Flat or low-slope roofs come with a different set of needs and considerations than higher pitched roofs. This common roof type for commercial or multi-story buildings has limited run-off qualities when compared to sloped roofing, which makes waterproof protection a priority. The most common type of flat roof construction methods used to be the built-up roof, often referred to as BUR.
If you have an older flat roof building, it was likely constructed using this method. If you need to repair it, understanding how build-up roofing works is important. If you need to replace it, there are several new flat roof systems that can be used, but modern BUR systems can be an option too.
Let’s take a deeper dive into what goes into a built-up roof system to give you a better understanding of this type of flat roof construction.
This article covers:
- A quick history of built-up roofs
- The composition of built-up roofs
- Built-up roof installation
- The benefits of a built-up roof
- The challenges of a built-up roof
- Built-up roof maintenance tips
Built-up Roof Systems: A Quick History
Built-up roofs have been around since the 19th Century, but as can be imagined, modern construction materials and methods have altered how this tried-and-true flat roof option is built today.
When first introduced, BUR roofing used built up layers of felt soaked in coal or pine tar and held down with black pitch and layered in with gravel. These built-up layers is where BUR gets its name. The use of these materials is also why a BUR roof is sometimes referred to as a gravel roof or a tar roof. The process formed a tight and redundant waterproof seal on flat roof commercial buildings. Later the process was modified with the introduction of duck cloth that was infused with asphalt to form the many built-up layers. Since that time though, the built-up layers have incorporated the use of modern manufacturing materials and adhesives to achieve the necessary waterproof protection with less weight on the buildings and less hazard to construction workers.
What Is a Built-up Roof System?
As the name suggests, BUR is a built-up or multi-layered roofing system that is composed of layers of roofing material that is fused onto the surface of a flat or low-pitch roof. The layers form a continuous roof covering, sometimes called a membrane ,that creates a watertight seal between the top of the roof and the interior of the building, protecting it from the elements.
This multi-layered approach creates what is known as redundancy, meaning that if one layer of the roof wears away there are many more layers beneath it to continue offering waterproof protection. This redundancy gives you time to repair the worn areas of the roof without causing property damage.
The Composition of Built-up Roofs
The layers or piles in built-up roofs are made of asphalt and reinforced fabrics or roofing felts. Asphalt is a petroleum-based aggregate material often used in roofing or road pavement. It is made of crushed stone, gravel, sand, and bitumen, which is a sticky black petroleum by-product that binds the materials together. It is a durable product that can withstand wear and tear from extreme temperatures, wind, storms, ice, snow, hail, and even fire.
The roof is “built-up” by alternating layers of each of these materials that are fused onto the roof deck. This built-up process starts with a base sheet that is often composed of fiberglass saturated with asphalt. It is adhered to the roof deck with either a self-adhesive backing, a specially made roofing adhesive, or with hot asphalt. This base sheet provides a waterproof foundation for the layering system.
Once the base sheet is adhered to the roofing deck a layer of asphalt is mopped onto the roof to form a seamless roofing membrane across the entire roof expanse. This asphalt layer comes in three different types, hot asphalt, cold asphalt, and torch-down. Each of these asphalt types comes with its own application method.
The next layer of a built-up roof is the roofing felt or ply sheet layer. These reinforced fabric layers are made of glass-fiber or organic mats. Glass-fiber roofing felts are made of glass fibers that are either woven or bonded together. Organic mats are made of wood-based materials, such as paper or wood fiber. Glass-fiber ply sheets are stronger, more moisture resistant, and work easier with asphalt than organic mats, but organic mats offer more flexibility. All and all, glass-fiber is more commonly used for BUR roofs.
The top of the built-up layers is covered in a final surfacing layer. This is composed of an aggregate material, most often gravel, to add an extra layer of protection on top of the roofing membrane. This layer is added on top of hot asphalt so it embeds into the roof. A much more uncommon practice is smooth BUR, which leaves this top surfacing layer off.
Other surface layers options include prefabricated sheets composed of mineral granules, or reflective coatings often made of aluminum.
Built-up Roof Installation
There are a variety of built-up roof installation methods, even though the basic construction is the same. In any case, it is a labor-intensive process that requires specific expertise to execute both soundly and safely.
All methods start with a roof inspection of the roof’s underlying structure. Starting with a structurally sound roof is imperative to the success of any roofing project. Once the inspection is complete and the roof is deemed clean, dry, and sound, then the base sheet can be applied, followed by three to five alternating layers of asphalt and roofing felt, and then the surface layer is applied on top of the final asphalt layer.
Built-up Roofs: The Benefits
Some of the advantages of a built-up roof include:
- Durability
- Water-resistance
- UV protection
Durability
The redundancy that is embedded into the composition of BUR roofing gives a built-up roof longevity. These layers are strong, made of durable materials, and can often last over 50 years. The multiple layers provide excellent weather resistance, including extreme temperature changes, ice, snow, hail, high winds, and harsh storms.
Water-resistance
Built-up roof systems are known for their excellent water-resistance ability. The seamless membrane achieved through the built-up process provides a tight, water-proof seal that creates a superior barrier to water intrusion and leaks.
UV Protection
The top surface layer on built-up roofs provides high-quality UV protection by deflecting harmful UV rays. This deflection can extend the roof’s lifespan, and provide energy efficiency properties too.
Built-up Roofs: The Challenges
As all construction products and methods have both pros and cons, so does the built-up roof. Some of the disadvantages of a built-up roof include:
- Installation
- Smell
- Weight
- Repair costs
Complicated and Difficult Installation
A built-up roof is difficult and time-consuming to install. It requires professional expertise in this specific roofing method as well as special equipment that must be hauled up to the roof. It also requires the use of hot materials, heated to 400 degrees Fahrenheit, that must be pumped up to the roof. It also requires the use of gas torches and open flames, increasing the risk of injury or fire. The installation needs to be planned and executed by professional roofers specifically trained and practiced in the method.
This difficult installation process can make a built-up roof a more expensive flat roof option. It is labor intensive and often dangerous, and the special roofing expertise required for installation does not come cheaply. This all adds to the built-up roof cost.
Intense Smell
Built-up roofs smell of asphalt and tar, which can be acrid and petroleum-like. This smell can be strong and irritating. It can also linger. When a building’s HVAC system is in place on top of the roof, this smell can also permeate into the building’s interior, increasing the irritation.
Heavy Weight
The multiple layers that go into built-up roof systems make them heavy. These layers can weigh between 1200-2500 pounds per square. This can put strain on a building’s structural integrity, especially in the removal process, due to settling. It is something to look out for if you are removing a build-up roof.
Repair Costs
Everything that makes built-up roof installation costly makes repairs costly too. The redundancy inherent in the BUR system can make repairs challenging, requiring cutting through several layers to address damage or other integrity issues in the membrane. Damage can also be difficult to spot, which makes regular maintenance and inspection imperative.
Built-up Roofs: Maintenance Tips
The first rule of all roofing maintenance is to plan for regular inspections. It is the only way to catch damage and problems early and extend your roof’s lifespan. Beyond that, make sure to trim overhanging branches, look for early signs of damage, especially blistering, cracking, splitting, or shifting on the roof’s surface, and keep up on small cracks or splits with patching.
Final Thoughts on the Built-up Roof
Whether you are looking to repair or replace your built-up roof, this long-standing flat roof option offers up both pros and cons. If you suspect a problem, the best thing to do is get a proper inspection and hire a professional roofer who specializes in built-up roof installation.
