Pros, Cons and Common Defects of Built-Up Roofing Systems

As the name implies, a Built-Up Roofing system or Built-Up Roof (often simply abbreviated to BUR), is a roofing system which consists of alternating bitumen and fabric layers, installed on low-profile (flat) roofs. BUR membranes can be installed on top of rigid insulation and cover boards, or directly onto roof decks. They are constructed by laying alternative layers of reinforcing fabrics (“plies”) with layers of asphalt or bitumen. The number of plies in a roofing system (e.g. 2-ply vs. 4-ply) refers to the number of layers within the roofing membrane, and “multi-ply” is commonly used for roofing systems with several layers.

Bitumen is a naturally occurring oil-derivative. When heated with a gas torch or kettle the material can be used as an adhesive and it is melted, then poured or mop-applied onto the roof and ply and built up in layers. Instead of asphalt or bitumen, BUR systems can be adhered to the substrate using cold-applied adhesives, especially when using roll-applied SBS Modified Bitumen asphalt (detailed later). One of the older types roofing systems available, BUR systems have been used for many years in the USA and are renowned for their versatility and wide-ranging applications. This article will inform you of the pros and cons of BUR systems, and help you understand potential defects that can arise from poor installation, workmanship and/or neglect. It will help guide you through what membrane defects to look for when inspecting a roof and help guide you through repair and replacement options.

Rolled Bitumen – Is it a BUR System?

Technically, yes. In the 1960’s and beyond with advancing technology, roofing and chemical manufacturers began developing styrene-butadiene-styrene (SBS) modified bitumen membranes. These consist of a 2-ply system of a lower-layer flexible mesh ply, topped with a bitumen top layer. These rolled systems can be applied to insulation/cover decks or roof decks in a similar way to traditional BUR systems, with melted bitumen and heating of the roof-facing side of the membrane with a gas torch, or with cold-applied adhesives. Note: this system is not the same as rolls of bitumen asphalt felt, which are nailed to the roof deck/substrate and can sometimes be seen on roofs as a common low-cost repair.

Pros, Cons and Common Defects

Built-Up Roofing systems have a reputation for providing durable and versatile roofing coverings which require little maintenance. However, like all roof systems they have their pros and cons. Table 1-1 summarizes the typical pros and cons of BUR systems and the text below identifies some common defects and areas to watch out for.

Table 1-1 – Pros and Cons of Built-Up Roofing Systems

Pros Cons
  • Durable
  • Low maintenance
  • Suitable for use in many areas and weather conditions
  • Roofs can easily be overlaid
  • Long lifespans
  • Heavy compared to modern single-ply membranes
  • Installation flood coating is messy and can make future replacement more challenging
  • Identifying leaks/areas of water ingress can be difficult

Blisters and Tar Boils

A blister (or blisters) on a BUR system is indicative that moisture or air has become trapped between the ply and bitumen layers. As temperatures change, pressures within the blister(s) alter and the air expands and contracts resulting in a loss of adhesion between the membrane and the substrate. Cracking of the membrane can also occur which allows further water into the roofing system and building. Tar boils (also known as “blueberries” or “blackberries”) are small spherical tar deposits on the surface of a roof which occur when moisture vapor enters the roofing system, heats up, and pushes out the tar/bitumen from the consolidated membrane. To resolve these defects, the damaged areas must be cut back until dry areas of the roof are found, and the roofing system built back up. A large number of blisters or tar boils may be indicative that the roofing system has suffered extensive water ingress and needs to be fully replaced or overlaid.

Tar Boils on BUR System

Cracks and Open Joints

Cracks and open joints are a common area where water ingress can occur. Over time, the asphalt/bitumen dries out and deteriorates, especially when exposed to ultraviolet (UV) light from sunlight and cracks within the membrane can occur. Open joints are a natural weak spot, especially if sufficient overlaps of ply layers were not provided during installation. If cracks or open joints are in isolation on the roof, damaged areas can be cut back and replaced, or the roof overlaid in its entirety. Alternatively, if the roof is a SBS Modified Bitumen roof and does not contain an aggregate ballast (discussed further in “Deterioration of Ballast” below), the roof can be treated with a roof recovery system. Roof recovery systems typically comprise liquid-applied solar-reflective coatings, or more substantial mesh and liquid applied composite layers which are installed across the entire roof. Installed at the mid/end-point of a roof’s life, they are aimed to prolong the roof’s life prior to replacement.

Fish Mouth Open Seam

Deterioration of Ballast

Bitumen and asphalt compounds inherently suffer from deterioration from ultraviolet (UV) light from sunlight. Prolonged exposure to UV light causes bitumen and asphalt to dry out, crack and become brittle. To overcome this, traditional BUR systems are installed with a top layer of stone aggregate which protects the system from sunlight. Modified SBS systems typically don’t have this layer of aggregate installed on top of the membrane, but the rolls of asphalt are finished with a fine layer of crushed stone or aggregate which serves the same purpose. Due to weather and natural deterioration of the ballast material, it is important built-up roofs are periodically checked, and areas of missing or deteriorated stone ballast are replaced. For Modified SBS bitumen roofs, the condition of the top layer of crushed stone/aggregate should also be regularly checked and if worn away, missing or severely deteriorated; a roof recovery system, overlay, or full replacement of the roof should be considered.

Missing Ballast and Cracking to Cap Sheet

STOP – Safety Moment

Although commonly made from fabric materials, plies within roofs on BUR systems (including Modified SBS systems), have historically been manufactured from asbestos fibers. Although encapsulated within the bitumen and asphalt layers, if the material is cut or removed it can release asbestos fibers which are harmful to human health. Additionally, bitumen, asphalt and adhesive compounds are formed from petroleum/oil products which are harmful and release organic compounds and fumes, especially when heated. Finally, heating of asphalt/tar carries a burn and fire risk and roofing works are completed at heights where injury or death from falling from height can occur. We therefore recommend that roofing systems should generally be inspected, tested, removed and installed by competent trained roofing professionals.

Roof Warranties

Two main warranties exist relating to roofs – a material warranty which covers the roofing product, and a contractor warranty which covers the installation by a roofing contractor/business. Usually, both warranties are provided on completion of a roofing project. Typically, roofing materials (the product which is installed) are guaranteed by a roofing manufacturer for a substantial period (say 20 to 25 years) against defects caused during the manufacturing process. A contractor warranty is typically much shorter (usually 1-2 years) and covers installation-related defects. The contractor warranty should detail what items are covered, and activities that will void them. It is important to ensure that copies of the warranties are obtained, and that building inspections and maintenance activities do not invalidate any warranties in place.

Related Websites – National Roofing Contractors Association

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