repairs because asphalt and coal-tar pitch are not compatible and contact between the two should be avoided. If you are in doubt, perform a volubility test.

The SOLUBILITY TEST is performed by pouring white gasoline into a container to which a small amount of unknown bituminous material is added. The amount that will stick to the head of a nail will be sufficient. The mixture is then agitated to determine the volubility of the unknown material. If the material mixes readily, giving a homogeneous mixture, it is an asphalt cement. If a mixture with stringy particles in suspension results, it is a tar, since tars are insoluble. If the unknown material is not readily soluble but forms black globules (balls), it is an asphalt emulsion. Built-up roofing should be inspected semiannually for cracking, alligatoring, low spots, and water pending; exposed bituminous coatings; and exposed, disintegrated, blistered, curled, or buckled felts.

CRACKING AND ALLIGATORING. - Smooth- surfaced, asphalt built-up roofs on which the surface mopping is relatively thin usually show definite alligatoring of the surface coating within 3 to 5 years. Alligatoring is always most severe where the asphalt coating is thickest. If allowed to proceed, alligatoring will develop into cracking, as shown in figure 7-9. Once the surface coating is cracked, water penetrates the membrane; and the roof deteriorates rapidly. Consequently, maintenance is necessary to prevent cracking.

The type and extent of maintenance depend on the future use of the structure. On smooth-surfaced, organic felt roofs of relatively brief expected use (4 years or less), remove all dust and dirt by sweeping, vacuuming, or air blasting and apply a thin coat of asphalt primer. After the primer is dry, one or two coating materials (asphalt or asphalt emulsion) may be applied by brushing or spraying at a rate of 3 gallons per square (100 square feet).

If the asphalt coating is alligatored but not cracked and the felts are not exposed, the primer may be omitted. If an asphalt emulsion coating is to be applied to such surfaces, dust and dirt maybe washed off with a stream of water from a hose. The emulsion can be applied to a damp but not a wet surface.

On organic (rag) felt roofs intended for prolonged use (over 4 years), the cleaning and priming requirements are the same as for roofs of relatively brief expected use. After the primer has dried, apply one coat of asphalt emulsion at a rate of 2 gallons per square. Immediately after applying the emulsion while it is still wet, embed strips of fibrous glass mesh (woven or nonwoven) in the emulsion, lapping the strips 2 inches. While the first coat of emulsion is still wet, apply a second coat of emulsion at a rate of 1 gallon per square over the fibrous glass strips. After the second coat of emulsion has set firmly, apply a final coat of emulsion at a rate of 2 gallons per square. If the asphalt surface is alligatored but not cracked and the felts are not exposed, the primer may be omitted.

EXPOSED BITUMINOUS COATING. - When the bituminous coating on a mineral-surfaced built-up roof is exposed, as shown in figure 7-10, brush loose gravel or slag from the bare area. Cover the bare area with hot bitumen poured at a rate of 70 pounds per square, and embed fresh gravel or slag. Old gravel or slag may be reapplied when the dirt and dust have been screened from it.

EXPOSED FELTS. - Smooth-surfaced, asbestos- felt built-up roofs may be surfaced originally with hot asphalt or with a cold-applied asphalt emulsion. After 4 or 5 years of exposure (sometimes earlier with cold-applied coatings), light gray or even white areas appear, indicating that the felts are partly exposed. Because the asbestos felts are constructed mainly of

Figure 7-10. - Exposure of bituminous coating.

Figure 7-9. - Cracking and alligatoring.