huangyhg

galvanized roofing and rot
i am currently looking at two sets of drawings (home plans)
that call for standing seam galvanized roof. one asks for a
plywood roof deck with tarpaper underlayment, another with
1x6 boards with 1" spaces and a tarpaper underlayment or rosin paper, i have been told that a metal roof can "sweat"
and possibly rot the wood structure.which will work best,
or is thier any concern? thanks.
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in warmer climates metal roofing can sweat. condensation can form below the sheet metal and over time, rot the sheathing below if directly installed thereupon. 30# felt is recommended to protect the sheathing, not the sheet metal. temperature differences between the inside and outside that occur with external climatic temperature changes coupled with the drastically different heat transfer properties of the steel and wood bring about this issue.
the osb or plywood sheathing is easier to install and will probably be cheaper due to labor costs. the spaces in 1x6 stripping will provide some ventilation if the sheet metal is installed directly on the wood, but the felt helps mitigate the noise of rain and provides a little extra layer of leak protection.
standard practice in southern california is tarpaper covered with felt, then the roofing material. most residential roofing are shake substitutes or shingle.
plywood underlayer is used where loads need to be spread out, particularly with heavier roofs such as eaglelite or miravista.
felt is sometimes for steel shingle roofs, but that's because the tarpaper is separated from the steel by slats that the steel is mounted on.
my aluminum roof sweats quite a bit. i suspect most other materials also sweat, but their water absorption is high enough to disguise the effect. aluminum and steel don't absorb, so the sweat is more obvious.
ttfn
1" strapping, or boards comes from the days when all cedar shingle and shake roofs were done that way. the idea, and a good one, was to allow the shingle to dry out after wetting. not a bad idea actually when considering wood. todays sealing of wood to limit air infiltration and reduce energy costs is proving not to be such a great idea from the woods perspective. i live in a 102 year old house thats very sound, on the other hand we do a lot of forensic work in houses that are 5 to 20 years old that are finished because the wood was unable to dry after wetting ... so the steel roofing will 'sweat' but be sure to allow a path for the water to drain away and also a means for the roof system, re wood, to ventilate itself and dry should it become wet.
request galvalume substrate which has plus or minus 50% aluminum barrier coating as well as zinc sacrificial coating,instead of galvanize. galvalume offers 20 year coating warranty, galvanize begins the corrosion process immediately.
suggest tarpaper over minimum 1/2" osb or plywood deck. this system is considered a hydrokinetic system, designed to breath. condensation between metal and tarpaper either vaporizes or drains to eave and or valley trim. plywood deck reduces substantially noise from rain etc. also solid deck will reduce oil can tendency, inherent in standing seam

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