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The tar paper is to keep water that makes it through the roofing material
to be able to run out underneath and not enter the attic space. It is a second
barrier to falling water not condensation. This is the reason it starts at the
bottom and is overlapped by the next layer applied as it goes up the roof. So to
answer your question yes, (no), no.
Metal roofing will heat in the sun causing expansion and then cool at night
causing contraction and over time the holes where the fasteners pierce the metal
will be enlarged and can let in some rain. This is the reason I only use light
colored roofing, preferably white. It is also the reason you want the tar
paper.
Metal roofing over OSB with aluminum backing and a full ridge vent is an
excellent choice for roofing.
Depending on your location and if you are collection drinking water from
your roof you may also want to look at some of the ceramic coatings available to
coat the roof. This reduces the expansion and contraction of the metal and helps
to prevent leaks in the future as well as extends the life of the roof. The draw
back is cost.
Clark
building a metal roof on top of OSB sheets
(actually using "solarboard" which has an aluminum-foil backing for
radiant-heat blocking). we'll have an un-insulated, well-ventilated
(open-ridge) attic underneath.
question: is it useful at all to put a layer of
tar paper between metal and wood (to limit condensation on wood surface ??
or??). if yes, is there a need for a (ventilation) space above tar
paper?)
thanks
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