If you go for a sail catcher like David, then you can have very thin topping lifts, because the sail catcher will prevent  the chafe, as long as they are UV resistant.  Otherwise, however, I am in complete accord with Arne. 

The topping lifts are probably the most important length of line on the rig and the most difficult to replace at any time, let alone in extremis.  On Badger, we fitted 6mm line for the topping lifts: this didn't chafe the sails and was theoretically strong enough.  However, after only a couple of years, the UV had attacked it sufficiently that one of them snapped after gybing, causing the reefed sail to tumble down on deck.  There was too much wind for full sail and we were lucky that we had two sails and could sail under the other one to find anchorage and repair the lift, having no engine.  (We were also fortunate to be coastal sailing at the time.)  As soon as we could, we replaced them with thicker rope, and I put 10mm on Fantail's rig.  I don't suppose UV damage is much of a consideration in Scandinavia, but the importance of the topping lifts in the rig cannot be over-emphasised.

Arne Kverneland wrote:

According to PJR’s table your topping lifts should be 10mm. I prefer 3-strand rope for these, as they let me splice them. My guess is that the most slippy  monofilament rope is kindest to the sail.

Good luck!

Arne

 

I would follow Hasler and Mcleod’s advice and use ‘ample  strength’ for the topping lifts.

They give three reasons (PJR p.178)  -  shortened by me, here:

·         The weight of the bundle. I will add that when sailing deeply reefed, many of the sheetlets will work on the furled bundle, and thus increase the load on the topping lifts.

·         Small diameter topping lifts cut and chafe the sail more than larger diameter versions.

·         The consequences of a broken topping lift at sea is very serious.

Here is what I wrote about topping lifts in my “Junk Rig for Beginners”

“..Lazy Jacks...

Most sailors have seen some sort of lazy jacks in use on gaffers and Bm-rigged boats. Little do they know that these “modern gadgets” have been in use in China for well over 2000 years. Unlike the western rigs, where the lazy j. are nice to have, on the Chinese JR they are absolutely essential, as the JR reefs by lowering the sails into the lazy jacks. This means that the lazy jacks must be made from stout chafe-resistant rope and be checked, maintained and replaced with the same regularity as the halyard.”

According to PJR’s table your topping lifts should be 10mm. I prefer 3-strand rope for these, as they let me splice them. My guess is that the slippiest  monofilament rope is kindest to the sail.

Good luck!

Arne