SB: Re: metal roofing

["Robert W. Tom" <ArchiLogic@yahoo.ca>]

8/30/01 1:13:19 PM, "N Leone" <nlpub@hotmail.com> wrote:

> I have heard that the underside of the metal can
>   attract condensation that might then drip down onto your ceiling
>   insulation bales.  

N.;

If the air has any ambient humidity, metal roofing will of course 
experience some condensation, usually  overnight as temps
cool.

Since most non-soldered-joint metal roofs should have a 
slope of at least  4/12, condensation will tend to run downwards 
till it hits a purlin (ie every 18") and/or run right down to the end 
of the sheet and out at the overhang.
(ie Most (Canadian) roofing profiles are such that there is very little 
actual contact area between the U/S of the steel and where it contacts 
the sheathing (be it skip sheathing (ie purlins) or solid sheathing and 
the edge of the steel should overhang the edge of the last purlin by
an inch or so to allow water to drip off.

So if you get moisture in the roof cavity it tends to be distributed 
moisture, mostly on the purlins instead of streams or puddles 
of water in/on the insulation.

1x4 purlins @18" o/c are quite adequate for rafters at 24" o/c  
and the effect is not unlike the purpose of using skip sheathing beneath
wooden shingles & shakes to allow drying.

It might be useful to note that I tend to use site-built, 24 inch deep,
parallel chord trusses instead of solid lumber rafters so that a minimum
of 18 inches of insulation can be accommodated while still providing
a good ventilation airspace between the top of the insulation and the 
underside of the roofing steel. 

And, in this part of the world, one usually installs a continuous 
polyethylene sheet on the interior side of the insulation as part of an air 
barrier system. (It also functions as a vapour diffusion retarder. )
Please note the descriptor "continuous" (ie no leaks).

If you've ever spent any time in a well-insulated attic space beneath 
a metal roof you will know that it can get hotter than a lawyer's behind in 
Hell once the sun comes out so that any moisture is easily 
cooked off early in the day.  And the sun does usually come out every 
day in this part of the world.

I've done more metal-roofed buildings (with skip sheathing (purlins))
than most people have teeth  (and  hairs on their head in some cases) 
and I've yet  to see one (roofs that is, not heads) with moisture problems 
due to condensation on the underside of the metal roofing. 

Then again, my experience has been limited to a fairly narrow
geographic/climatic range of MooseLand where summers are 
hot and humid and winters are usually cold enough to freeze the 
nuts off of an iron bridge.
 I have no idea of what happens with metal roofs in deserts or in humid, 
primarily-cooling climates.

--- * ---
Rob Tom
Kanata, Ontario, Canada
<ArchiLogic@yahoo.ca>

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