How many plies do I need?

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.

What type of felt is used in BUR?

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.

How long does a BUR roof last?

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.

Can I use cold-applied adhesive instead of hot asphalt?

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.

How heavy is a BUR system?

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.

What is the alternative to gravel ballast?

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.

Built-Up Roofing (BUR) Calculator

Estimate built-up roofing materials: felt plies, asphalt flood coats, gravel ballast, and base sheet for multi-ply BUR flat roof systems.

Roof Area

sq ft

Number of Plies

Gravel Finish?

Felt Rolls

Includes 15% overlap waste

Interply Asphalt

5,000 lbs

Flood Coat Asphalt

3,000 lbs

Total Asphalt

941 gal

8,000 lbs

Gravel Ballast

10.0 tons

System Weight

610 lbs/square

Planning notes, formulas, and examples

About the Built-Up Roofing (BUR) Calculator

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.

When This Page Helps

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.

How to Use the Inputs

Enter the total flat roof area.
Select the number of plies (3-ply, 4-ply, or 5-ply system).
Review the felt roll count, asphalt quantity, and gravel tonnage.
Add 10–15% overage for felt overlap waste and edge laps.
Coordinate delivery timing with the roofing crew's schedule.

Formula used

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

Example Calculation

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.

Tips & Best Practices

The standard application rate for interply mopping is 25 lbs of asphalt per 100 sq ft per ply.
The flood coat requires 60 lbs of asphalt per 100 sq ft for proper gravel embedment.
Gravel embedding in the flood coat protects the asphalt from UV degradation.
New BUR can be installed over one existing BUR layer if the existing roof is well-bonded.
For modified bitumen cap sheets, replace the flood coat + gravel with a torch-applied or self-adhered cap sheet.

Multi-Ply System Components

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 Application

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 Alternative

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.

Sources & Methodology

Last updated: February 8, 2026

Frequently Asked Questions

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.