Cob: Metal Roofing

[Shannon C. Dealy]

On Thu, 6 Mar 2003, Darel Henman wrote:

[snip]
> But, steel is a dumb choice for a roof, just as corrugated metal is.
> Expensive and is a thermal bridge (very high thermal conductance).
[snip]

Have to strongly disagree with this one, modern enameled metal roofs are
about the cheapest type of roof you can put on when labor and life
expectancy are figured in (50+ years), at least in this area (Oregon).
They are also environmentally friendlier than most conventional /
non-natural roofs, since at the end of their life they can be easily
recycled.  As far as the thermal bridge aspect, this frankly doesn't make
sense, since it doesn't bridge to anywhere, the sheathing, roof support
structure, insulation, and vapor barrier, are all below the roof,
and more to the point, in most roof designs for modern houses, there are
actually vents to allow air flow between the outer most layers of the roof
and the insulation in order to prevent moisture build up (building code
requirement), this alone by-passes most of the insulation value that might
be provided by using a material other than metal.  The primary function of
the outer most layer of the roof (metal, shingles, tile, etc.) is to block
rain, hail, wind, etc. from going into the building from above and/or
damaging the underlying parts of the roof system, it's insulation value
is of limited value in most modern structures.  From the above and other
considerations (such as weight and the amount of structure required to
support it), metal comes up superior to three tab shingles, cedar shingles
and tile (and I have worked on all four of these roofing systems).  Of
course for a roofing system like thatching (another roofing technique I
have some experience with - and someday Mark I will finish that thatching
article - reeeaaal sooooon now :-)  the above statements don't apply
because it is breathable, provides it's own insulation and there is only
one "layer" to the roof (though for fire safety purposes the interior
of the roof should be covered over to block most airflow to the underside
of the thatch).  When all things are considered - cost, code, building
officials, strength and materials required to support roof, ease of
getting materials and experienced roofers for the particular style, as
well as environmental aspects - for many if not most people on this list a
metal roof is likely to actually be the best choice.  Of course for
someone living out in a rural area of a third world country, with
significantly different costs for materials relative to their income, lack
of building codes, and greater availability of roofers experienced with
natural materials, a thatch, wood shingle, or other roof approach will
probably make alot more sense.

Shannon C. Dealy      |               DeaTech Research Inc.
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