Thus, if the tiles are 12 in. square, the strips are nailed so that their centers are spaced 12 in. apart; while if the tiles are 16 in. square, the strips should be spaced 16 in. on center. Strips are nailed also along each edge where the ceiling meets the walls. These strips are used to secure the outer edges of the last rows of tile. There are various mechanical ways of fastening ceiling tiles. A number of manufacturers offer special clips or related fastening devices which hold the tile firmly in the completed installation. Most lumber supply dealers can show you several varieties. Generally speaking, these clips are designed only to be nailed to wooden furring strips. These strips are nailed to the ceiling joists and spaced as noted in the preceding paragraph. Fig. 9.15. Installing ceiling tiles. Flanged tiles (Figure 9.15) represent another kind of fastening idea. These tiles have edges similar to the edges of tonguedandgrooved or shiplapped lumber. The first row of tiles is fastened by driving casing nails through a tongue or groove in such a way that the edge of the next tile will cover the nails. The tongue of the next tile is then inserted into the groove of the first tile and the nailing is repeated, and so on across the ceiling. Staples may be substituted for the small nails or brads. A stapling machine capable of driving them can be rented from some building supply dealers and also from firms specializing in the rental of tools. INDOOR REMODELING 461 Before applying tiles, the ceiling should be measured to determine whether an even number of rows or courses can be laid. Even though calculations indicate that an even number will result, it is best to begin in the center of the ceiling and work outward, for, if the walls are not plumb or the corners square, such defects will be absorbed by the last course of tiles. If the dimensions of the ceiling indicate an odd number of courses in either or both directions, the center line should be drawn so that there will be odd courses of equal width along opposite walls. In this connection it is well to remember that a most satisfactory method of finishing off the edges of a wallboard or tile ceiling is by means of a cove molding. Therefore, when measuring the number of courses which can be applied, it is possible that deductions for parts of the total coverage afforded by both moldings may preclude the necessity of additional small odd courses. Fig. 9.16. Finding the center lines of a ceiling. To insure straight courses, the center points of the opposite short walls of the room (A and B, Figure 9.16), are measured and marked, and a chalk line snapped between them. The line Л В is then divided in half at 0. Since the walls of the room may not be at right angles to each other, no attempt should be made to secure a line at right angles to А В by snapping a line between the center points of the longer walls. The familiar 345 method can be applieM by measuring a line ОС equal to 3 ft.