plate when the rail flange 14 contacts the washer surface and expansion due to the weather where ambient temperatures may vary from sub 0 u00b0 to 130 u00b0 f occurs the washer will freely turn while the fastening screw remains firmly fixed the screws contemplated for this system will normally be 3 4 inch in diameter and 57 8 inches in length the screw head being hexogonal and 11 2 inches in width and 3 8 of an inch thick the washers used are preferably 23 4 inches in diameter 1 4 inch thick and have a one inch diameter hole to receive the threaded body of the screw the screw holes in the base plate are preferably 7 8 inch in diameter after assembly the bottom surface of the washer will engage the top shoulder of the lower rail flange the channel 2 has a width which is desirably 1 8 inch greater than the width of the base of the rail 15 also the channel has a height which is suitably 1 8 inch greater than the height of the base at its corners so that about 1 8 to 1 4 inch gap normally exists between the bottom of the washer and the top of the flange 14 these gaps give the rail room.