Built-up roofing, also called BUR, is the most common roofing material used on low-slope roofs. It is composed of alternating layers of reinforcing fabric and bitumen (asphalt) and is finished with a top layer of aggregate, such as stone or gravel. BUR is preferred for low-slope, or “flat,” roofs because it creates a continuous sealed surface. By contrast, roof shingles are not sealed and require a fairly steep roof slope to shed water effectively.
Types of Built-Up Roofing
Built-up roofing is nothing new. The concept been around for over 100 years, although the material and its installation certainly have evolved during that time. Modern built-up products incorporate a rigid insulation layer for improved energy efficiency.
Most built-up roofing is comprised of three parts: bitumen material, ply sheets, and one or more surfacing materials. The bitumen can be either “hot,” meaning it is heated so that it liquefies during installation, or it can be “cold,” which is more like an adhesive and is not heated. Cold built-up roofing can be sprayed or applied with a squeegee. It does not give off toxic fumes during application and is not dependent on weather. It also offers better performance than hot built-up roofing.
The ply sheets of built-up roofing are special fabrics that are reinforced with fiberglass or organic materials. Each ply sheet layer is laid over hot or cold bitumen to bond it to the roof. Ply sheets are commonly produced in a standard width of about 36 inches.
Surfacing materials form the top layer and may consist of small stones or finer gravel, depending on the application. This layer provides a finished look and helps protect the layers below from sunlight and damage from flying or falling debris. It also makes the roof safe to walk on. One special type of built-up roofing, called ballasted roofing, uses large stones (about 1 1/2 to 2 1/2 inches in diameter) for the surface layer. With a ballasted system, the lower layers are not adhered or fastened to the roof structure, and the heavy surface layer keeps the roofing in place.
Built-Up Roofing Lifespan
Depending on the climate and the specific materials used, built-up roofing has an average life expectancy of 15 to 30 years, but some constructions can last up to 40 years. In general, built-up roofing tends to fare better in warmer climates than in cold regions. This lifespan makes built-up roofing comparable to composition (asphalt) shingles, which last between 15 and 30 years, depending on their quality.
Other roofing materials can last much longer. For example, standing seam metal roofing has a life expectancy of about 50 years, and slate roofing can hold up for 100 years or more.
Pros and Cons of Built-Up Roofing
Built-up roofs tend to provide excellent waterproofing and ultra-violet protection. Thanks to the aggregate top layer, they are also fire-resistant. Built-up roofing is generally low-maintenance and therefore costs little to maintain over its life.
On the downside, built-up roofing can be slow to install and, with the exception of cold bitumen processes, installation involves hazardous fumes. Overall, installation costs are relatively high, and some types of this roofing can be susceptible to wind and water damage.
Basic Repairs of Built-Up Roofing
As with all types of roofing, damage to built-up roofing should be repaired as soon as possible to prevent further damage to the roofing or the construction materials below the roofing. Several common problems can be remedied with relatively simple repairs.
- Open joints: To repair joints or seams that have separated, add some cement under the open seam and make sure to hold it down so it can adhere to the substrate. If this doesn't work, try cutting a large piece of felt and place it over the open joint. Secure it with nails, then cover the nails with roofing cement. Finally, spread some gravel over the cement and let it dry.
- Blisters: Small blisters can be easily repaired using a knife to cut the blister. Allow the spot to dry as much as possible. If the top layers are damp, keep cutting down until you've reached a dry layer. Remove the felts (plies) and install new felt over the area. Apply the asphalt, and cover it with chippings. You can also use liquid-applied coatings on top of the repaired area if chippings aren't available.
- Undulations or waves: Repair built-up roofing undulations by simply adding layers on top of the area to level it. Make sure the substrate is in good condition before making any repairs.
- Cracks: Cracks on the asphalt surface should be an easy fix. Start by cleaning the area of any gravel and debris. Apply a coat of asphalt cement over the area and install some roofing felt. Make sure there are at least 4 inches of overlap to guarantee that you're covering the area. Repeat this process again and apply a final coat of asphalt cement. Apply gravel on top of the cement.