Our terrace is nailed (by the previous owner at least 10 years ago) with what I believe are stainless grooved nails with a convex head, if I remember correctly they are 75 long. They hold great and I have never had to nail down a single one in the 6 years we've lived here. The picture is taken inside the conservatory but it's nailed both outside and inside.
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Edit: It's something similar to this, but I think the head is more rounded
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