Built-up roofs (BURs)
This type of roof consists of bitumen (asphalt or coal-tar pitch) sandwiched between two to five layers of roofing felts and is usually covered with a mineral aggregate embedded in the top surface. The bitumen is the waterproofing agent, stabilized and reinforced by the roofing felts. The felts restrain the bitumen from flowing in hot weather and help resist cracking in cold weather. The aggregate surfacing generally consists of gravel, slag, or crushed rock. Its purpose is to protect the bitumen from the damaging effects of the sun’s infrared and ultraviolet rays. These rays, through a combination of heat and photochemical oxidation, accelerate the aging of bitumen, resulting in premature brittleness and cracking. Depending on the number of layers in the builtup roof and the quality of construction, the projected life can vary from ten to twenty years.
An aggregate-covered built-up roof is difficult to inspect, principally because of the mineral-aggregate topping. Nevertheless, look for telltale signs of past, current, and potential problems. Walk around the roof systematically so that you cover the entire roof area. Look for patched areas, cracking, blistering, surface erosion, alligatoring, and wrinkling. In particular, look for cracks at the joints of roof projections and roof-mounted structures.
Usually, this type of roof has a flanged metallic strip along the perimeter, a gravel stop. It provides a finished edge for the built-up roofing and also prevents loose aggregates from washing off the roof. Look for cracks in the joint between the gravel stop and the roof. All cracks in the roof cover should be sealed. Record the location of any cracked or patched areas on your worksheet so that you can check the ceiling of the rooms below these areas for water stains.
Not all builtup roofs have a top surface covered with aggregates. A smooth-surface roof treatment has a weathering surface that is an asphalt-saturated roofing felt. Since the top surface is exposed, defective or deteriorated sections are visible. To maintain the weathering surface in reasonably good condition, the surface should be covered with a protective coating such as an asphalt-based emulsion every three to six years.
Incidentally, when walking on the roof, be careful not to step on any blisters—weak spots in the roof covering, usually the result of air or water vapor being trapped between the layers of roofing felts. Depending on a blister’s brittleness, the weight of someone walking on the blister might cause the roofing felt to crack.
In the 1970s, the notion that a single ply of rubber, plastic, or modified bitumen material could provide the same weather protection as a built-up roof gained acceptance. Initially, single-ply membranes were used on commercial and apartment buildings; today, they are also used on residential structures with flat roof decks.
Several methods are used for installing a single-ply membrane. The membrane can be loose-laid over the roof deck and attached only at the perimeters. It is then held in place everywhere else by the weight of gravel or concrete pavers placed on top. Another method is to adhere the membrane to the roof deck fully with a self-adhesive or an adhesive applied at the jobsite. Single-plies can also be installed as a partially adhered system by using mechanical fasteners or applying an adhesive at predetermined intervals.
When inspecting a single-ply roof membrane, look specifically for open joints at seams that are not properly adhered; check for membrane erosion and punctures; and look for cracks, splits, and open sections around the perimeter walls and roof projections. As with a BUR, if the membrane is covered with gravel or paving blocks, a visual inspection of the weathering surface is not practical. In this case, pay particular attention to the condition of the ceilings in the rooms directly below the roof.
This type of covering is often found on older structures in urban areas and over porch roofs. It is also found on pitched roofs of low-cost homes and outbuildings. It consists basically of an asphalt-saturated roofing felt applied directly over the roof deck and provides only single- or two-ply coverage. Quite often, the outer surface is coated with hot tar to seal joints and small cracks. Roll roofing should be inspected for blistered, cracked, eroded, and torn sections. Be careful not to walk on blistered sections. Sometimes the lapped joints between the strips curl and lift, making the joints vulnerable to water seepage. Check the joints between the roof and the parapet wall and other roof projections. These joints are vulnerable to cracking and periodically require resealing. (See FIG. 2-10.) If any of the above items are noted, they should be recorded on your worksheet, since they indicate the need for some maintenance.
Sheet-metal roof coverings are usually made of galvanized iron (sheet iron coated with zinc), terne metal (steel-coated with a mixture of lead and tin), or copper. A galvanized roof with a heavy zinc coating will last many years without painting. However, a lightly galvanized roof will rust and must be kept well painted. A terne metal roof, if kept well painted, can last for over forty years. A copper roof will also last for many years and does not require painting. For the most part, sheet metal is no longer used to cover the main roof on residential structures. It is used to cover porches or other exterior side projections such as sunrooms or garages. Occasionally, a sheet-metal roof covering might be found on the main roof of a Victorian structure. When you are inspecting a sheet-metal roof, look for cracks or open joints at the soldered seams; because of the large amount of expansion that takes place, the seam is a vulnerable joint for leakage. Also, look for exposed corroding sections of metal; they must be scraped and coated with paint or a bituminous compound such as asphalt cement. In some cases, problem conditions that have occurred over the years have necessitated coating the entire roof with a bituminous compound as a means of sealing corroded areas. Be aware that this type of roof coating requires periodic maintenance.