1 you said some metal roofing should never have firring strips i have certainly seen that with some of the very thin steel or weak aluminum decking which could never stand even a foot of free span over firring strips without crumpling buckling under someone walking on it so they would have to lie flat on the water barrier over the sheathing.
Metal roof over shingles without furring strips.
Manufacturers provide 30 year warranties on metal roofing products.
Some consideration for vent stacks and chimneys is required during the design process.
Consult the guide for your shingle brand to determine the specific method of installation required to preserve the warranty.
If talking strip metal roofing like standing or raised seam then i can t imaging getting an even final surface or proper fastening without firring strips not only because putting the metal right on the shingles gives no way to level out the uneveness of the old shingles but also the underlying shingloes and underlayment will get awful hot.
Use furring strips to build a slat board on the roof then mount the steel sheets to the slat board.
This is by far the most common way to install a metal roof.
If trapped between the metal roofing and old roofing moisture can build up and cause mold and rot.
Metal roofs depend on thin strips of wood known as furring to allow air to circulate and prevent rot.
Most metal shingles will be attached to the roof using metal shingle clips but metal roofing shingle installation can be system specific.
Furring strips are required if you plan to place a metal roof on top of existing shingles.
Spacing of furring strips for metal roof when installing a metal roof over an existing roof insulate the spaces between the wood strips with rigid insulation to prevent condensation before.
As long as the plywood under the shingle roof is in good condition this way can provide many benefits.
Installing steel roofing over existing roofing materials is fairly simple.
3 install 1 4 purlins down over the shingles before installing the metal roof the third way to install a metal roof over shingles is by applying 1 4 purlins over the shingle roof.