Cob: Metal Roofing

[Shannon C. Dealy]

On Thu, 6 Mar 2003, Darel Henman wrote:

[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).
>
> This is not a corrogated metal or metal roof you are talking about
> above, but a manufactured enabled metal. The question was about a metal
> roof, not an enameled material.

It is essentially the same from a thermal perspective, and from a life
span perspective, even the cheap corrogated stuff is usually good for 20
to 30 years.  In the generic sense, I can't speak for the person who
started this thread, but around here most people when they talk about a
metal roof, mean the enameled kind which is the overwhelming majority of
"metal roof" installations at least in this area.

> > They are also environmentally friendlier than most conventional /
> > non-natural roofs, since at the end of their life they can be easily
> > recycled.
> Not as easily as adobe tile and green roofs or thatches.  A lot of
> energy is require to so call re-cycle it.

I did specify "most conventional / non-natural", which pretty much leaves
out green roofs and thatches.  As far as the tile, certainly it's better,
but it does have two significant down sides which are it's massive
weight which requires major structural reinforcement in the design of the
building to support, and cost (both for the tiles and the additional
materials and engineering for the building to support their weight).  Not
necessarily a big deal, but it does mean significantly more materials
going into the building structure.

> Also there is a lot of junk metal coming out that contains a lot of
> metal refinery refuse and might contain  heavy metal particles, which
> are reportedly bad for health.

In the case of the enameled metal this is a non-issue, since the enamel
will seal it in.

>
> > 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.
>
> Since you couldn't make sense let me help you make sense.  Any skin,
> like a roof, it will bridge heat to or from the inside of the structure
> the outisde air.  Just as our skin does.  Go outside on a very cold day
> and lift up a metal block or even a wrench.   Put that down and pick up
> a piece of wood.  You'll notice the difference is great.  The metal
> conducts or suck energy about a thousand times faster than wood.  You
> would also never of course put on a metal jacket in the winter.   But we
> will use layers of clothing to keep us warm.   Metal roofs will also,
> depending on its emissivity factor radiate heat out at night.  It will
> radiate it inside and conduct it inside during the day.
>
> Having insulation under it helps, but it pulls more energy out through
> the insulation that an less conducive roofing surface would do.  Every
> bit of insulative material helps.   Perhaps in somes areas where the
> weather is very moderate it doesn't mater.  Local factors for a
> materials usage then come into focus.
[snip]

You missed both major points I was making here, the design of modern roofs
requires ventilation UNDER the outer skin, so much of what would be gained
by the shingles on top is lost due to air flow underneath, and in
addition, the relative insulation value of a layer of shingles is dwarfed
by the insulation value of the actual insulation layer below.  I'm not
claiming it has no insulation value, only that the net value it provides
in a conventional modern roof is minimal to non-existant, which makes it
little better than the metal roof.

>
> > 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.
>
> This we know, but shingles and tile do not conduct thermal energy at
> anything like metal does.  Tile and shingle roofs would transfer heat
> nearly a thousand times less than metal.   Most modern structures you
> mention above are not designed well I agree with you.  And no one should
> add additional burdon on the insulation in a roof but use a material
> that would or could even add to it.  That is the best solution that can
> be found.

I did not claim that the structures were not well designed, the
ventilation under the roof is a necessity in order to prevent moisture
build up and damage, but the effect of this ventilation is to massively
reduce the insulation value of the outer shell of the roof.  As far as
them transferring a thousand times less heat, this may be true, but again,
the net result is a tiny percentage of the total insulation value of the
roof, because much of the heat loss the material would theoretically
prevent is being bypassed by allowing outside air to flow under the roof,
thereby negating much of what would be gained.

Perhaps you will see my point if you examined it from this perspective,
imagine a roof that sits in the air 10 feet above the insulation and
vapor barrier, with nothing in between them, no walls, just air flowing
through, the roof keeps the rain off but doesn't do much else.  In this
case, clearly the roof material makes absolutely no difference in the
insulation value to the house regardless of what it is.  Now if you put
this roof down onto the house, but put in vents to allow air to continue
to flow through between the roof and the insulation, you will continue to
get much (though not all) of the same effect, which will continue to kill
much of the insulation value, and while you will gain some (though more is
likely to come just from the layer of air), what you get is going to be
very small relative to what the insulation below provides.

>
> > 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).
>
> You seem to be describing a very thin metal with an enamble on it.
> It's hard to see that it would be less weight that shingles or cedar
> shingles, though I haven't seen this conglomate item your mentioning.
> Even though it would pull energy through your insulation faster than a
> less conductive material would without a doubt.

It is comparable to the the old corrogated roofing that used to be used
all over the U.S. (and still is in use though not as much) on farms for
out buildings and even the main house.  The main differences are the
enamel and the metal is generally flat except at the edges where the metal
has been folded to form an interlocking mechanism for attachment to the
next piece of the roof material, and yes it weighs far less per square
foot than any type of shingle I have worked with, though my mentioning of
the weight issue was aimed more at tile roofs, since the weight of three
tab and cedar shingles is not much of an issue.

[snip]
> > 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.
>
> If they want to spend a lot of money on heating and cooling, your
> right.  But one must consider this as an ancillary cost.  Cement might
> be even cheaper.  Would you use it?  Money is not the only factor to
> consider.
[snip]

This simply isn't true, you statement reads like the material the
outer layer of your roof is made out of is the primary source of
insulation in the roof, when frankly, you get far more insulation value
out of the plywood underneath the singles than you get out of the singles
themselves, and when the roof vents are taken into account, the effective
insulation value of even the plywood is dwarfed by the insulation layer
installed below (and there is nothing that says you can't use plywood
under the metal roofing material).  Unless your roof has little or no
insulation under it, the outer layer will have very little effect on your
heating and cooling bill.


Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
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