Standard sizes
ship frames and decks 1 mm cardstock (if you do not have 1 mm laminate what you have got to the right thickness.)
It is probably coded with"*" on the 160gsm bits
** double up
*** treble up
hope this helps
barry
The rules described by Barry may be standards for Polish ship models but they do NOT APPLY for the 1/250 scale "Wilhelmshavener" (or "Jade" or "Lehrmittelinstitut") ship models. The latter are designed to be built without laminating baseplates, bulkheads, and decks. Sounds flimsy but is actually adequate.
Parts which are supposed to be doubled or tripled are usually printed inside one half of a doubling-"box". You cut out the whole box, fold in the middle, glue the halfs back to back, and then cut out the parts - now double strength. Don't know what I am talking about? Don't worry you'll understand when you see one.
If you find written German instructions look for the phrase "auf Abfallkarton verdoppeln" (= laminate to scrap card) indicating the rare occasion where laminating is recommended.
As a kid I built a couple of Wilhelmshavener ships, always according to the instructions, and they all turned out OK. Nowadays, suffering from advanced modelers syndrome, I would double the decks to reduce the risk of sagging. Of course this would require to shave about 0.2mm off the top of all supporting bulkheads to compensate for the increased thickness of the decks.