When pressurizing a hull the thing to remember is that the deck has somewhere around 9 square feet of area (actually more, this is a conservative estimate), or about 1300 square inches. That means that for every 1 psi of pressure added the hull-to-deck joint has to withstand 1300 pounds of force. At 5 psi internal pressure you're applying around 6500 pounds of force on the deck joint.

Any time you're dealing with pressure, even apparently low pressures like 4 or 5 pounds you have to consider the amount of surface area that the pressure is acting on.