What chemical to kill moss on metal roof

I'm looking for a chemical that will kill moss without reacting with the metal roof please.

The roof is made of some sort of metal and has a grit finish bonded to it. The finish is quite fragile so the moss can't be scrapped off as the grit comes away with it. I often use Algon but can't in this instance.

Suggestions please friends ?

Basically any moss killer weed killer type of thing sprayed on will kill it and over time it will come off the roof , biocides would do the same job but take longer .

They will probably be aluminium which tends to be inert to most things apart from oxygen

 I'm no chemical expert though but finding out what the tiles are made of would help.

I usually spray on Ferrous Sulphate or Iron Sulphate and a few days later the moss has turned brown. You get it from gardening centres or on the net. I feel it better just to power wash the moss off because the roof comes up like new.

Its hard to know what metral it is so I need to be careful what specific chemical I use this time. Power-washing isnt an option unfortunately. The grit is too fragile. I wish though!