Hi Eric,
Wow, that area would be a fine weight if felted – getting on for 40kg?
This stuff is called Easy-Trim. Make sure you buy the Non-Breathable (ie fully waterproof) variety. There’s about 20 stockists in Tyneside (such as Dove and Eurocell) so you’ll find it anywhere. It’s in 45m rolls of 1m width and costs about £20 per roll. It’s literally 1/20th the weight of felt.
https://www.easy-trim.co.uk/products/roofing-underlay/breather-membranes/non-breather-membranes/
You’ll get funny looks from the builders when you ask for the Non-Breathable because the norm is to use ‘breathable’ under the likes of tiles/slates.
I hurriedly nailed a covering on the roof at the end of the day just to keep things waterproof and it survived the last ‘hurricane’ without so much as a ‘flap’ or tear. The first covering is still on. I’ve got 30m left!
Not sure what snow and Ice will do but I’ll probably put another layer on and then see.
Regards
T