I don't know about that where I live there is quite a bit of hail damage so
houses get new roofs all the time. It is common practice to put a second
layer over the first then if there is hail again to tear off both layers.

I guess it would depend upon the reason for the roof replacement

Owens Corning web site varies depending upon the shingle but for your basic
3 tab this is from the web site

applied over recommended decks. If other decks are used, the

resulting construction may not qualify as Class A.

Regardless of deck type used, the roofing installer must

ensure that the attic ventilation meets or exceeds FHA Minimum

Property Standards.

Note: All roof structures, especially Mansard style construction, must

have complete through ventilation from bottom to top to prevent heat

build-up or entrapment of moisture-laden air that can cause premature

shingle failure.

(B) Handling: Use extra care in handling shingles when the temperature

is below 40°F. Shingles can be broken easily in cold weather

or their edges damaged in hot weather.

(C) Fastening: Owens Corning recommends nails as the preferred

If old asphalt shingles are to remain in place, nail down or cut away

all loose, curled or lifted shingles. Sweep the surface clean of all

loose debris just prior to applying the new roofing. Ensure proper

size and length of fasteners. If roofing over old wood shingles, cut

back the old shingles at eaves and rakes and apply wood edging

strips. Some local building codes may require the use of a No. 30

asphalt saturated felt over the old wood shingles prior to reroofing.

Consult local building code authorities. Surface must be smooth

before shingles are installed. Make deck smooth by nailing down all

loose and curled shingles, protruding nails, etc. Install beveled wood

feathering strips, if necessary.

When roofing over existing shingles with a 5" (5-5/8" metric) tab

exposure, the following procedure should be used for smoothest

finished appearance and ease of alignment:

http://www.owenscorning.com/around/r...structions.asp



Wayne

"Tony Hwang" wrote in message
news:t9LAc.806289$Ig.387090@pd7tw2no...
Tom Baker wrote:

"Jeff R." wrote in message
...

Our Condo Association is in the process of reviewing bids to have our
units
reshingled
with 30-year fiberglass (GAF or Tamko) over a single layer of shingles.
One
of the
bidders strongly suggested that we strip the existing shingles first,
claiming that if we
didn't we would decrease the useful life of the new shingles by as much
as
50%! Other
people I've spoken to say that 2-4 years is more likely.

Obviously we would like to avoid the additional cost of removing the
shingles but not if
it means we would only get 15 years out of the new shingles. Can anyone
provide more
insight as to the actual impact of NOT stripping first?

Thanks in advance, Jeff R.

--- Other Info ---
We are in the Midwest.
Believe the existing shingles are asphalt organic felt (does this
matter?)
Some of the existing shingles have started to curl slightly.


If the shingles have begun to curle, you may have problems beyond old
shingles.
Also, check the manufacturers web sites.
Don't hesitate to e-mail them with questions.

TB
Hi,
Just proper thing to do is to remove old shingles. Then you can check
integrity of sheathing underneath too.
They're trying to save cost of removal and hauling it out.
Tony