Understanding Built-up Roofing

In this guide, we'll discuss...


Overview

Built-up roofs (BURs) have alternating layers of bitumen, like asphalt, and reinforcing fabrics, known as “felts."

BURs can be composed of two, three, four, or more layers. The greater number of layers, the more durable the roofing becomes.

Built-up roofing has been used in the commercial roofing industry since the mid-1800s. It’s a tried-and-true type of roofing that was the commercial roofing system of choice up until the 1970s when newer single-ply roofs hit the market, such as EPDM and TPO roofing.

Today’s options are more advanced than ever before. Modern bitumen materials contain added polymers designed to reduce fumes/odors, increase durability, and improve its waterproofing properties.


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Built-up Roof Materials and Installation

Materials

Built-up roofs are composed of layers of materials. The most common materials used in BUR systems are:

  • asphalt
  • coal tar
  • cold-applied adhesives

Ballasted asphalt is used regularly because it provides a phenomenal finish surface and acts as a fire retardant. Cold-applied systems are also popular because they do not carry toxic fumes and can be applied with a squeegee.

The materials being used for your project will depend on your roof's dimensions and how much you’re willing to spend.  

Installation Methods

The installation process consists of mopping hot bitumen material onto the surface of the roof, and felt is then applied on top. There are also cold-applied adhesive bitumen materials available, which eliminate the need for steaming hot bitumen. This is a safer option while also maintaining the same durability and waterproofing abilities of traditional hot bitumen.

LEARN MORE: The Complete Commercial Roofing Guide


Gravel BUR

Advantages & Disadvantages of Built-up Commercial Roofing

Advantages

  • Proven roofing technique – used for 100+ years in the U.S.
  • Resistant to UV rays, wind, hail, and rain
  • Affordable roofing over the long-term
  • 50+ year serviceable life is possible with proper installation and maintenance
  • White surface cap sheet available to help reduce AC energy needs during the summer; white sprays and white gravel are other “cool roof” options

Disadvantages

  • Steeper initial cost compared to most single-ply roofing systems
  • Although designed to last for decades, a poorly installed BUR system may only last 10 years or less
  • Heavier than other commercial options, especially ballasted built-up roofing

Eco-friendly Options

Built-up commercial roofing systems aren’t traditionally known as eco-friendly. However, recent advances in BUR materials and design are helping to point this type of low-slope roofing toward the “greener” end of the spectrum.

Many commercial buildings, particularly in the southern and western United States, are choosing to install a white cap sheet or lay white gravel on top of the BUR to help control cooling costs in the building. White roofing helps reflect as much as 80% of the light and UV rays from the sun.

Additionally, modern built-up roofs can outlast most single-ply roofing systems if installed correctly. A roof that lasts 30, 40, or 50 years is definitely an eco-friendly choice over opting for a roof that has to be replaced every 15 to 20 years.

Learn more about another eco-friendly roofing option: The Homeowner's Complete Metal Roofing Guide


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Cost of Built-up Commercial Roofing

The chart below depicts the average cost of installing a built-up roof on a 2,000 square foot rooftop in four major cities across the country. The exact price of your particular installation can vary depending upon the materials used and complexity of the project.

BUR Pricing in the U.S. for 2,000 sq. ft. Roof

City Material / Installation Cost
Chicago, IL $7,100 - $9,700
Atlanta, GA $6,000 - $8,000
New York City, NY $8,300 - $11,300
Los Angeles, CA $6,900 - $9,400

These prices reflect the average cost for materials and installation of a built-up commercial roof. However, various factors can affect the overall cost, including:

  • your geographic location
  • the complexity of the roof
  • how many layers are being installed

applying cement over flat roof

Built-up Commercial Roof Repairs and Maintenance

Maintenance

An annual maintenance plan is the key to maximizing the usable service life of a built-up roofing system.

BURs are nearly maintenance-free, but it’s recommended to keep the roof clear of tree limbs, leaves, and other foreign debris. It’s equally important to ensure there are no areas of the roof prone to collecting rainwater. Remove ponding water from the roof within 24 hours of noticing it, and call your roofing contractor to have the issue assessed.

Repairing a Built-Up Asphalt Roof

1) Prepare the Repair Area

The first step in repairing your BUR asphalt roof is to clean the surface of the damaged area. This ensures the patch adheres properly to the surface. If your roof is covered with gravel, remove the gravel at least 4” to 6” inches beyond the patch area. Use a brush to sweep away any loose dirt and dust.

2) Prime the Repair Area

Once the area is free of debris, apply a liquid asphalt primer to the surface. Applying primer is important because some BUR systems may be old and lack necessary oils to ensure a proper binding. The primer allows the patch to bind itself to the roof surface increasing long-term performance.

3) Cut the Fabric Patch

Once the primer has dried, cut two different layers of asphalt-impregnated cotton fabric. The patch should extend at least 3” past the desired area. The first layer should be 4” wide, and the second 6”.  

4) Cement the Patch

Apply a bed of asphalt flashing cement at a uniform rate over the patch area, extending just beyond the area where the fabric will be applied. Next, apply the 4” cotton fabric into the cement using a trowel. Repeat the steps with the 6” fabric patch, ensuring it extends over the first layer in all directions.

5) Apply a Finish Coat

Finally, apply a finish coat of asphalt mastic to the patch. If you cleared away gravel, return the gravel to the repaired area to conceal the repair and maintain a uniform appearance to the roof.


How to Find a Qualified BUR Contractor

Not all roofing contractors specialize in built-up roofs, but Hometown works with some of the best specialists in the business. We make it easy for customers to compare roofing contractors. You’ll have access to a variety of options and receive free quotes for your project.