Never, ever, nail felt! This makes holes in it, unsurprisingly. It should be
stuck using the appropriate adhesive which should be laid in stripes up and
down the roof at, say 1 foot intervals. If you plaster the whole roof with
adhesive it is likely that when the felt gets holes in it the rain water
will gather, unable to get out by gravity, whereas with 'vertical' glue the
water, if any penetrates the felt, will run out to the gutter. The you glue
a top sheet on top.

Rob Graham