We commonly run into opportunities to install Modified Bitumen and Built-Up roof systems where the facility requires high traffic areas and especially when connecting to smaller roof areas with a high density of service equipment. The flexible nature of the “modifiers” allow owners to select a roof that is a thicker product and is more flexible to building movement and metal-to-membrane interfaces than a BUR system.

There are two primary categories of Modified roofs: APP (Atactic Polypropylene) and SBS (Styrene Butadiene Styrene).

APP Modifieds

The primary components of APP Modified roofing materials are a polyester mat because polyester accommodates the APP modified asphalt’s elongation properties. The reinforcement material is dipped into a hot modified bitumen mix, then goes through a rolling cylinder, cooled, and then wound into a roll. APP membranes are applied using a torch. The back of the sheet has extra asphalt on it which, when heated, bonds to the substrate. This is especially convenient for the smaller, more cut up roof because less room and equipment is needed on site to torch-apply a membrane than is necessary for application using hot bitumen.

SBS Modifieds

Early on is was determined that the SBS modified asphalt could be stretched up to six times its original length and that, unlike the APP, it would return to its original size when allowed to relax. There are a wide range of reinforcements used in SBS roofing materials. These include fiberglass or polyester mats and scrims, or combinations of both. The fiberglass mats range in weight from 1.0 to 2.5 pounds per 100 square feetor around 50 to 125 grams per square meter. Polyester reinforcements range in weight from 3.5 to 5.0 pounds per 100 square feet or 170 to 250 grams per square meter. The type of reinforcement used depends on the material’s performance requirements. SBS membranes can be hot asphalt applied, torch applied, or cold process applied.

What is the general nature of the materials used?

Rolls of modified bitumen membrane come in widths of 36″ (0.9 m) to approximately 39″ (1 m) and cover an area of approximately 100 square feet to 112 square feet per roll. Surfacings for these roll materials consist of a smooth surface, or mineral granules, aluminum, copper, or an aggregate such as gravel or slag that is set in hot asphalt.

Modified Bitumen roof systems consist of one, two, or three-ply systems. The type of substrate will often determine the type of system to be installed. Modified membranes can also be installed in conjunction with built-up roof materials (such as multiple plies of fiberglass felt) to form a “hybrid” roof system. Modifieds have proven performance on residential, commercial, and industrial applications. Often there are a few challenges with Mod Bit systems which might include blistering, limited roof traffic, difficulty detecting leaks, longevity and wear, poor performance in ponding water, and no scuff or tear resistant qualities as is more common with other membranes.

What are the main differences between a traditional BUR and a modified system?

Both are layered systems; layers (plies) of roofing materials stacked on top of the other with asphalt between each layer- hence the name “built-up roofing systems.”

Most types of roofing systems are constructed on site under varying circumstances by crews of varying skills. One of the major benefits claimed for BUR systems is the redundancy these multiple layers provide which absorb the differences of installation, climatic conditions and crew skill-sets to produce a time-proven roofing system. When a BUR system is installed, it can be assembled over many types of roof decks. Concrete, metal or plywood are the most common roof decking types.  In many instances, insulation or other construction boards are laid over the deck first and the roofing is installed over that substrate. If the bottom ply, or layer of either system, is attached directly over the structural deck, it is usually mechanically fastened to the deck. The exception is structural concrete.  This is done to divorce the bottom layer of roofing from any movement of the building caused by building settling or structure.  If insulation or other construction boards are applied to the deck first, they are mechanically fastened and the BUR system can be fully adhered to the substrate.


The traditional Built-Up Roofing system (BUR)

The “traditional” Built-Up roofing system has been around for a long time. The term “roofing” invokes visions of dirty men mopping hot asphalt high up on roof-tops. Indeed, this is what the application of a “traditional” roof consists of. The ply sheets used were often made from organic fibers – including asbestos – and saturated with asphalt. These old roofs lasted a long time, due in part to the asbestos containing felts and a better grade of asphalt. The use of asbestos was banned in the late 1970s and technology has provided greater uses for the asphalt residue of distilled crude oil. Today’s asphalt is basically the “bottom of the barrel” – literally – with diminished properties compared to asphalts in the past.

The ply sheets used in today’s standard built-up systems usually have a tensile load carrying fabric of fiberglass sandwiched between the layers of asphalt which form the upper and lower surface of each ply. Typically, the minimum number of layers in a traditional built-up roof system is three (four layer systems are more prevalent and are preferred), whereas a modified system can consist of two or more layers.

Standard roofing plies are typically stiffer than the modified materials. This means the transition from, say, a horizontal plane to a vertical plane has to be mitigated by installing a cant strip in the junction. This is generally a strip with one face at a 45 degree angle so the transition is in two 45 degree steps. This is preferred over a single 90 degree angle which has the potential to crack along that fold line as it ages.

Asphalt does not like being exposed to UV.  Therefore, the surface of the installed system needs to be protected from these destructive rays.  This is often accomplished by embedding granules into the surface. This is called a cap sheet and is done during production of the sheet at the factory. However, reflective coatings or applying larger gravel into a flood coat of asphalt will also perform the same protective function.