavier than single-ply roofs, requiring structural assessment to support ballast weight. Installation involves hot processes or adhesives, presenting fire safety considerations and environmental concerns if not managed properly. Maintenance is essential to monitor for surface granule loss, blisters, ridging, or ply delamination. Periodic inspections, prompt membrane repair, and gravel replacement help sustain performance and prevent leaks.

Installation Process In Brief

Installers prepare the substrate, ensure it is dry, clean, and sound. A primer or crack-prevention membrane may be applied. Base sheets or boards are installed, followed by multiple plies of saturated felt with hot asphalt or cold adhesive. Each ply is set and rolled to remove air voids. The final surface layer—cap sheet and ballast if used—provides UV protection and weather resistance. Proper detailing around penetrations, edges, and vents is critical to prevent leaks at joints and terminations.

Maintenance And Longevity

Typical BUR lifespans range from 15 to 40 years depending on climate, roof design, ballast, and maintenance. Routine inspections should target blisters, cracks, ridges, and stray granules, which can indicate underlying issues. Cleaning debris, repairing blisters, and resealing seams extend life. Periodic re-cap sheets or gravel replacement may be necessary to maintain UV protection and waterproofing. In higher-traffic or industrial settings, more frequent assessments are advisable.

Cost Considerations And Value

Installation costs for built up roofs vary by region, accessibility, ballast requirements, and choice of cap sheet. Although BUR can have higher upfront costs than some single-ply systems, its durability, proven track record, and potential for repairability can yield long-term savings. When evaluating value, consider energy performance, expected service life, maintenance needs, and the cost of re-coating or re-ballasting over time.

Environmental And Energy Aspects

Many BUR systems incorporate recycled materials in cap sheets or use sustainable insulation underlayments. Reflective cap sheets reduce heat absorption, supporting energy efficiency in hot climates. Conversely, some older BUR installations may require careful management of asphalt fumes and volatile organic compounds during installation. Proper ventilation and adherence to local environmental regulations help minimize environmental impact.

Applications And Suitability

Built up roofs are well-suited for flat or low-slope roofs on commercial buildings, schools, and multifamily housing. They perform well in varied weather, including heavy rain, snow, and wind, when properly constructed. For roofs with heavy foot traffic, rooftop equipment, or high UV exposure, BUR’s durability and ability to tolerate mechanical damage make it a practical choice. In retrofit projects, BUR can be installed over existing roofs if the substrate condition is suitable.

Choosing A Contractor And Quality Assurance

Selecting a qualified contractor is essential for BUR success. Seek references, review completed projects in similar climates, and verify licenses and insurance. Request detailed proposals outlining material types, ply counts, ballast options, and expected maintenance plans. Ask about fire safety measures during installation, adherence to code requirements, and warranty terms that cover both waterproofing and workmanship.

Common Questions About Built Up Roofs

• How many plies does a BUR typically have? Most systems use three to five plies, depending on climate and design requirements.
• Do BUR roofs require maintenance? Yes, regular inspections and timely repairs are essential for longevity.
• Is BUR fire-resistant? The system provides fire resistance through its thick, layered construction, but local codes and materials determine rating levels.
• Can BUR be installed over existing roofs? Often, yes, if the substrate is in good condition and suitable for ballast or attachment methods.