Cob: Metal Roofing

[Darel Henman]

Here's to correct some faults in what Shannon wrote about roofs, which
we both have installation experience in.  But she did not consider the
thermal performance issues, which I will expand on.


"Shannon C. Dealy" 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).

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.


> 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.
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. 

> 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.  

> 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.

> 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.


> 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 :-) 

I am looking forward to that.  I just visited several thatched roofs
last week.


> 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.

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.

>  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.

True.  Please finish that thatch article.  There's little practical
information out there on it.

Darel