On a close-boarded roof, if the slit is horizontal, cu1 a piece of canvas;1/8 in. wider than the slit (say 3 1/8 in. square), then push the point of a trowel or something similar into the slit and upwards to free the canvas from the boards for a few inches. Give both patch and torn part a coat of thick paint, and push the former about halfway under the upper edge of the slit. A few taps of the hammer will make the patch lie flat and close up to the corners; then nail the edges down with copper tacks 1 in. apart, and paint again. If the slit is vertical, make a horizontal cut across the top 3 in. long, forming a T. Paint and tack the vertical portion and proceed as described above, making the patch long enough to cover the lower end of the slit.