Flat Roofs: Built-Up Roofs (BURs) and Modified Bitumen
Built-Up Roofs (BURs) consist of multiple plies of roof felts laminated together with bitumen. BUR material can consist of bitumen-saturated felt, coated felt, polyester felt or other fabrics. A surfacing is generally applied and can be asphalt, aggregate (gravel or slag), emulsion or a granule-surfaced cap sheet.
BUR is one of the oldest and most reliable ways of installing a new roof. It was first known as composition roofing and started in the 1840's. BURs come in two basic types - asphalt and coal tar - and three basic components – (1) the waterproofing component, (2) the reinforcing component, and (3) the surfacing component which is used to protect the other components from the elements. There are many different types of materials used in Built-Up Roofing. Some are:
- Base Sheets:
- Asphalt-Coated Organic Base Sheet – Perforated and Non-Perforated
- Asphalt-Coated Glass Fiber Base Sheet
- Asphalt-Coated Glass Fiber Venting Base Sheet, Types I and II
- Felts and Fabrics:
- Asphalt-Saturated Organic Felt – Type I ( aka No. 15) and Type II ( aka No. 30)
- Coal Tar-Saturated Organic Felt
- Smooth-Surfaced Asphalt Roll Roofing – Types I, II, III, and IV
- Asphalt- or Coal Tar-Saturated Cotton Fabrics
- Asphalt- or Coal Tar-Saturated Woven BUR Lap Fabrics
- Asphalt-Impregnated Glass Felt – Types III, IV, and VI
- Coal Tar-Impregnated Glass Felt – Type I
- Thermoplastic Fabrics for Built-Up Roofing – Types I, II, III, and IV
- Asphalt – Types I, II, III, and IV
- Coal Tar – Types I, II, and III
- Lap Cement – aka Cold-Applied Liquid Adhesive, aka Solvent Based (Cutback) Asphalt – Type I (Grades 1 and 2), and Types II and III
- Gravel or Slag
- Asphalt Roof Coatings – Asbestos and Non-Asbestos
- Aluminum-Pigmented Asphalt Roof Coatings – Non-Fibered, Asbestos Fibered, and Fibered without Asbestos
- Emulsified Asphalt – Fibered and Non-Fibered
- Mineral-Surfaced Asphalt Roll Roofing (Organic)
- Mineral-Surfaced Asphalt Roll Roofing (Glass Felt)
Other items not listed are flashing materials, mastics, caulking materials, fasteners, and roof insulations, to name a few.
The service life of a flat roof is dependent on many factors: geographical location & weather conditions, foot traffic, materials used, conditions under which the roof was installed, slope of roof, type of surfacing material, etc. Under ideal conditions, a 3-ply built-up roof should last at least fifteen years, a 4-ply should last at least 20 years, and a 5-ply should last at least 25 years. We have seen ten year old 4-ply roofs that needed to be replaced and twenty year old 3-ply roofs that were still functioning.
Modified Bitumen (MB) is asphalt that has had modifiers added to it to give it plastic or rubber-like properties. The most common types of modifiers being used are APP (Atactic Polypropylene) and SBS (Styrene Butadiene Styrene). 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.Surfacings for these roll materials consist of mineral granules, aluminum, copper, or an aggregate such as gravel or slag. Modified Bitumen roof systems consist of one, two, or three ply systems. The type of substrate will often determine the type of system being 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. West American Roofing recommends that all modified roofs be installed on slopes not less than 1/4" per horizontal foot in order to achieve positive drainage.
APP Modifieds: In order to create roofing grade asphalt, asphalt flux is air-blown at elevated temperatures which converts the flux to roofing grade asphalt. In the early 1970’s, the Italians, lacking the blowing equipment, were looking for a product that would convert asphalt flux into a usable roofing product. They discovered that if Atactic Polypropylene (APP) - a by-product of propylene polymerization - was added to asphalt then it gave the asphalt some plastic properties. They found that by adding about 30% of APP modifier, they could stretch the modified asphalt up to fifty percent of its original length before it would break. Next came the need to make it into a usable roll product. Some type of reinforcement would be needed. They looked into various reinforcement materials and decided on a polyester mat because polyester would accommodate the APP modified asphalt’s elongation properties whereas the more commonly used woven glass mats would not. The reinforcement material is dipped into the 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 was especially convenient for the smaller, more cut up roofs because less room and equipment is needed on site to torch-apply a membrane than is necessary for application using hot bitumen.
SBS Modifieds: While APP was being looked into in southern Europe, northern Europe was experimenting with a different type of modifier called Styrene Butadiene Styrene (SBS). The French and Germans found that if they added 10%-15% of SBS rubber to asphalt, the asphalt’s characteristics changed to those of the rubber additive. They learned that they could stretch the SBS modified asphalt 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 feet or 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.