I had no problem with my first Chesy 18 built with 4mm and no glass on the deck - I'm 5'10 and 240 lbs. However, my second Chesy 18 was built totaly with 3mm, the finished boat came in at 38 lbs. After a couple of wet re-entries I put a 4 inch crack in the rear deck. I added a rear deck beam and had no problems after that. I also glassed the inside cockpit area and the hull was strong enough for me to stand up!

Just a tip for 3mm decks (and maybe 4mm) - don't leave it out in the hot summer sun for long periods of time. Mine sat for a day or two and the rear deck sunk in over 1 inch. The air contracted. When I released the rear hatch cover - WOOSH, then the rear deck popped back into shape. No problem with the front deck. I guess my hatches were pretty tight - I only used mini-cell window insultation from Home Depot.