On Tue, 08 Jun 2010 17:28:04 -0400, frank1492 wrote:

In small buildings (such as a shed) what is the downside to covering
the entire roof with aluminum flashing before shingling? Perhaps
expensive? That's the only thing I can think of.
Any other negatives?

There is no point. Flashing is to keep water from coming in seams and edges,
and wicking up from the bottom. It'll do nothing in the middle. If you are
really anal, use "Ice and Water Shield". This stuff is a heavy rubbery sort
of rolled roofing that sticks to the deck and seals the nail holes. It's
commonly used to seal the bottom 4' of a roof deck against ice dams. The
stuff is expensive, but will be a lot cheaper than flashing, which will do
nothing.

I want to protect the wood indefinitely.

That's a *long* time.