Metal roof over sagging 2x4 purlins

Guest User
We are in the process of installing a new vertical panel (Fabral's Grandrib) over an existing metal roof on our old barn. It is a gable roof; we are using 21 ft. panels. After spiking our 2x4 purlins 2 ft. on center into the rafters, we discovered that we had a problem with rising and sagging. On the west side, there is a significant rise in the purlins, in the middle there is a significant dip, and it levels off on the east side. We shimmed underneath the 2x4s in the sagging area, but the difference between the rise and the dip is still over 3/4 of an inch. We now plan to cut different thicknesses of plywood and screw them onto the dipped 2x4s to make the transition to the high spot more gradual and less appearant. Is this a good idea? In addition to this, we also have variations from 2x4 to 2x4 from top to bottom. We determined this with a straight edge. We need to get our new roof on ASAP. Is it worth it to go through all the trouble of leveling our purlins? How much of these dips, rises, and variations will be visible after the new roofing is installed? What is the tolerance of the steel to bending around any imperfections, and how would these imperfections affect the longterm watertightness of the roofing? Thanks.
Todd Miller
Isaiah Industries, Inc.
If you do not get the purlins reasonably level, it will be visible when the roof is installed.
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