What is Built Up Roofing? (BUR Definition + Installation Process)

What is Built-Up Roofing?

Modern built-up roofing systems are made of layers of asphalt with alternating layers of felt going in between. Built-up roofing systems are also commonly topped with a layer of aggregate such as stone or gravel for a final layer of protection. 

The alternating layers of asphalt (also called bitumen) and felt are applied directly to the roof deck insulation. Built-up roofing provides a continuous membrane over a roof’s entire expanse, making it exceptionally waterproof. As such, it is almost exclusively seen on low slope or flat roofing systems where water drainage would otherwise be an issue. 

Today, built-up roofing is one of the most viable options for modern low slope and flat roofing systems. The following post will discuss built-up roofing systems, including their pros, cons, and standard installation methods. 

Built-Up Roofing Definition

Built-up roofing is more or less what it sounds like: a roofing system that utilizes multiple material layers to protect the roof decking. However, there are even different kinds of built-up roofing. It’s important to understand that built-up roofing systems are a viable option for most low slope and flat roofs. 

BUR Roof Systems History

BUR systems have been around for a very long time. In fact, according to some sources, there is evidence that rudimentary built-up roofing systems have been around since the 1800s. The extensive history is a good thing because, in all that time, the materials, applications, and designs have only gotten more refined. 

Built-up roofing systems have changed over the years. In 2022, there are more options and features than before. For example, modern built-up roofing systems often incorporate a layer of rigid insulation to improve the energy efficiency of the roofing system and the entire building.

Built-up Rooftop

Modern BUR systems typically utilize a rigid insulation layer for increased energy efficiency and reduced energy costs. In addition, modern BUR systems have a wider variety of features since their initial emergence.

Components of A Built Up Roof

The essential elements of built-up roofing have remained the same in recent history. While modern innovations and new materials have been introduced, your basic built-up roofing system will consist of felt layers, asphalt layers, and surfacing material. 

  • Asphalt Layers: The asphalt layers we have already described are similar to the material used to make asphalt shingles. 
  • Felt Layers: The felt layers are usually composed of a fabric reinforced with fiberglass. They can also be infused with other organic materials. The felt layers bond with the asphalt layers through either a cold or hot bonding process. 
  • Surface Layers: The final layer is made up of surfacing material, typically some gravel or fine stones. The job of the surfacing layer is to make the roof safe to walk on and provide a rigid top layer of protection for the felt and asphalt layers underneath. 

What Are the Different Types of Built-Up Roofing?

While additional layers of insulation can be included in the construction of a built-up roofing system, you will generally have a few different types. They include:

Hot Built Up Roofing

The term “hot” refers to how the layers are installed. The asphalt layers are heated, so they liquefy and form a bond with the felt layers. In general, though, hot built-up roofing doesn’t perform as well as cold built-up roofing in extreme weather. The installation is generally more complex and weather-dependent. 

Cold Built Up Roofing

With cold built-up roofing insulation, the asphalt layers are bonded with the felt layers with an adhesive. As a result, cold built-up roofing is more weather-resistant than hot built-up roofing, and there are no toxic fumes to worry about. Cold installation can also be done in virtually any weather, provided it’s not raining or snowing. 

Ballasted Roofing

Ballasted built-up roofing is not installed using heat or any adhesive even. Instead, the layers of asphalt and felt are kept in place with a top layer of heavy stones. The top-layer stones can be up to 2 inches in diameter and provide enough weight to keep all the layers in place. 

Pros and Cons of Built-Up Roofing

Like most roofing materials, BUR systems present benefits and downsides, or more simply put, pros and cons. Check out the advantages and disadvantages of built-up roofing below:

Benefits of Built-Up Roofing (Pros):

  • Lifespan: A built-up roofing system can last as long as 40 years. That’s on the high end, though. A built-up roofing system’s average lifespan is more like 20-30 years.
  • Low Maintenance: Once your built-up roofing system is installed, it will need very little maintenance. 
  • Fire-Resistant: One of the best things about built-up roofing is that the top aggregate layer makes it very resistant to fire. 
  • Energy-Efficiency: Some built-up roofing systems can be outfitted with a reflective top layer that bounces UV rays off your building, keeping your building cooler in hot weather. 
  • Weather Protection: Since there are no breaks in the layers of a built-up roofing system, it provides excellent leak protection. 
  • Impact Resistant: The aggregate top layer of built-up roofing systems is very rugged and will not puncture easily. 

Downsides of Built-Up Roofing (Cons):

  • Lengthy Installation: Because of the multiple layers that need to be applied, built-up roofing installation can take a long time. 
  • Fumes from Hot Installation: If you opt for hot built-up roofing, you may be exposing your building to toxic fumes.
  • Ponding Water: Ponding water can build up on any flat roofing system, including built-up roofing. The roof can begin to sag if nothing is done about the ponding water. Sagging can lead to depression in that roof section, making it more likely that ponding will become an ongoing issue. 
  • Cost: Compared to other kinds of flat and low slope roofing systems, built-up roofing installation can be expensive. 

Fundamental Built-Up Roofing Repairs

Suppose your BUR system fails at some point in its expected lifespan. In that case, there are repair options that may extend its longevity. Check out some primary repair options for your built-up roof system in 2022.

Spray Foam Roof Coating

Spray foam roof coatings minimize tear-offs while helping to maintain a seamless membrane. In addition, SPF coatings include closed-cell insulation while increasing R-Value by nearly 7 per thickness inch. As a bonus, SPF-sprayed roofs become sustainable post-warranty.

Silicone Roof Coating

Smooth built-up roof systems may benefit from a silicone coating. The primary appeal of silicone coatings is its low cost compared to other repair or replacement options. A silicone coating ranges between $2-$4 per square foot, which essentially cuts your expenses by more than half.

However, you shouldn’t apply a silicone coating to a gravel BUR.

Built-Up Roofing Lifespan

A BUR roofing system typically lasts between 20-30 years though it may last up to 40 years with proper maintenance. Like every roofing system, longevity depends on variables like installation quality, climate, UV exposure, and maintenance regularity.

Bult-Up Roofing Challenges in 2022

Built-up roofs are challenging because of cracks, blisters, and ridges that emerge after exposure to the elements. In addition, unresolved gaps can split inside the membrane, a phenomenon known as alligatoring. 

Moisture infiltration can create blisters that become easily viewable on your roof system. Left untreated, these blisters can spread into other parts of the membrane. 

Similarly, thermal expansion can cause ridges and separated insulation. Replacing damaged insulation is critical in this instance.

Protecting Your BUR Membrane System in 2022

Regular inspections are the easiest way to protect your BUR roof membrane in 2022. Once roofers identify a small problem, you should address it ASAP rather than allowing the situation to linger and cause more extensive damage.

In addition, BUR systems should have reapplied UV protective layers every five years to maintain protection from damaging sunlight. However, an inspector should examine the seals and flashing before reapplication to ensure proper fitting. If cracks emerge, you’ll want to reseal them before applying the new UV protection layer. 

Similarly, damaged flashing must be replaced before reapplication.