Most fibreglass boats are built as two mouldings, the hull and the deck. The deck sits on a flange round the top of the hull and is sealed and fastened with bolts, screws or rivets. A toe rail or rub rail usually covers the seam. Safe Skipper describes these joints as designed not to come apart, often covered up and sometimes glassed in.
When the sealant ages or the fastenings work loose, the joint leaks. Water runs down the inside of the hull into lockers and bunks, far from where it came in.
What a surveyor checks
Inside, follow the joint through every locker you can open and look for stains, salt, rust streaks from the bolts and soft or rotten wood. Outside, look along the toe rail and at fittings near the edge, such as stanchions, cleats, chocks and genoa tracks. Safe Skipper points to fittings that have taken excessive strain. Then load the joint where you can: one that opens or moves under a loaded cleat has failed.
In the report
"Port side, hull-to-deck joint at the genoa track, saloon lockers: salt staining and rust streaks below three through-bolts, active drip after hose test. Remove the track, rebed the fastenings and reseal the joint locally." See hull-to-deck joint leak.