For tall chimneys a cast iron cap is generally considered the most durable finish for the top. The usual shape of such caps is that shown in Fig. 227. Such a cap completely protects the mortar joints from the weather and prevents the bricks in the upper courses from becoming loose. If the chimney is corbeled out as shown the cap also acts as a drip to protect the sides of the chimney, at least near the top. The inner lip of the cap should extend down into the chimney from 8 to 12 inches. If the cap is not larger than 4 feet square it need be but ¼ of an inch thick; if larger than this the thickness should be increased to 3/8 inch.
If the cap is 3 feet square or greater, for convenience in handling and casting it should be made in two or four sections, which should be bolted together, flanges being cast on the under side for this purpose.
Chimney Ladders. - It is sometimes desirable to have a ladder built inside of large brick flues, or shafts, and on the outside of tall chimneys to serve as a ready means of reaching the top. Such ladders are usually made of ¾-inch round iron bars, bent to the shape shown in and no frame is necessary. In all other stones, and in cement walks, the hole should be protected by a cast iron frame at least 4 inches deep. The frame is generally cast with a projecting ring about 2 inches wide and ½ inch thick, which should set in a rebate cut in the stone and filled with soft Portland cement. The frame is also made with a ¾-inch rebate for the iron cover. The cover is made of cast iron about ½ inch thick and should have a roughened surface on top. The covers are sometimes made with holes, into which glass bull's eyes are cemented to admit light to the vault. Both solid and glazed covers are generally carried in stock by the larger iron foundries, and in sizes from 16 to 24 inches in diameter.
Fig. 228 and placed in the wall of the chimney, or flue, when built. For easy climbing the rungs should be placed 12 inches apart between centres, and should be about 18 inches wide and project 6 inches from the wall.
Coal Hole Covers and Frames. - When coal vaults are placed under the sidewalk the architect should specify iron frames and covers for the holes made for putting in the coal. If the vault is covered with granite flagging a rebate may be cut in the stone to receive the cover,