18mm will be plenty. Loft timbers are not strong enough to support heavy loads, including use as habitable rooms, and even the weight of flooring adds to the load.
If you tell us the dimensions of your loft timbers, and preferably also post some pics showing method of construction, we can tell you more.
If you are lucky you will be able to carry an 8x4 (2440x1220) sheet through the hatch. that's the cheapest way to do it.
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