"T i m" wrote:
Is it easy for a 'felt noob' to tell if it's 'heavy duty' (before
someone offloads some cling film on me as the 'good stuff')? ;-(

In my experience shed manufacturers only stock 2 types of felt, standard
shed felt and heavy duty shed felt. A roll of heavy duty felt is about two
or three times as heavy and therefore larger in diameter than standard felt.
If you see both types side by side they are unmistakable.

Looking around the web I'm unsure if I just go straight on with one
layer, multiple layers (of the same stuff) or if to use sub layers,
battens, tacks, heat, glue or tape etc?

For a shed roof you normally cover in a single layer of felt, overlapping
the strips, and either glue it or nail it down. I would study how the
existing felt was laid and just copy that with the new stuff, including the
direction that the strips were laid, either from end to end or side to side.
If battens and galvanised felt nails were used, and the existing felt lasted
for years, then it must have been done right in the first place. Some people
disapprove of using galvanised felt nails but if you have wooden battens you
don't have any choice. If you don't want to use galvanised felt nails then
don't have battens and use glue under the edges. Regarding a 3 layer system,
unless the shed was built to support the weight of 3 layers of felt then
that's a bit risky. A single layer of heavy duty felt, fitted correctly,
should last at least 10 to 15 years.

To save reiterating any old / established stuff, is there a good faq /
guide that would help me please?

No faq that I know of, there is a bit about felting a shed roof in this
Homebase guide:
http://www.homebase.co.uk/wcsstore/homebase/en_US/images/p0//HBHowToArticles/articles/howto_erectashed.html

All the best ..
T i m

And all the best to you too, good luck with the shed roof. Don't try it on
a windy day!