installation over poor condition asphalt

Jim Reynolds
Putting galvanized exposed fasteners panels over asphalt 3-tab shingles. On 2-story barn with top deck at 12/3 pitch and side decking at 12/16 or 12/18. Rafters are on 2 ft centers (most of them!) And decking on top area sags between rafters. This part of roof can't be seen by anyone but the birds. Least cost option - do I really have to tear off the old shingles and decking? Can I just install horizontal purlins and fasten the metal directly to them? Heat transfer into barn is of no concern. Also with the shallow pitch on top, what do I need to do special. Btw I intend to use panels with 2 1/2 inch ridges. Thanks
Todd Miller
Isaiah Industries, Inc.
First of all I always suggest underlayment over the old shingles. You do not have to remove the existing necessarily. I would suggest sending photos of the current to your roofing manufacturer and see what they think. Finally, one way to help even out the roof is to install vertical battens before your horizontal battens. Make sure that the roofing you install is approved for installation over battens.
Eric Novotny
An informed customer is the Best Customer!
You can go overtop in most scenarios. Like Todd said, make sure you have an underlayment across the shingles and horizontal purlins should work but make sure they have drainage channel in them if you ever get water behind the roof. 2:12 is fine for most roofs with standing seam if you have a some pitch to it. You can run mechanically locked standing seam down to a 0:12 if you want. Make sure you have some screening at the top and bottom to keep the critters from nesting under the roof.

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