Lots of excellent suggestions above.

A few I'd add:

In my experience, 90% or more of "roof" leaks are actually flashing
problems - it's pretty hard to screw up shingle application (not that
some people don't manage):

http://www.homeevaluationservices.com/sliproof.JPG

but it can be quite difficult to flash intersections between the roof
and something else properly.

For starters, are you doing a "tear off", or re-roofing over the
original shingles?

IMO you may get satisfactory results with a re-roof if it's a
"simple" roof, but in my experience if there is any intersection
with a vertical surface (such as the side of a dormer) it can be VERY
difficult to (re)flash such intersections properly during a re-roof.
And even on a "simple" re-roof you want to pay close attention to
the flashing at all roof penetrations - for example many of the
vent-stack flashings that depend on a rubber collar around the stack
for water seal are old enough so that the "rubber" seal may have
deteriorated:

http://www.montanahomeinspections.co...g_flashing.jpg

and such penetrations which will require re-flashing, not just a dab
of roofer's cement to seal cracks.

Other frequent problems I see on both tear-offs and re-roofs include:

- Chimney problems not corrected before roofing. Have the chimney
inspected *before* roofing, and coordinate any repairs with both the
mason and roofer - you don't want end up seeing something like this:

http://www.joehallroofing.com/info/infofiles/cheap.jpg

which will require both masonry work (which can damage roofing) and
re-flashing (which will mean removing some newly installed shingles).

- Incorrect chimney flashings to masonry chimneys. In most areas of the
country the accepted industry best practice is "step flashing", a
series of individual flashings which extend underneath the shingles and
up the side of the chimney and are then covered with a "counter
flashing" that is set ("let in") to the mortar joints.

http://www.copper.org/applications/a.../images/57.gif

There are other ways to properly flash a chimney, but what you do NOT
want to see is roofer's cement slobbered over a "flashing", it will
start cracking, often within a year:

http://www.homeevaluationservices.com/27.jpg

or worse yet, no flashing at all:

http://www.realestateinspectorsgroup...eyflashing.jpg

- Incorrect flashing at dormer sides - generally if done properly
this is a step flashing (in this case, as seen before siding is
installed):

http://www.gfsprague.com/house_pics/...tersection.jpg

junctions with other materials, such conventional stucco or EIFS, have
somewhat different flashing methods.

As you can see this flashing is difficult to inspect during a re-roof,
and one way to save money on a tear-off is to re-use instead of
replacing it.

- No kickouts on "blind" gutter ends. Big potential damage on this
one as you may be directing water into walls. "Easier to show you
than tell you":

http://www.nachi.org/forum/attachmen...8&d=1151175817

http://i74.photobucket.com/albums/i2...o1/Stucco3.jpg

http://i74.photobucket.com/albums/i2...StoDetail2.jpg

- Gutters and downspouts not properly (re) installed and wetting
foundation. Use common sense he the water should flow off the roof
and into the gutters, not overshoot them or flow between the gutter and
the structure, once in the gutters it should flow toward the downspouts
without over flowing the sides at any point, and once in the downspouts
it should exit at least 5' from the foundation at a location graded
such that the water does not flow back toward the foundation. If you
have any doubts, have the roofer direct a garden hose onto the roof
area in question, and watch the results.

Michael Thomas
Paragon Home Inspection, LLC
Chicago, IL
mdtATparagoninspectsDOTcom
eight47-721-0776