Roof rafters are the structure underneath the exterior roofing materials. Rafters form the roof line on a home, porch or building as a sloping support out of lumber, beams or timber. The rafters install from the highest point of a roof on the ridge to the edge of the structure. To calculate rafter spacing, measuring the corrugated tin roofing sheets and the overlap necessary for installation.
Place the tip of a measuring tape on the side of a sheet of tin roofing material. Stretch the tape across to the opposite side to measure the total width of the sheet.
Measure the width of the first hill and valley on the tin sheet on one side. Multiply this number by two and subtract it from the total sheet width. For example, if the corrugated tin is 42 inches wide and one hill and valley is 5 inches wide, multiply 5 times 2 to obtain 10 inches and subtract it from 42 to obtain 32 inches.
Divide the number by two to obtain the centre of the sheet of tin. Thirty-two divided by 2 is 16. Space the rafters on the structure on 16 inch centres so the centre of the tin can be screwed into the rafter.
Sheets of tin overlap each other so that the convex hill of one sheet covers the concave valley of the next sheet. The junction of the two pieces screws down onto the rafters. The most common on centre rafter spacings are 16 and 24 inches.