Standing Seam dimpling over SIPs, noisy

Hi,

I recently installed 24ga black standing seam (snap lock) over SIPs with a 3/8” ventilation mat. The mat is basically a cedar breather, and was specifically marketed for metal roof. I installed myself after a company rolled my panels on site.

I am having pretty bad oil canning, where instead of a slight ripple I am getting deep dimples when the sun hits. I noticed this while installing and checked fasteners nearby and I didn’t seem to find any obvious over-tightening. They are nail flanges, not clips. I could live with dimples if it weren’t for the audible pops every time a cloud rolls over and relieves the panel from the sun momentarily. It is continuous throughout the summer day.

The underlying surface is flat, as all SIP panels are continuous 4’x 18’ panels joined with splines. Below the ventilation mat is 30w felt.

The roof looks great without sun.

Any ideas?

Unfortunately having trouble uploading photos.