The application of hot asphalt to built-up roofs has traditionally been done with cotton mops. Old timers prided themselves in their selection of cotton and their skill at making their own mops. In the old days cotton was purchased by the hank, bundled together in large bales. Today cotton may be purchased either as ready-made mops or in the traditional hank to be made into mops on “rainy days.” Owens Corning’s development of fiberglass created the possibility of substituting fiberglass in place of cotton for hot mopping roof decks. The first use of fiberglass for hot mopping a roof was implemented shortly after the second World War. The immediate benefit of fiberglass as a roofing mop is its longer life due to its heat resistant qualities.
The acceptance of fiberglass roofing mops has been slow; today however, fiberglass mops are used on one-half of all roofs hot mopped in United States. The change from cotton to fiberglass by roofers is occurring more rapidly today and forecasts predict that within another decade most roofers will have been exposed to fiberglass roofing mops and will make the change from cotton to fiberglass. The following information will examine both cotton and fiberglass roofing mops and will present information that will enable one to see the benefits of using fiberglass roofing mops.
Cotton is an organic fiber that is tough and absorbent. The use of cotton as a kitchen mop easily demonstrates these qualities. For roofing, a 2 1/2 lb. cotton mop is the average. Cotton mops, however, do range in size from 1 1/2 to 5 lbs. The durability of a cotton hot mop is solely dependent upon the quality of the cotton and the temperature of the asphalt. Cotton varies from very soft to hard; the latter more durable in hot asphalt.
A major portion of hard cotton in the past was scrap cotton obtained from the tire cord manufacturers. Today, there is little demand for cotton tire cord; therefore, there is little hard cotton scrap for roofing mops. This greatly increases the cost of good hard cotton. Softer cotton is generally what the roofer must use.
Most of the cotton roof mops today are made from scrap from the garment industry. For instance, a blue cotton roofing mop is scrap from the blue denim clothing industry. To make the situation even worse, the clothing industry is using more and more cotton blended with synthetic fibers. When this material is used for hot mopping it burns faster than pure cotton would.
After a short time in hot asphalt, cotton starts to char and burn. The cotton strands will separate near the top of the mop, leaving long and undesirable strings of cotton on the roof deck. A roofer may use 2 to 5 cotton mops per day, per hot crew, depending upon the quality of the cotton and the temperature of the asphalt.
The cost and availability of cotton for roofing mops is dependent upon the quality of the yearly harvest and the demands of the different major cotton markets.
Fiberglass is a non-organic fiber which has high heat resistant qualities and rather low abrasiveness and absorbent qualities. The use of fiberglass as a kitchen mop as compared to a cotton mop would be very impractical.
WEARING… As a roofing mop, however, fiberglass must wear out rather than burn up as does a cotton mop. This enables a roofer to re-use a fiberglass mop for several days. The length of time a fiberglass mop can be used depends upon the surface being roofed and the type of fiberglass material used in the mop. Since fiberglass has low abrasiveness, rough surfaces tend to increase wear more so than smoother surfaces. Fiberglass wears at the end of the strand, becoming shorter until there is not enough length for good pick up or spread. The more heavier plied fiberglass wears longer than lighter plied yarn or roving.
PICK-UP… Since fiberglass is non-absorbent, asphalt is retained within the mop through surface tension. Because the heavier plied strands have less surface area pound for pound than lighter plied yarns and roving, pick up and wear are opposed to each other. Heavier plied yarns either sacrifice pickup or require more strands and consequently more weight to match the pickup of lighter plied strands. On the other hand, lighter plied yarns and roving, because of their increased surface area, have high pickup but wear out faster. A practical solution to this problem is to mix these materials to reduce wear and increase pickup as much as possible.
PICK UP…Can be further increased if the fiberglass strands are spaced apart so that there is room for the asphalt to bridge from strand to strand. This principle can be easily observed through an examination of a paint brush. The bristles are spaced so that paint is suspended in between the rows of bristles.
FLOW…Because fiberglass roofing mops are non-absorbent, they allow asphalt to flow off the strands easier than cotton. This coupled with fiberglass’ ability to conduct heat provides a more uniform flow and distribution of asphalt on the roof deck. Fiberglass wears off, gets shorter.