I have a 400 square foot roof that is leaking. It is an almost flat
roof next to a more slanted roof over my garage. I got an estimate from
a roofer for $2800 who said the roof was shot. He said he wants to rip
off the roof and the roof of the garage. There is nothing wrong with
the garage roof but I guess the junction between the two roofs could be
the source of leaks. I thought I'd save some money and do the work
myself and just lay shingles over the existing roof. I went to Home
Depot and was told that putting new shingles over old shingles was a
bad idea because the old shingles would swell and crack and damage my
new shingles. So I decided to rip off the roof myself. I started
writing down UPC numbers of tar paper and shingle when a roofer came
along and told me that I was wasting my money by doing it myself. He
told me that roofing is an intricate and delicate business. He said
that you need equipment that heats up the tar. He said cold tar won't
last. I was just planning to spread tar that Home Depot had near the
shingles. He said he would just lay shingle over the shingles that were
already there for $1700.00. I told him that two people had told me that
the shingles underneath will swell up and break the new shingle. He
said he has put shingle over other shingle before and hasn't had any
problems. He said if I wanted him to rip up the shingle that was
already there he would do it for an extra $500.00 so the total would be
$2200.00.
What should I do? Should I repair the roof myself or not? If I do, do I
need hot tar or can I get away with cold tar, if I do need hot tar do I
need to rent special equipment to heat the tar? Is it OK to just lay
shingle over preexisting shingle or should the preexisting shingle be
ripped up? I've also noticed that there are coatings for roofs. Should
I just put one of those on instead of new shingles?

OK....too much going on here. Let's break it into pieces:

1) You said there were shingles on the flat roof, but you talked about
spreading tar over something. Over what? Over the shingles? Is that how the
existing roof is set up? Tar over shingles??? When you said "tar", did you
mean "tar paper"?

2) It's generally not recommended to put more than 2 layers of shingles on a
roof, and some municipalities have regulations which prohibit roofers from
doing this. My understanding is that it's because of the extra weight, not
because the first layer will swell. I put a 2nd layer on my garage 15 years
ago, and there was no swelling.

3) Where a part of the roof meets another surface, like the wall of a higher
part of the house, or chimney and vents, you need to do various kinds of
creative and skillful artwork with bent shingles, flashing, and/or caulk. If
you do it wrong, you may still end up with leakage, if that's where the
problem is. If winter's upon you and you don't have time to do a little more
research into doing it right, you may, in fact, be better off having a
roofer do the whole job.

Doug's advice is right on. The critical junction is between the low
slope roof and the sloped garage roof. I'm assuming shingles on the
garage and probably a membrane or tar&gravel on the flat section. Dont
just dump tar on and hope for the best. Where are you located?

Do not do business with the guy you met at homecheapo, no one can give
an estimate sight unseen. Better yet, I can do it for $100 less. grin

Where are you located? If in SE PA I can refer you to somone who will
give you an honest assesment.

Doing a roof job correctly is not something for an amateur. Experience
and knowing what materials to use in various applications, how to
correctly do flashing, etc takes experience.

I would get quotes from several competent roofers. Talk to neighbors,
friends, etc to find out who does good work. Check the references and
previous jobs of whoever you choose.

You did not indicate what state you are in.
If you are in Florida, don't do it yourself!
The next time there's a hurricane, your insurance company will research
any roof repairs and if permits were not pulled , you'll end up not
being covered by your windstorm policy, homeowners, etc.
All insurance companies here now have access to building records &
permits.
Any work you do without a required permit, could leave you not covered
by them, if disaster hits.
[believe me, the insurance companies here do check, & are within their
rights to decline paying you if (example) you have a storm & you file a
claim for roof damage.]