I am planning to install 26 gauge metal roof on my son’s 80 year old farm house over existing shingled roof (wood decking). I am concerned about the (1) type of finish/coating, (2) what I should use for lathing, and (3) can install lathing and leave the old shingles on the roof? This will not be a standing seam roof.
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alot of people do install over shingles, but they is some concern that the roughness of the shingle will wear the paint off the back side. They is also concern about lathing strip. although it will create a channel for moisture, it will also create a channel or air to get under during a storm. Mine was installed on plywood, tar paper then metal, flat no strips.
I've done over a dozen like this
leave the shingles iof there is no sign that leaks have introduced rot, otherwise you need to strip to fix the joists and sheathing.
But if you can go over then run 1x4 or strapping, whatever is available in your area. Even 2x4s are OK and have some advantqages. For instance, one I reroofed was on a very wavy opld log cabin. Settling of the logs had made sure the rafters were not in the same plane, if they ever were. So to straighten that up, use shims and 2x4s.
Do these on 16" to 24" OC horizontally. The rib channels willl provide for ventilation.
screw them to the rafters as much as possible, by striking chaulk lines to aim for. Keep things straight, because you want the fasteners to line up stright when you step back to look it over. ( don't step back to admire it until you are on the ground though)
Then go
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