What Is a Built Up Roof? Everything You Need to Know

Have you heard about built up roofs, but struggled to find good information about them? Maybe you have a built up roof (or suspect you have one) and it needs to be repaired or replaced, but you can't find any contractors with the experience (or willingness) to work on it. 

While built up roofing used to be the world's most popular flat roof system, it's relatively uncommon these days. Fortunately, at GIDEON, w've worked extensively with built up roofs, and in this article, we'll give you the complete run-down on this old (but excellent) roof system.

We'll talk about what built up roofing is, how it's installed, it's pros and cons, how much it costs, when it's a good choice, what you need to do to maintain it, and how it compares to other roof systems like TPO and spray foam. 

Let's dive in. 

Table of Contents

• What is Built Up Roofing?
• History and Evolution of Built Up Roofing
• The Components of a Built Up Roofing System
  • The base sheet
  • Bitumen (i.e. Asphalt)
  • Roofing felts or ply sheets
  • Surfacing materials
• The Built Up Roof Installation Process
• Pros and Cons of Built-Up Roofing
  • Advantages of BUR
  • Drawbacks of BUR
• When is Built-Up Roofing a Good Choice?
• Budget considerations
• Maintenance and Repair of BUR
• Comparing BUR to Other Roofing Options
  • BUR vs Single Ply Roofing
  • BUR vs Metal Roofing
  • BUR vs Modified Bitumen Roofing
  • BUR vs Spray Foam Roofing

What is Built Up Roofing?

Built Up Roofing (BUR) is a long-established roofing system used on low-slope or flat roofs. Its name comes from how it's installed, which involves layering, or "building up," multiple layers of material to create a solid, continuous roof covering or "membrane". These layers (called "plies) comprise alternating layers of bitumen (either asphalt or coal tar) and reinforcing fabrics. Once the roof has been "built up," it's topped with a layer of aggregate (usually gravel), which helps protect the underlying membrane from the elements, including hail and UV rays. It was once common practice to leave the roof without any aggregate topping, (known as a Smooth Tar Built-Up) , but this is rare nowadays.

History and Evolution of Built Up Roofing

BUR goes back to the 19th century, making it one of the oldest roofing methods still in use. Back then, coal tar and pitch (byproducts of the coal gasification process) were the primary elements of the waterproofing membrane. As technology advanced, asphalt (a byproduct of the oil refining process) began to replace coal tar, as it was more durable and easier to install. Over the last 150 years, improvements in materials and methods have continued to improve the strength, performance, and ease of installation of BUR systems.

The Components of a Built Up Roofing System

Built Up Roofing Systems have four primary components:

• The base sheet
• Asphalt
• Roofing felt
• Surfacing materials

1. The base sheet

The base sheet is the first layer applied to the roof deck and is often nailed or fully adhered directly to the roof deck. This layer provides the initial waterproof barrier and serves as the foundation for the rest of the BUR system. Base sheets can be made from various materials such as fiberglass, and are often saturated with asphalt for added strength and water resistance. Some base sheets have a self-adhesive backing, while others need to be adhered using asphalt or a special roofing adhesive.

2. Bitumen

Bitumen is the primary waterproofing material in a BUR system. Roofers mop it onto the roof while it's hot, until it forms a seamless membrane. There are two main types of bitumen used in BUR systems: asphalt and coal tar.

Asphalt is more common because of its superior weathering characteristics and ease of application. There are three different types of asphalt, depending on the application method: hot asphalt, cold asphalt, and torch-down.

Coal tar, on the other hand, is more resistant to water penetration and is used on roofs exposed to harsher weather conditions or in chemical environments. Coal tar is also self-healing, that is, abrasions or cuts will naturally repair themselves. 

The best coal tar roofs see 40-50 years of service life, and old-time roofers will tell you proudly that coal tar is "the best damn roof you could ever install." But these roofs are nasty for the applicators. Whether it's tearing them off or putting them on, installers may suffer chemical burns on their skin, eyes, or even lungs. As a result, coal tar has largely fallen out of favor.

3. Roofing felts or ply sheets

Ply sheets, also known as roofing felts, are the reinforcing fabrics sandwiched between the layers of bitumen. They give the built-up roof strength and stability and are typically made of glass-fiber or organic mats. These glass-fiber mats are preferred for their superior strength, resistance to moisture, and compatibility with asphalt. Organic mats, made from wood-based materials like paper or wood fiber, are less commonly used due to their susceptibility to moisture, but they're more flexible.

4. Surfacing materials

The surfacing material is the final layer of a BUR system, protecting the underlying layers from hail, mechanical damage, and UV radiation. The most common surfacing material is gravel, which is embedded in the top layer of asphalt while it's still hot.

However, there are other options, like mineral surface cap sheets, which are prefabricated sheets embedded with mineral granules, or reflective coatings such as aluminum or elastomeric coatings. These not only protect the roof but can also enhance its energy efficiency by reflecting sunlight and reducing heat absorption.

The Built Up Roof Installation Process

Overview of the installation process

The installation of a BUR system is a labor-intensive process that requires a high level of expertise. It begins with a thorough inspection and preparation of the underlying roof deck to ensure it's clean, dry, and structurally sound.

The first layer, the base sheet, is applied directly onto the roof deck. This is followed by alternating layers of asphalt and roofing felts. The asphalt, usually hot, is mopped onto the roof, and then the ply sheet is rolled out on top of it. This process is repeated, typically for three to five plies, to create a built-up, laminated roof membrane.

The final step is applying the surfacing material. If using gravel, the installers will embed into the final bitumen layer. If using a cap sheet or a coating, they are applied as per their specific installation instructions.

Pros and Cons of Built Up Roofing

Advantages of BUR

Built-Up Roofing is lauded for several key advantages:

• Firstly, it provides excellent durability. With multiple layers of bitumen and reinforcing fabrics, BUR systems offer robust protection against water, wind, and other weather elements.
• Secondly, BUR systems are known for their superior water resistance. Thanks to the seamless nature of the finished product, water intrusion is significantly minimized.
• Furthermore, BUR systems provide excellent UV protection. The top surfacing layer, whether it's gravel or a specialized coating, deflects harmful UV rays, which can extend the life of your roof.

BUR is such a tough and waterproof system that many of these roofs are still in service after 50 or even 100 years.

Drawbacks of BUR

Despite its strengths, there are also some challenges to consider with BUR.

One major drawback is the difficulties involved in installation. It's a time-consuming process that involves hot materials and specialized equipment, which is why it should only be undertaken by real roof mechanics highly trained to function as a team.

These roofs are smelly. The building's HVAC units sitting on top of the roof will draw the smell of the asphalt into the building, which can be unbearably intense. 

These roofs also require pumping the asphalt (heated up to 400 degrees Fahrenheit) up to the roof, and this hot asphalt frequently drips through the cracks and seams of the decking material. 

Installers often use propane gas torches with an open flame to adhere mineral cap sheets to the top layer of the membrane, which increases the risk of personal injury and building fire.

BUR systems can also put a lot of strain on the building structure due to the multiple layers and the gravel surfacing.This is an oft-overlooked issue when removing old asphalt roofs on old brick buildings. Most of the time, there are 2-3 separate roof layers weighing anywhere from 1200-2500 lbs per square. When this weight is removed, the building structure relaxes. This can cause walls, window ledges, and other parts of the building to crack.

Finally, while the multi-layered system offers durability, it also means that repairs can be more challenging and costly compared to some other roofing systems. Damage may not be apparent until it's penetrated several layers, and repairing it requires cutting through the built-up layers.

Also - while blemishes and leak-causing penetrations can be found easily on a Smooth BUR - on a BUR with gravel ballast, damage is difficult to see. Because of this, it's all too common for an inexperienced service tech to fail to fix a leak time and time again.

When is Built Up Roofing a Good Choice?

BUR is a good fit for most flat roofs. Its multiple layers make it extremely waterproof, and its toughness makes it very resistant to abrasions and foot-traffic damage. In fact, it isn't common to see "walk track" systems (special roof paths for foot traffic) installed on these roofs because foot traffic is unlikely to damage the roof as easily as it can on a Single-Ply membrane. If a screwdriver or other sharp object pierces the top layer, the layer below it will still stop water, unlike a TPO membrane, which has only one layer of protection. 

BUR works well on concrete substrates because they don't require fasteners (used on many single-ply systems), and they seamlessly seal and waterproof the concrete during the base sheet application. 

Many BUR roofs were built at the turn of the century and just after WWII and are still in use due to their strength and redundancy.

Budget considerations

These days, built up roofs are quite costly due to lack of skilled roof technicians, the high number of man-hours required for installation, the increased number of OSHA regulations, and the high cost of liability insurance. These factors have contributed to the rise of single ply systems like TPO and the fall of BUR. In many cases, BUR roof systems will increase your cost by up to $4-6 per square foot over a comparable single-ply system.

Maintenance and Repair of BUR

Regular inspection and maintenance are vital to prolonging the lifespan of your Built-Up Roof. Clear debris promptly, as it can lead to water ponding and, eventually, costly leaks. Trim overhanging tree branches to prevent damage from falling limbs and limit the amount of leaf litter on the roof.

Regular inspections can help you spot early signs of wear and tear. Look for signs of blistering, cracking, or other forms of surface damage. Keep an eye out for shifts or changes in the roof's surface, as they may indicate potential issues underneath.

Common repair methods and preventative measures

When repair is needed, it often involves patching small cracks or splits in the surface. In the case of more extensive damage, a professional may need to remove and replace entire sections of the roof.

If you have an older roof that is nearing the end of its life but is still in good condition, consider applying a protective coating to the roof's surface, like an elastomeric coating. This can stop the aging of the underlying roof in its tracks, extending its life by another 15-20 years.

Comparing BUR to Other Roofing Options

BUR vs Single Ply Roofing

When comparing built-up roofing (BUR) to single-ply roofing, there are several key differences that building owners and property managers need to take into account.

Single-ply roofing systems, which consist of sheets of rubber and other synthetics that are ballasted, mechanically attached, or glued to the insulation layer, are lightweight, flexible, and easy to install. This makes them a popular choice, particularly for roofs with many protrusions or unusual shapes.

However, the inherent simplicity of single-ply systems can also be a disadvantage. They consist of just one layer of protective membrane, making them potentially more susceptible to damage from sharp objects, foot traffic, or harsh weather.

On the other hand, the multi-layered structure of BUR provides a level of redundancy that can effectively guard against leaks. The aggregate top layer also offers excellent resistance to UV radiation, which can extend the roof's lifespan and potentially lead to energy savings.

BUR vs Metal Roofing

Metal roofing is another popular option for commercial buildings. Metal roofs are appreciated for their durability, fire resistance, and minimal maintenance needs. They can also be more environmentally friendly, as metal is recyclable, and metal roofs often reflect sunlight, reducing heat absorption and energy costs.

However, metal roofing systems have their own set of challenges. They are generally more expensive to install than BUR systems. Metal roofs can also be noisier in rain and hail, and they can dent if large hail or debris hits them.

BUR vs Modified Bitumen Roofing

Modified bitumen roofing is a type of asphalt-based system similar to BUR but with key differences. These systems typically include a base layer and cap sheet, similar to the multiple layers found in BUR, but unlike BUR, in which the bitumen is mopped on in multiple layers by hand, modified bitumen is factory-manufactured into rolled sheets, which are then applied to the roof using heat, adhesives, or mechanical fasteners.

While the fewer layers and larger, pre-manufactured sheets can make modified bitumen roofing easier and quicker to install than BUR, they don't offer quite the same level of redundancy. If a puncture or tear occurs in a modified bitumen roof, water is more likely to infiltrate it.

BUR vs Spray Foam Roofing

BUR is an inherently tough roof. It can take a lot of foot traffic and abuse. A spray foam roof is inherently easy to puncture and can only take light foot traffic and little to no abuse - unless a more robust elastomeric coating is applied, in which case it can withstand 4-inch hail. 

Both roofs can be repaired pretty cheaply. Both systems also require highly skilled technicians to install the materials and assemblies properly, which is the reason neither system carries a large market share. Both roof systems can also be coated and recoated to extend the useful service life.

Overall, spray foam roofing comes out ahead due to its seamless monolithic properties, ability to self-flash walls and protrusions, and insulating capabilities.

The Upshot

"If ain't broke, don't fix it." BURs have been installed for 150 years because they work. They work so well that "flat roofs", which were extremely uncommon prior to the industrial revolution, became the standard for commercial buildings. Their multiple plies have kept the largest and flattest of roofs high and dry for many, many years, and they may do the same for yours. 

If you have a built up roof that needs maintenance or replacement, or if you're interested in installing one on your building, give GIDEON a call. In a free 30-minute consultation, our nationally-recognized roofing experts can provide you with customized recommendations for your project.