Posted 4/18/2016 19:30 (#5250461 - in reply to #5250229) Subject: RE: New Standing seam roof on house--over shingles???
SE SD
Very common practice in the Midwest to put down 1x4 every 32" and leave shingles on, whether there on or off is personal preference that will adjust the price of course.
My opinion 1 layer would just beef up your vapor barrier.