What is a Built-Up Roof System?

What is a Built-Up Roof System?

May 30, 2019

Built-Up Roof system or BUR system is a common built-up Commercial Roofing system provided by Parsons Roofing Company. Bitumen is the first adhesive layer and roofing felt is installed in multiple plies over the base layer of a commercial roof. The top layer consists of stone, gravel, crushed slag, or other aggregates.

Typically, a built-up roof lasts anywhere between 15 and 30 years. BURs work well in warm climates since the BUR overlays are designed to cool the roof. Multiple layers also increase resistance to surface damage, which means they incur fewer built-up roofing repairs.

A built-up roof lasts anywhere between 15 and 30 years even in warm climates when installed correctly by Parsons Roofing Company, the Best Commercial Roofing Company.

Built-Up Roof System and A Few Options for a Commercial Roof

Built-up roofing as a concept has been around for a while.  The two main built-up roof types are modified bitumen and ballast roofs.

While the former type of roofing is installed onto the entire roof deck using adhesives, the latter system involves fastening the roofing membranes to the deck at different spots. Modified bitumen roofs have a mix of rubber and asphalt, which are finished with a top layer of gravel.

On the other hand, the top layer of ballast roofing is a mix of smooth river stone and larger gravel applied on a single-ply roof.

#1 Built Roof System Option – Modified Bitumen BUR

Typically, bitumen-built-up roofing systems have three or more component layers.

They consist of the base asphalt layer with modifiers, the ply sheets, and the top surface. Modified bitumen roofs use asphalt, which acts as primary waterproofing protection, but is modified with agents like synthetic rubber to create a uniform matrix.

This improves the puncture resistance and strength of the asphalt. It also enhances its physical properties like elasticity by increasing its resistance to permanent deformation during stretching under loads. 

The multiple layers of ply sheets can be of reinforced fabric, felt, fiberglass, polyester, or, in some cases, plastic core materials. These internal sheets offer tensile strength, weather resistance, and stability to the final finished roofing structure.


The top surface protects the lower layers from damage due to harsh sunlight and rain or debris. This layer also holds and releases heat, thereby keeping moisture and heat from damaging the underlying layers of your commercial built-up roof.

The internal sheets in Modified Bitumen BUR offer tensile strength, weather resistance, and stability to the final finished roofing structure.

Modified bitumen built-up roofing is of two types: Styrene-Butadiene-Styrene (SBS) and Atactic Polypropylene (APP). Each has its own advantages, and an experienced contractor can help you find the right roofing system.

  • SBS offers improved flexibility and increases its contraction and expansion capabilities.
  • APP can improve the stability of your roof over time.

While SBS is elastomeric bitumen and it can stretch, APP is a plastomeric bitumen that is made flexible with an elastic-plastic. However, APP roof covering is more UV-resistant.

#2 Built-Up Roof System Option – Ballast Roofing

A ballasted roof keeps the roof membrane separate from the decking material. That means it is not anchored or adhered to in any way. However, it is ballasted, generally with gravel. A ballasted roof has a loose-laid bottom-most sheet over the roof deck.

Multiple membranes are then added to this sheet and the top-most layer has large stones. Although the lower layers do not adhere to the roof structure, the heavy stones on the top layer keep them in place. To prevent excess movement of these layers, they are fastened at a few places on the deck. The material expands and contracts during hot and cold weather, which makes it highly durable.

Ballast roofing provides an aesthetically pleasing overall look. You can also use a ballast just for the walkway on the roof, which gives a firm surface to walk on and access the whole area in case of any repairs. However, individual ballast stones might come off during storms, which is very dangerous. For this reason, contractors are now using paving stones instead, which are more stable and do not move even in high winds. 

Ballast roofing provides an aesthetically pleasing overall look.

Ballast roofing is a fire-resistant, sustainable, and energy-efficient roofing system, as it absorbs heat but doesn’t let it pass through. The upkeep and BUR repairs are simple, as you can easily remove the ballast stones and fix the membrane.

What is a Built-Up Roof System Made of?

The bottom-most ply of the BUR system, also known as a “base sheet,” is fastened onto the roof deck or insulation to allow for vapor movement and prevent blisters (soft bubbles due to trapped moisture). Factory-made fully adhering base sheets are also available. These prevent membrane tearing if the insulation moves during a rainstorm.

Built-up roof systems are the most common commercial roof for commercial buildings.

The most used base sheet material is asphalt. Between these layers, there are roofing felts or ply sheets. These are reinforcing fabrics consisting of glass-fiber or organic mats that increase the durability of the roof and resistance to thermal shock and moisture.

For the surface layer, contractors apply aggregate material like gravel, mineral granules, or stone. In some cases, they may use mineral cap sheets or mop the surface with hot asphalt, elastomeric sealants, or aluminum roof coatings. These act as the main defenses against dampness and the elements and reflect UV radiation.

Advantages of a Commercial Built-Up Roof System
  • Long-Lasting – In some cases, the roof lifespan will reach 40 years without needing any major BUR repair.
  • Waterproof – The seamless and continuous surface of the BUR system makes it waterproof.
  • Low-Maintenance and Durable – Built-up roof systems are low-maintenance and can withstand heavy foot traffic.
  • Versatile – Built-up roof systems are able to be installed over any type of commercial roof deck.
  • Ultra-Violet Protection – These “cool roofing systems” can protect your commercial property from UV radiation.

Proper inspection and maintenance increase your roof’s life span.

Built-Up Roof System Repair and Commercial Roof Maintenance

Although BUR systems generally require routine maintenance, you should follow up with a proper inspection schedule. Typical repairs include:

  • A series of small cracks in the roof surface form due to the shrinking of asphalt. If it goes unnoticed, these cracks grow into splits and spread.
  • Soft bubbles form in the roof membrane as a result of trapped moisture. This is referred to as blistering. You need to identify the water source and correct these blisters as soon as possible.
  • Damages happen due to thermal expansion or when the underlying insulation separates from the roof deck. If unchecked, these can cause tears to the roofing materials.
  • No matter how minor the damages are, get your commercial built-up roof repairs completed as soon as possible. Avoid more severe losses by getting the best commercial roofing contractor to inspect your roof.
Contact Parsons Roofing Company to Get Your Roof Inspected and Avoid Major BUR Repairs

The team at Parsons Roofing Company of Atlanta specializes in handling all built-up roof repairs and maintenance. Prevent significant damages with timely inspections. Contact Us to request a roof inspection.

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