Estimate built-up roofing materials: felt plies, asphalt flood coats, gravel ballast, and base sheet for multi-ply BUR flat roof systems.
Built-up roofing (BUR), commonly called "tar and gravel," is a time-tested flat roofing system consisting of alternating layers of reinforcing felt (plies) and bitumen (asphalt), topped with a flood coat and gravel ballast or a cap sheet. BUR systems have been used for over 100 years and remain a cost-effective choice for commercial flat roofs.
This calculator estimates materials for a multi-ply BUR system: the number of felt rolls, gallons of hot asphalt per mopping (interply and flood coat), and tons of gravel for ballast. The estimate is based on roof area, number of plies, and standard application rates.
Accurate material estimation for BUR is critical because hot asphalt must be ordered and delivered in a timely manner from an asphalt plant, and running short during application wastes heating fuel and crew time.
Precise calculations are essential for meeting regulatory requirements, passing inspections, and ensuring the long-term structural integrity and safety of the completed project.
BUR installations require coordinating felt rolls, asphalt kegs/tankers, and gravel delivery. This calculator ensures all materials are on-site in the right quantities for efficient installation. Consistent use of this tool across projects builds a library of reference data that improves estimating accuracy over time and reduces reliance on individual experience alone.
Felt Rolls = (Roof Area × Plies) / Roll Coverage Interply Asphalt = Roof Area × Plies × 25 lbs per 100 sq ft Flood Coat = Roof Area × 60 lbs per 100 sq ft Gravel = Roof Area × 400 lbs per 100 sq ft
Result: 45 rolls felt, ~500 gal asphalt, 10 tons gravel
For a 5,000 sq ft roof with 4-ply BUR: Felt at 4 sq per roll = 5,000 × 4 / 400 = 50 rolls (with waste). Interply asphalt = 5,000 × 4 × 0.25 = 5,000 lbs. Flood coat = 5,000 × 0.6 = 3,000 lbs. Total asphalt ≈ 8,000 lbs (≈500 gallons). Gravel at 400 lbs/sq = 20,000 lbs = 10 tons.
A BUR system consists of: 1) Base sheet (nailed or adhered), 2) Interply felt plies mopped with hot asphalt, 3) Flood coat of hot asphalt, and 4) Gravel aggregate embedded in the flood coat. Each layer serves a specific purpose, creating a monolithic waterproof membrane.
Hot asphalt must be applied at 400–450°F for proper bonding. Equipment includes an asphalt kettle on the ground and a hot asphalt pipeline or buckets to the roof. The crew must coordinate felt rolling with asphalt mopping to maintain the proper temperature window.
Modified bitumen (mod-bit) uses factory-made sheets with polymer-modified asphalt. They are installed by torching, hot-mopping, or self-adhesion. Mod-bit provides a more consistent membrane thickness than field-applied BUR and is becoming the preferred choice for many contractors.
Most specifications call for 3-ply or 4-ply systems. 3-ply is the minimum for a 10-year warranty. 4-ply is standard for a 15–20 year warranty. 5-ply is premium and used for critical buildings. Each additional ply adds cost and weight but also lifespan and puncture resistance.
Traditional BUR uses fiberglass-reinforced felt (Type IV or Type VI). Organic felts are rarely used anymore due to moisture absorption. Fiberglass felt is lighter, stronger, and does not absorb moisture.
A well-installed 4-ply BUR system with a gravel surface coat lasts 20–30 years. Regular maintenance (recoating, patching, and keeping drains clear) extends the lifespan. Many BUR roofs exceed 30 years with proper care.
Yes. Cold-applied BUR systems use solvent-based or water-based adhesives instead of hot asphalt. They eliminate the need for asphalt kettles and reduce fumes, but material costs are higher and application rates may differ.
A 4-ply BUR with gravel weighs 500–700 lbs per square. This includes felt (50–80 lbs), asphalt (100–150 lbs), and gravel (300–400 lbs). Without gravel (cap sheet finish), weight drops to 200–300 lbs per square.
A modified bitumen cap sheet can replace the gravel and flood coat. This is called a "hybrid" or "modified BUR" system. Cap sheets are lighter, provide a walkable surface, and can have a reflective granule coating for energy efficiency.