For over 100-years, Built-up Roofing has been used in the U.S However, it is often referred to by another name–-tar and gravel roofing. Unlike the single-ply roofing that is so popular today, this form of roofing is created by alternating layers of bitumen and fabrics that come together to create a “membrane.” Then it is finished with an aggregate layer or coating. BUR typically starts with a rigid insulation layer, and then there are felts, fabrics, or mats that are saturated with bitumen, most commonly asphalt (which works as a better fire retardant than the others), cold-applied adhesive, or coal tar.
When you see roofers spreading a gooey black substance on a roof with a mop or mechanical spreader, you’ve probably been watching them create a built-up roof. The finish of a built-up roof often resembles a rocky beach because the gravel, small rocks, and slag are mixed in with the asphalt; these roofs can also be smooth. Typically, BUR roofing is used on flat or low-slope roofs and is seen on many school buildings and industrial structures, and it creates a continuous sealed surface. By contrast, roof shingles are not sealed which is why they require fairly steep roof slope to shed water effectively.
Built-up roofing systems have many options including the types of mats that reinforce the ply sheets and the type of bitumen used. These materials have an impact on the cost of the project but the decision of what types of varieties are not all decided by budget. Hot asphalt, for example, can be quite noxious and does not make a good solution for environmentally friendly projects.
Types of Built-Up Roofing
BUR may have been around for over 100 years, but the material and its installation certainly has evolved during that time. For example. modern built-up roofing products use a rigid insulation layer for improved energy efficiency.
Most built-up roofing is made up of three parts: bitumen material, ply sheets, and one or more surfacing materials. The bitumen layer can be either “hot,” meaning it is heated so that it liquefies, or it can be “cold,” which means it acts more like an adhesive and is not heated. Cold built-up roofing can be sprayed or applied with a brush or squeegee. It does not give off toxic fumes during application and does not require special weather conditions for installation. It also provides better performance than hot built-up roofing.
The ply sheets of built-up roofing include special fabrics that are reinforced with fiberglass or organic materials. Each sheet layer is laid over the bitumen to bond it to the roof. Most ply sheets are produced in a standard width of approximately 36 inches.
Surfacing materials form the top layer usually are made up of small stones or fine gravel, depending on the application. This layer gives a finished look and helps protect the layers below from UV rays and damage from debris. This also makes the roof safe to walk on. There is a special type of BUR called ballasted roofing, which uses large stones (usually 1.5-2.5 inches in diameter) for the surface layer. With a ballasted system, the lower layers are not fastened to the roof structure, and only the heavy surface layer keeps the roofing in place.
Lifespan of Built-Up Roofing
Depending on the climate and weather, and the specific materials used, BUR has an average life span of 15 to 30 years, but some constructions can last up to 40 years. In general, built-up roofing tends to hold up better in warmer climates. This lifespan makes built-up roofing comparable to composition (asphalt) shingles, which can last between 15 and 30 years, depending on their quality and the installation.
Pros and Cons of Built-Up Roofing
Built-up roofs offer excellent waterproofing and ultra-violet protection. Because of the aggregate top layer, they are also fire-resistant. BUR is generally low-maintenance and therefore keeps costs down over its lifespan.
The cons are mostly centered around the installation. Built-up roofing takes a lot of time to install and, if you are not using cold bitumen processes, installation involves hazardous fumes. Overall, installation costs are relatively high, and there are certain types of this roofing can be susceptible to wind and water damage.
Built-Up Roofing Basic Repairs
As with all types of roofing, damage to built-up roofing should be repaired as soon as possible by a roofing contractor to prevent additional damage to the roofing or your home. Several common problems can be remedied with relatively simple repairs.
• Open joints: To repair joints or seams that have separated, use cement under the open seam and make sure to hold in place so it can adhere to the substrate. If this doesn’t work, you can cut a large piece of felt and place it over the open joint. Secure it with nails, then use roofing cement to cover the nails. Finally, spread small stones or fine gravel over the cement and let it dry.
• Blisters: Small blisters can be easily repaired using a knife. Cut the blister and allow the spot to dry as much as possible. If the top layers are damp, continue cutting down until you’ve reached a dry layer. Remove the felts and install new felt over the area, apply the asphalt, and cover it with chippings. Liquid-applied coatings can also be used on top of the repaired area if chippings aren’t available.
• Undulations or waves: Repair built-up roofing undulations by simply creating additional layers on top of the area to level it. Be sure the substrate is in good condition before making any repairs.
• Cracks: Cracks on the asphalt surface are often an easy fix. Start by cleaning the area of and then apply a coat of asphalt cement over the area and install roofing felt. Be sure there are at least 4 inches of overlap to guarantee that you’re covering the area. Repeat this process and apply a final coat of asphalt cement. Apply fine gravel on top of the cement.
Built-up roofing is a time-tested and true roofing method, especially with all the advances in technology that make it an even more effective and efficient solution for building structures. It typically requires little maintenance, and built-up roofs are easily repaired with conventional, time-proven materials, resulting in extended service life.