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Modified Bitumen Roofing: A High Performance Option

No one cares about the roof… until water is dripping on your new computer.  A leaking roof is one of the leading causes of mold growth, inventory damage and destruction of building components such as walls, ceilings, finishes and furnishings not to mention computers.  While roof systems often account for less than 50 percent of the total building enclosure, defective roofing easily accounts for the lion’s share of building construction litigation.

Historically, built-up roofing (BUR) was the mainstay in low-slope roofing.  Many practitioners and owners were devoted to it for a variety of reasons, not the least of which were the durability and the redundancy of multiple layers.  Nevertheless, BUR has lost considerable market share due to costs, safety, environmental concerns, lack of skilled applicators and being just plain messy. 

A modern day alternative is modified bitumen roofing, also known as “modifieds,” or “mod bits.”  Modified bitumen roofing has evolved as a high performance option to built-up roofing.  The waterproofing component in both systems is asphalt. While the quality of a BUR system is highly dependent on the skill of the guy mopping the hot asphalt (a dangerous job), with modified bitumen roofing the asphalt is applied to the reinforcing mat in the factory resulting in a more consistent product.

The reason it is called “modified” is because the asphalt is modified with elastomeric polymers that add elasticity and durability to the sheet. Installation of these systems is very similar to BUR but with far fewer safety concerns and far less mess. Modifieds can be applied with hot asphalt but are often applied with adhesives or with the “torch down” method thus eliminating much of the mess and improving safety. Roofing contractors who have historically installed BUR are familiar with modified bitumen roof systems and have easily transitioned to those systems.

Many of Terracon’s clients prefer modifieds.  Mecklenburg County Government in Charlotte, NC has utilized modified bitumen roof systems on the majority of their facilities including office areas, television stations, gymnasiums, courthouses, detention centers and natatoria over the past 10 years.  Each case is unique, requiring the modified systems to be designed to accommodate the varying conditions such as high interior vapor drive for pools and gyms. Modified systems meet code requirements for most low slope applications including fire and wind uplift classifications.

Ease of Use

These facilities are almost always occupied during construction and the work proceeds without significant interruption to day-to-day operations. Mecklenburg County facilities have various roof-top equipment requiring routine maintenance. Modified systems can withstand routine maintenance foot traffic without impairing long-term performance. Walkway materials, supplied by the mod bit manufacturers can be easily applied for areas of heavier traffic. The systems are durable and require low maintenance, which is a significant advantage for the County. The fumes from the cold adhesive are manageable and typically not a problem for the County or the building occupants and are certainly far less than the fumes from the old BUR asphalt kettles.

Various roof systems are available and most have their place in the construction industry provided they are carefully selected and detailed for the conditions encountered.  Modified bitumen is a viable option where durability, redundancy and low maintenance are critical and should be given careful consideration.


Louis HallLou Hall is a senior engineering in our Charlotte, N.C., office.