Re-felting lean-to shed roof - sealing an awkward join

I built a lean-to shed/greenhouse on to the end of a Marley garage 10 years ago. I'm about to re-felt the roof and read Thermo's advice about using garage grade felt.

Two questions:
1. Do I need both garage felt underlay and garage felt top sheet?
http://www.wilkinsonplus.com/invt/3223340
http://www.wilkinsonplus.com/invt/3222340

2. Where the lean-to joins the garage, trying to seal the felt under the the corrugated garage roof overhang was very difficult. Used roof sealant, no room for flashing.

Want to do a better job, but don't know how. Wondered if this may work. Roof gradient very shallow - only option to accommodate greenhouse dimensions. Run off from garage can be substantial. Want to avoid water blow back in windy weather and capillary action.

Grateful for any ideas.

if its only a shed then no need to use the underfelt layer. the top layer of the marley stuff is just like shed felt but much thicker. up to you really.

as for the join, can you take the corrugated roof off easily?

if not then you could face up the corrugations with mortar as suggested, but i would use something like aquaseal to coat the mortar in, or there is a liquid flat roof repair you can get in a samll tin (cant recall the name its about £30 for a 1 litre tin) Very good stuff and would suit that purpose

Thank you for quick reply. Impossible to take corrugated roof off, so will follow your other advice. Have looked up about the liquid you suggest - found Isoflex, Rustoleum Dacfill, Aquaroof liquid rubber so far and will keep reading.

Could I just ask if you would suggest I run the mortar in line with the end of the corrugated roof or set it back a bit as in my drawing?

Guidance much appreciated.