Locating and Fixing Deck Leaks

By Tom Burden, Last Updated: 9/10/2019

clear hatch installed on a boat

I thought installing a new hatch would stop the leak in the upper left corner, but the leak is not coming from the hatch! It has migrated from one of the stand-up lead blocks at the base of the mast. My next project will be to re-bed those blocks.

You’re sleeping in the V-berth after a really enjoyable day on the water, and are awakened by the soft sensation of raindrops on the foredeck. Isn’t it a great pleasure to hear the pattering of rain directly overhead, with you warm and snug below deck.

Then it starts again—that drip from the corner of the hatch directly above your pillow. It’s the pesky and annoying leak that you have not been able to find and fix. You know it is coming from the hatch, but you’ve already taken the hatch out and re-bedded the whole thing, and it’s STILL dripping onto your head. Makes you want to set up an umbrella!

The dockside wisdom is that this is a major leak, based on the premise that a leak that drips onto your crew’s head is a minor leak, but a leak that drips onto the owner’s head is a major leak.

Leaks in the deck happen to just about all boaters, including Bill Erkelens. He’s a pro at preparing racing sailboats for the Volvo Ocean Race and the America’s Cup. “I do this for a living and my boat still leaks!” says Erkelens. “This year, I’ve tracked down every leak in the deck. This time I’ll get it right.”

More Than Just a Nuisance

We’re not discussing leaks from below here—the kind that can suddenly cause your boat to sink—but instead the water that finds its way through leaks from rub rails, hatches, ports, cleats, cockpit drain fittings, windshield seams, spotlight bases and other avenues for penetration.

These are leaks that can stain bulkheads and headliners, damage electronics and cause upholstery to get soggy and full of mildew. They make your boat a wet, smelly environment. They need to be hunted down and fixed.

brown stains on a shelf indicating damage to plywood deck overhead

Telltale brown stains on this shelf indicate that there’s damage to the plywood core in the deck overhead.

Cored Decks

Deck leaks also represent a threat to your boat’s structure, because they can cause decks to become soft and spongy, with cracks that weep brown water that stains topsides. This is because most fiberglass boats have decks with a wood core, and keeping water from infiltrating that core is a key to keeping your boat’s deck intact. Let’s see why below.

Most fiberglass boats were built with some kind of a sandwich construction. Two thin layers of fiberglass comprise the “bread” in this sandwich, with the “meat” being the 3/8” to 3/4” core. It’s most commonly end-grain balsa or marine plywood. Cores in racing sailboats or powerboats may be closed-cell foam or high-tech honeycomb.

Places Where Water Gets In

In theory, when your boat came from the factory, all of the openings in the deck were beautifully sealed and watertight. In the real world, even boats built by the best manufacturers may have badly caulked deck hardware and these locations are vectors for water intrusion.

DIY projects from the past: These are the most common culprits for deck leaks, with badly applied or inappropriate sealants failing to keep water out. Stress cracks in the deck or cabin top, created by impacts from heavy objects like anchors or rigid dinghies, or by flexing of the fiberglass structure, are another source. Runabouts and center consoles sometimes get water leakage into the transom, eventually compromising the stiffness and integrity of its balsa or plywood core.These structural problems may have more serious implications than just leaky decks, and may need to be checked out by a surveyor or other marine professional.

Traveling leaks: Traveling leaks are the most challenging to find, as the water migrates inside the deck. The water may be leaking in underneath a stand-up genoa halyard block next to the mast, but it travels through voids in the deck, and drips onto your head from the deck hatch three feet away.

Stanchions: Sailboats have lifelines, and the vertical stanchions that support them are subjected to a lot of stress. Folks lean on them, and they’re used for attaching spring lines or fenders. Often they’re thru-bolted with small, inadequate washers, without the use of a proper backing plate. Then, as they become loose and start to wiggle, the fasteners are over tightened, crushing the deck’s wood core.

Ports and hatches: Any hardware in the deck or cabin, whether it is a deck hatch, ventilation grill or a port, has probably been installed with caulk and/or fasteners. The consensus is that you will get about seven to 10 years out of a deck hardware installation, before the sealant begins to fail and the hardware may begin to leak.

Locating the Source of a Migrating Deck Leak

The Internet is full of helpful-sounding advice on how to track down the source of a mysterious leak. The most popular method seems to be pouring water from a hose over the suspect deck fittings. You move the hose from one fitting to the next, while an assistant watches below and notes when the leak starts and stops. Hopefully, you can then identify exactly which cleat, stanchion, pad eye or other fitting is the source of the leak. This method is also used with the addition of colored water, using several colors of food coloring, and pouring different colored buckets of water on different hardware.

Some folks have tried hooking up the output side of a vacuum cleaner to the inspection port of a small dinghy, sealing everything up, and blowing air into the sealed hull. They then brush soapy water around the outside of every fitting, looking for the telltale soap bubbles as air escapes. I tried this method on my Laser dinghy decades ago, and it worked. I saw bubbles emanating from the hull/deck joint, which I repaired with WEST epoxy.

What to Do When the Core is Wet

If the deck has leaked for a while, water may have saturated the end-grain balsa or plywood core in the hull or deck. This can be a serious problem, and one that may require professional help. Localized problems with wet deck cores are common on many older boats. Extensive damage is a major concern, with the potential need for expensive repairs.

If you’re considering the purchase of a used boat, you will probably get to watch a surveyor tap, tap, tapping on the hull and deck with a little mallet. The surveyor is listening for a nice hollow ringing sound, indicating a dry, structurally intact core. A dull thud indicates a damaged, waterlogged core. Surveyors also take readings with a moisture meter, and drill small holes to take samples of the core.

The WEST System video below shows the basics of how to replace a small section in the core of a sailboat deck.

example of reaming out the balsa core using the bent nail method

Reaming out the balsa core using the “bent nail” method. This is step one of hardware bonding. Image courtesy of the Gougeon Brothers.

Fixing the Leaking Deck Hardware

Now that you’ve found the source, your next job is to remove the offending hardware, clean all surfaces where the item mates with the deck surface, and re-bed the hardware.

The folks at Gougeon Brothers (makers of WEST System products) have developed an excellent method for sealing fastener openings in your deck, called hardware bonding. The video below shows how it’s done. It is one of many excellent online resources they offer on westsystem.com for the do-it-yourself boat owner.

Hardware bonding is a pretty simple process. First, you drill out the top of your hole oversize, about three times the diameter of the fastener. Next, use a homemade tool, a bent nail placed in your drill, to remove about 3/8” of the core. Next, you cover the bottom of your fastener hole with masking tape and inject a mixture of thickened epoxy, filling the hole from the bottom with mayonnaise-thick goo. Finally, re-drill your fastener hole the correct size, and you have a sealed and epoxy-reinforced opening.

example of filling the reamed out core and drilling the hole for fastener

Hardware bonding steps two and three: Fill the hole with thickened epoxy, then after it dries drill your fastener hole.

Replacing and Bedding the Hardware

After sealing up the holes in your deck to protect the core, caulking becomes less critically important, but is still necessary to prevent leaky hardware. To start, make sure that both of the surfaces are clean and dry. Peel or scrape away every bit of the old sealant and wipe down the mating surfaces with a solvent such as acetone or Life-Calk Solvent and Cleaner.

Use the Right Sealant

Many folks use silicone sealant for bedding deck hardware, but polysulfide or butyl rubber tape are perhaps better options. This stuff works great and is easy to clean up. For more on which sealant to use, check our Advisor, How to Select Sealants & Caulk.

butyl caulking tape

Butyl Caulking Tape is a popular alternative to liquid sealants.

Use Enough Sealant

Apply a liberal coating of the caulk so that it will squeeze out around the entire perimeter of the joint. If you don’t see that “good squeeze-out” the joint will likely leak. A thick enough glue line is the key. This is not the place to economize.

Snug the fasteners and don’t over-tighten them: pull the two mating surfaces together so they’re both bedded in the sealant, and the caulk squeezes out evenly. Then leave the components alone until the sealant cures. You can then remove the excess by slicing it off with your razor blade or utility knife. If you over-tighten everything, so most of the sealant ends up on your cleanup rag, and not in the joint, your assembly will leak.

Compress the Seal

Once the sealant is fully cured, you can finish tightening the fasteners, ideally by tightening the nuts, not the bolts in your thru-bolt assembly (turning the bolts will break the seal around each one and allow water to penetrate). Compressing the sealant ensures a watertight bond, even if the sealant does lose its hold.

That’s our best advice for helping you deal with those leaky decks so you can enjoy a dry day or night and keep your deck intact. Good luck!