On Fri, 4 Mar 2005 14:08:49 -0600, manhattan42
scribbled this
interesting note:

John Willis Wrote:

So far as I know, National Building Codes allow up to three roofs on a
house. Just because it is up to code does not mean it is ideal. A
house with more than one roof installed is more likely to develop
leaks (because the roof exposed to the weather is not installed over a
uniformly smooth substrate) and I've never seen a second roof like
this last as long as it should. In every case I've seen, a house with
a second, third (or more) roofs installed require attention far sooner
than they would if there had been only one roof.

But that is not the observed fact by shingle manufacturers nor the Code
agencies. There is no more likelihood that a covered roof will fail
prematurely than one over a fresh substrate all things being equal.

Just because Code says something does not make it true. A laminated,
dimensional, five and five eighths inch exposure shingle, such as GAF
Timberline, installed over a five inch exposure wood shingle roof will
have leaks before an identical installation, except where the
substrate is smooth decking instead of wood shingles. Why? It has to
do with how the exposure of the faces of the different materials align
with each other. In the first example a careful observer will notice
horizontal waves across the plane of the roof-a kind of frequency
based on the difference in the exposure of the different materials.
Those waves represent areas where leaks are likely because the top
shingle is not flat, in fact it is somewhat dish shaped across its
horizontal axis, which means water tends to travel a bit farther on
that horizontal axis in those areas. If fasteners or the butt ends of
the shingles happen to be in the wrong place in those areas, leaks
will happen, and they will happen more quickly than they would if the
installation had been over a smooth deck. Why? Because over a smooth
deck that water would not have had the ability for that horizontal
travel as all the shingles lay flat and none are dish shaped. Physics
is physics and nothing code or the manufacturers say will change that
fact.

Which is why manufacturers offer FULL warranty of their products and
Code Officials allow the practice.

Code allows lots of things I personally wouldn't do when building my
own house. Just because it is supposedly safe and code allows it does
not mean it is ideal or can't be improved upon. As for the warranty,
in these cases the shingles haven't failed, the installation has, so
of course the manufacturers don't care if you install a second roof
over a first. All they warrant is the shingles will last a certain
length of time. They don't warrant anything about the installation!

Likewise, I have NEVER seen a second layer fail prematurely in my 25+
years as a professional contractor simply because it was a second or
third layer.
If they failed, it was due to issues completely unrelated to the number
of shingles.

All the above that I've written is also based on over 25 years of
professional experience in the field of residential roofing and
roofing contracting. BTW, if a three tab shingle is installed over
loc-tab shingles, it gets even worse and I've seen examples of this
kind of installation fail in as little as five years. The best
solution in those cases is to remove all the roofs and start fresh
again.

So just because one rips off and installs a primary layer instead of
installing a second one only does not guarantee that the job will be
superior and justify the added cost.

This much is very true. In fact I would rather have an excellent
installer put on a second roof than a poor installer put on a first!

Especially if half the time once
the roof has been ripped it has been found that the paper and plywood
is in good condition and didn't need it in the first place.

Yes, the issue is money, and more often than not it is money the
contractor wants to needlessly put in his own pocket at the expense of
a gullible homeowner.

If you get good performance from installing a second roof over an
existing roof, good for you. I've found that it is always better to
tear off a roof and start fresh.


--
John Willis
(Remove the Primes before e-mailing me)