Why should screws be in the flats?

I understand that nails used to nailed through the ridges, but now screws are to be placed in the flats. The flats are a water channel, which would make we think the ridges would be better for fasteners. Why do manufacturers recommend screws on the flats? How long till the washers on the screws deteriorate and begin to leak? Thanks		Bernie Draht 8/17/2011	Share Or copy this URL: https://www.metalroofing.com/spirit/comment/16370/find/
Today's screws not only have the rubber grommets but also protective metal caps. They are designed to last easily 20+ years. My understanding is that the recommendation by most manufacturers on most profiles is to fasten through the high points on the ribs. However you would want to check with individual manufacturers.		Todd Miller Isaiah Industries, Inc. 8/17/2011	Share Or copy this URL: https://www.metalroofing.com/spirit/comment/16371/find/
Thank you for your reply. That would seem to be the best fastening location to me. The practice here, and the local manufacturer insists that the fastening should be on the flats. The installer I have always fastens on the high points on the ridges.		Guest User 8/17/2011	Share Or copy this URL: https://www.metalroofing.com/spirit/comment/16372/find/
How much space should there be on screws on metal roofs up and down roof		Guest User 8/30/2011	Share Or copy this URL: https://www.metalroofing.com/spirit/comment/16373/find/
The spacing of fasteners will be specified by the roofing manufacturer based upon panel profile, metal thickness, and perhaps where you live.		Todd Miller Isaiah Industries, Inc. 8/30/2011	Share Or copy this URL: https://www.metalroofing.com/spirit/comment/16374/find/

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