Make sure that you are not placing the roofing screws too close to each other or too far apart. Generally, it is best to place each roofing screw about one and half to two feet apart, but no more than that.Click to see full answer. Also, how far apart should roof purlins be? 4 to 6 ft. Beside above, what are the disadvantages of a metal roof? Disadvantages of metal roofs Affordability. Metal roofs can be as much as two or three times more expensive than other roofing materials. Noisiness. Metal roofs can be noisy, especially during a heavy rain or hailstorm. Expansion and contraction. Inconsistency of color match. Performance. Also asked, how do I calculate how many roof screws I need? To calculate sheet length and quantity, click here. M) of your roof by multiplying LM by the coverage area. The coverage area for corrugated iron is 0.762m as denoted by the 762 in the product name. Finally, multiply the Sqr. M by 7 to find the number of screws needed.Where do you put screws in metal roofing?Next, you must make sure you are placing the screws in the appropriate place for a tight fit. Absolute Steel advises screws be installed down inside the flat of the panel and not in the top of the rib. See the illustration below. This is important as placing a screw in the top of the rib will prohibit a good seal.