Roofing slate, clay tile, and cement-asbestos shingles are rigid roofing materials generally used on permanent buildings with pitched roofs. These materials usually give good service with very little repair.
Slate is a comparatively dense, nonporous rock used on roofs to provide a durable covering. Only responsible and experienced slate roofers should be engaged to lay or repair such a roof.
The most frequently needed repair of slate roofs is the replacement of broken slates. When such replacements are necessary, supports should be placed on the roof to distribute the weight of the roofers while they are working. Broken slates should be removed by cutting or drawing out the nails with a ripper. A new slate shingle of the same color and size as the old should be inserted and fastened by nailing through the vertical join of the slates in the overlying course approximately 2 inches below the butt of the slate in the second course above. A piece of sheet copper or terneplate about 3 by 8 inches should be inserted over the nail head to extend about 2 inches under the second course above the replaced shingle. The metal strip should be bent slightly before being inserted so that it will stay securely in place.
Very old slate roofs sometimes fail because the nails used to fasten the slates have rusted. In such cases, the entire roof covering should be removed and replaced, including the felt underlay materials. The sheathing and rafters should be examined and any broken boards replaced with new material. All loose boards should be nailed in place and, before laying the felt, the sheathing should be swept clean, protruding nails driven in, and any rough edges trimmed smooth.
Asphalt-saturated felt should then be applied horizontally over the entire roof deck, lapping the sheets not less than 6 inches and over ridges and hips not less than 12 inches. The sheets should be secured along laps and exposed edges with large-head roofing nails spaced about 6 inches apart.
If the former roof was slate, all slates that are still in good condition may be salvaged and relaid. New slates should be the same size as the old ones and should match the original slates as nearly as possible in color and texture. The area to be covered should govern the size of slates to be used and, whatever the size, the slates may be of random widths, but they should be of uniform length and punched for a head lap of not less than 3 inches. The roof slates should be laid with a 3-inch head lap and fastened with two large-head slating nails. Nails should not be driven too tightly—the opposite of the method used for nailing wood shingles; the nail heads should barely touch the slate. All slates within 1 foot of the top and along gable rakes of the roof should be bedded in flashing cement.