CHAPTER XIII ROOFING, FLASHING, AND WATERPROOFING F-2' Roof-Slab Roof Drain Slope,1⁄2" to 1'-0" Fig. 1. Roof-Construction at Drain- 1. Design of Concrete Roofs. The design of concrete roofs follows the same methods previously described for floors, using ordinarily a live load of from 30 to 50 lb per sq ft, in addition to the weight of the construction, which must include any fill or surface-covering employed for the purpose of drainage or protection. The GRADE of so-called flat roofs is formed by a cinder or cinder-concrete fill, or by constructing the roof itself so that the surface has the required inclination. If the latter method is followed care must be exercised in so pouring and grading the roof-slabs that the concrete will have a uniformly even surface, and be free from pockets or other irregularities. It is particularly necessary to grade the roof adjacent to the outlets, which should be depressed about 1 in, as shown in Fig. 1, even when all the rest of the roof-surface has no pitch. 2. Insulation and Drainage of Roofs. In the construction of factories or similar buildings, where artificial heat is required, the necessity of preventing CONDENSATION OF WATER on the ceiling of an upper story, as well as matters of PROTECTION and DRAINAGE, must be considered in determining the type of roofcovering. If INSULATION only is desired, HOLLOW CLAY, or GYPSUM-TILE 3 or 4 in in thickness are good insulating mediums. Of the two, gypsum is usually preferable as the dead load is less and the insulating quality higher. Both types of blocks are laid end to end, breaking joints. The clay-tile should be of the scored variety and covered by 1 in of cement-mortar mixed one part of cement, to three parts by volume of sand, which serves as a base upon which to lay the membrane water-proofing. The gypsum-tile are similarly placed, but a coating of hot pitch is usually applied to the concrete roof-slab before laying the blocks. No mortar-coat is required over the top of gypsum as the blocks are more regular than clay-tile; a second coat of pitch is applied as a base for the membrane water-proofing. Both types of insulation should be thoroughly protected against INJURY DUE TO EXPANSION caused by temperature variations. This is accomplished by placing %-in joints filled with some suitable mastic along all parapets, chimneys, or other abutting surfaces. If BOTH INSULATION AND PITCH are necessary, a cinder-fill, composed of clean steam-cinders, is usually the most satisfactory, and superior to a cinder-concrete fill because it is a BETTER INSULATING MATERIAL, has a LIGHTER DEAD LOAD, and is LESS LIABLE TO CREATE TROUBLE BY EXPANSION. Cinder-concrete, however, is ordinarily used for the GUSSETS. When a cinder-fill is employed, a coating of cement-mortar, mixed one part cement to three parts by volume of sand, is laid 1 in thick over the cinders as a bed for the roof covering. The cinders are SLIGHTLY dampened, before hoisting to the roof, thoroughly tamped in place, and PROTECTED FROM RAIN as soon as laid. If the fill is counted upon for the PURPOSE OF INSULATION, the minimum thickness should not be less than 3 in and for pitched roofs, the minimum pitch is usually taken at about 4 in per foot for a tar-and-felt roofing. In order to keep the dead load (cinders alone weigh about 60 lb per cu ft) within reasonable limits, the DOWNSPOUTS should be so located as not to require a maximum depth of fill over 12 in. The surface of the fill should be graded around interior drains as indicated in Fig. 1 and -in expansion joints are provided to divide the roof-area into sections not over twenty feet square. When cinder-concrete is used these joints should extend down to the structural slab and be filled with a suitable mastic. Additional EXPANSION JOINTS are also provided wherever the roof-fill abuts the parapets, and should be at least 8 in in width, extending in all cases down to the structural slab of the roof and filled with asphalt or pitch. Where it is desirable to pitch the roof-surfaces, and at the same time to gain a greater degree of insulation than that offered by a cinder-fill alone, a cinder-fill may be placed over a layer of clay or gypsum block. In buildings where it is considered necessary to create a dead-air space below the roof, a HUNG CEILING can be employed and the drainage obtained by inclining the roof-slab or by any other means as described above. This method of insulation is most applicable to beam-and-slab construction in which metallath is supported upon light structural steel members attached to the soffits of the beams. In roofs of the girderless type, this method of insulation is somewhat more costly than in beam-and-girder construction, and is not generally employed. In any case where a hung ceiling is used, the design should provide for the wires or inserts supporting the frame of the ceiling to be placed in the concrete at the time of pouring the roof. Hung ceilings are more often encountered in SCHOOL-HOUSES and INSTITUTIONAL BUILDINGS than in industrial work for the reason that in the former class of construction it is customary to plaster the ceilings and a concealed space is often required for ventilating ducts. The advantages of a hung ceiling are its effectiveness in providing insulation at considerable less weight than the various forms of insulating mediums described above. (Under average conditions 12 lb per sq ft is allowed.) Its disadvantages are the cost, except when plastered ceilings are required, and the tendency to rust where moisture is present. CORK INSULATION is too expensive to be generally used except for cold-storage rooms and similar installations. It is always desirable to place the DOWNSPOUTS, or CONDUCTORS, on the interior of a building. Their cross-section as a function of the roof-area depends upon the severity of the rains to be expected in any particular locality. Ordinarily in the vicinity of New York City, and for roofs comparatively flat, about 1 sq in of cross-section should be allowed for each 100 sq ft of roof-area. The HEAD OF A CONDUCTOR should present an area several times that of the conductor itself, and be properly screened or otherwise protected against clogging. As a final precaution OVERFLOW VENTS should be provided through parapet walls at a suitable height to provide against possible failure of the roof through waterload. 3. Roof-Surfacing Materials. The choice of any particular form of roofing depends upon whether the roof is approximately flat or has a consider able pitch. In the former case, the purpose for which the roof is intended as for example, a promenade or recreation-area, often governs the choice of surfacing. In the case of pitched roofs, the architectural treatment usually determines the choice. The general considerations of first cost, cost of maintenance, and fire-resisting qualities, however, should always be carefully weighed. For FLAT ROOFS, and roofs of moderate slope, the most economical roof-surfacing is composed of from three to five layers of saturated felt, or similar membrane, laid with hot coal-tar pitch, or asphalt, and covered, except in the case of the asphalt, with slag or gravel. The most widely used method of roofing, employing coaltar pitch as a cementing material, is that known as the BARRETT SPECIFICATION which is as follows* (Fig. 2): "(a) This specification shall not be used where the roof incline exceeds one inch to one foot. *This specification has been transcribed without editing. "(b) The roof deck shall be smooth, firm, dry and free from loose material. If roof deck is inclined, it shall be properly graded to outlets. "(c) First. Coat the concrete uniformly with Specification Pitch. "(d) Second. Over the entire surface lay four (4) plies of Specification. Tarred Felt, lapping each sheet twenty-four and one-half (241⁄2) inches over preceding one, mopping with Specification Pitch the full twenty-four and onehalf (242) inches on each sheet, so that in no place shall Felt touch Felt. "(e) Third. Over the entire surface pour from a dipper a uniform coating of Specification Pitch into which, while hot, embed not less than four hundred (400) pounds of Gravel or three hundred (300) pounds of Slag for each one hundred (100) square feet. The Gravel or Slag shall be from one-quarter (1⁄4) to five-eighths (%) inch in size, dry and free from dirt. "(f) General. The Felt shall be laid without wrinkle or buckles. Not less than two hundred (200) pounds of Pitch shall be used for constructing each one hundred (100) square feet of completed roof, and the Pitch shall not be heated above 400 degrees Fahrenheit. "(g) The roof shall be applied by a roofing contractor approved by the Barrett Company." In connection with Paragraph (a), it should be noted that a pitch of % in to the foot is sufficient to insure the necessary drainage, and the Barrett Company gives the same guarantee if the roof is laid flat. For surfacing over wooden roofs a similar specification is used. Five plies of felt are applied, the first two being nailed to the wooden deck, with barbed nails through flat tin caps, and the last three plies laid with pitch, as described above. If the roof-slope is greater than 1 in to the foot, a special five-ply type is used, the first ply, only, being nailed to the roof. The following SPECIFICATION OF THE H. W. JOHNS-MANVILLE COMPANY illustrates a widely used type of membrane roofing, employing asphalt as a cementing material:* "Materials "(a) Asphalt Concrete Primer to be Johns-Manville Asphalt Primer over concrete. There shall be used approximately one gallon per 100 sq ft of roof surface. Over Gypsum there shall be used approximately two gallons per 100 sq ft of roof surface, applied in two coats. "(b) Asphalt Cement to be Johns-Manville Ajax Asphalt (combination of Trinidad Lake and other natural asphalts) cement. There shall be used approximately 90 lb per 100 sq ft of finish roof surface. "(c) Single Ply Sheets Asphalt Impregnated Asbestos Roofing Felt, each 32" wide and weighing approximately 14 lbs per 100 sq ft; to be Johns-Manville No. 2 Ajax Asphalt Impregnated Asbestos Felt. “(d) Liquid Asphalt Roof Coating to be Johns-Manville roof coating. There shall be used approximately one gallon per 100 sq ft of finished roof surface. "Application of Materials. "(e) Coat the concrete with Johns-Manville Concrete Primer to provide a proper bond between the concrete and asphalt allowing primer to dry. "(f) Mop the surface thus primed intermittently with Johns-Manville *This specification has been transcribed without editing. Ajax Cement heated to flow freely, and while the cement is hot embed into it sheets of Johns-Manville No. 2 Ajax in three ply construction. Expose to weather 101⁄2" of each ply, mop the surface between plies with hot Ajax cement and roll the felt closely behind the mop so that no missing of asphalt can take place. Over Gypsum the back edge of first ply to be nailed to roof slab with proper nails driven through flat tin caps in addition to mopping as above. "(g) After such materials have been properly applied, and the roof is otherwise complete, spread over it an even thickness of Johns-Manville Asphalt Roof Coating, and neatly finish it to provide an even black appearance. (Fig. 3.) "Flashings. "(h) Roofing materials shall be carried upon vertical surfaces 2". All flashings except those around ventilators, standpipes, exhausts, etc., shall be Fig. 3. Johns-Manville Three-Ply Ajax Built-Up Roofing over Concrete. Inter mittent-Mopping System composed of base flashings of special Asbestos flashing material, approximately 101⁄2" wide, cemented and nailed to vertical surface. Such flashings shall be counter-flashed with Johns-Manville Asbestole System." This type of roof is applied only by the manufacturers and the roof-deck is required to have a pitch of not less than 1/4 in nor more than 6 in to the foot. On wooden roofs, a similar specification is used, with four plies. The first ply is nailed through flat tin caps. It should be especially noted that the nailing of the first ply is required on all types of roofs where the slope is greater than 1 in to the foot, in which case provision must be made in the structural slab. When WEARING SURFACES are required, those composed of concrete slabs, brick, or tile are the most satisfactory. Concrete for this purpose, 22 or 3 in thick, should be laid over a waterproof membrane and divided into sections not over 20 ft square, and there should be %-in beveled expansion-joints between the sections, filled with a suitable mastic such as "sarco." The wearing surface |