It was an interesting exercise, albeit a pretty fruitless one.
Without a degassing chamber, it was always going to be risky - though any bubbles that might have formed in the mould would simply become small bumps on the final, cast shape, which could easily be cut/sanded away from the dental plaster. As it turned out, the RTV silicone we used came out in a single, big lump, rather than a long, thin, fluid stream as the YouTube tutorial videos suggest.
Maybe it's just a lack of technique here - we treated the silicone like dental plaster and just wanted it out of the mixing cup before it set, but the set-up time for silicone is hours, not minutes like plaster!
A bit of banging on the table and quite a few bubbles came to the surface. This either meant we'd made an excellent mould and managed to get all the air bubbles to the surface, or the entire silicone mixture is likely filled with air bubbles throughout and the mould is already ruined!
After making up some silicone and leaving it overnight, taking the roof section out in the morning was quite exciting. The silicone poured into the mould as a thick, gloopy goo - nothing like the thin, flowing liquid-like properties of the dental plaster we use when casting pieces. We weren't even sure whether the silicone would pick up the details because it was so thick and gloopy.
The silicone worked really well.
In fact, it worked rather too well, creeping underneath our individual layers of tiles and making a thin film of silicone between each course on the roof. This made removing the roof from the mould quite tricky, without ripping the really thin layers of silicone.
The end result was a not-so-satisfactory mould. There's no way it can be used as it is. And our beautiful wobbly tiled roof section now has loads of little bits of pink poking through between the tiles. So it looks like we're going to have to make a new roof section as well!
We don't really fancy making another 3" wide roof section again from so many individual tiles. But simply engraving a sheet of mdf/plastic doesn't really give a very satisfactory result - the roof still looks like a flat sheet once the building has been constructed.
So maybe there's a middle way.... maybe we can draw and laser cut strips of shingles, all connected along the top edge, to glue on in overlapping fashion to create our roof effect - in this case it'd still mean gluing about ten strips (rather than lifting a complete roof if we stuck to just laser etching) but that's still less work than we put into making a single row our course of tiles on this roof section!