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Stemming Troublesome Deck Leaks

Leaks around Deck Joints, Rails, and Railcaps

Deck joints don’t leak as much as they used to. Most builders have switched to less leak-prone types of joints, and most are more conscientious about fastening and sealing them.

This is not to imply that all older fiberglass boats have deck joints that leak. There are many perfectly sound, watertight joints today on boats built up to 30 years ago. Then, as now, good workmanship begat durability.

Still, if leaks along your deck joint indicate that the watertight integrity has been broached, the first thing to do is to trace the leaks carefully and find the source. The leaks have one of three sources, or a combination of them: the joint or interface of the hull and deck laminates; the mechanical fastenings used to hold overlapping laminates together; or the mechanical fastenings used to fasten a rail over the joint.

The Rail Fastenings

With a sort of “last in, first out” perversity, the mechanical fastenings of various rails tend to be the earliest source of leaks. This is not too surprising, as they usually pierce the deck joint after it has been assembled, and the fasteners get little or no benefit from the compound that was used to seal the joint.

Naturally, the rail gets bumped and kicked, which helps to loosen the fastenings, and if it’s a wooden rail, its alternate swelling and shrinking further assists the loosening process.

I wish I had a penny for every tube of silicone applied along the corner of a toerail or a guardrail to try to keep their fastenings from leaking. It doesn’t work; you must take the rails off and rebed them. Give each fastening its own individual shot of bedding-in the rail, under the rail, and in the hole through the fiberglass.

Hull-to-Deck Joint Fastenings
After the rail fastenings, in order of their tendency to leak, come any fasteningsused specifically to fasten the hull and deck mold ings together. In some boats, these will be a sizable bolt every 6 inches or so around the entire deck edge. In others, the deck fasteners may amount to but a scattering of light screws or rivets as needed to hold the overlapping laminates together until the rails are applied and the rail fastenings take on both jobs. Regardless of how many there are, these fiberglass-tofiberglass fastenings are often prevented from leaking only by bedding used to seal the toerail above. The problem is compounded by the fact that two single-skin fiberglass laminates are too thin to provide good holding for a screw without its point projecting through into the interior. Despite the fact that the joint, the fasteners, and the rail above may be bedded, a potential path for water is created through the laminate.

Because of the incredible ability of water to find its way through the smallest hole, in my experience at least, self-tapping screws are the least likely to leak of all the fastenings used in a deck joint. I have found tapping screws in older boats that were obviously original, with out so much as a drop of moisture around them. I can’t believe that screws would hold the joint together as well as bolts if the hull were distorted, but they have obviously never leaked for two or three decades. I presume that is because selftapping screws are wedged into a predrilled hole so tightly that they tend to prevent the passage of water down the interface. This tightness is probably augmented by the fibers displaced by the threads of the screw, adding to the pressure around it. Of course, it is only the great strength of stainless steel that permits forcing a screw into a hole that tight without breaking it off.

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Stemming Troublesome Deck Leaks