bruce_phipps wrote:
While I was up in the loft last evening, fitting some loft insulation,
I noticed a 6 inch rip in the roofing felt. I could view the roof tiles
through it.

There is no evidence of water ingress. Can I safely patch this up with
duck tape or do I need a more watertight solution?

If you want to do a proper job, get some felt and Ruberoid adhesive,
cut off a length of felt to fit between the rafters, preferably
pushed on to the top of the rafter at each side by 1/4" or so,
rehearse fitting it and tucking it over the top of the existing
lower unholed length of felt, cut away any felt that's hanging
down from the hole (leave as much as poss), liberally apply
adhesive to the underside of the holed felt, up and down the
edge next to the rafters, fit new felt and prop it in place with
battens arranged like a plasterboarder's "dead man" on each side
and the middle of the felt, leave to set properly, remove props.