How to Find Soft Spots and Wet Core in a Fiberglass Deck

Walk every part of the deck, cockpit sole and cabin top in soft shoes. Shift your weight from foot to foot. Healthy cored deck feels firm. A spot that gives, flexes or creaks underfoot is a soft spot. Mark each one with chalk or tape.

Tap with a plastic-faced mallet or a screwdriver handle in a grid, a few inches apart. BoatUS describes a solid "thunk" as healthy core and a dull, flat thud as a sign of trouble. Pay the most attention to the ring around every piece of hardware, since that is where water enters. Outline the dull-sounding area with chalk so you can see its size and shape.

Look for gelcoat cracks radiating from a fitting, a fitting that sits lower than the deck around it, rust or brown stains weeping from a seam, and sealant that is cracked or pulled away. Then go below with a flashlight. Water stains on the headliner, a damp locker, or a dark, stained bulkhead right under the soft area all point to the leak above it.

A pinless moisture meter is useful for mapping how far water has spread, but treat its numbers as relative. Surveyor Bill Sandifer (Good Old Boat) explains that the meter's "fiberglass" scale reads far lower than its "wood" scale, that readings change from one meter to the next, and that surface water and paint can fool it.

How to use one well:

• First find an area you believe is dry (away from any hardware) and note that reading as your baseline.
• Move the meter in a grid toward the suspect fitting. A clear jump above your baseline is what matters, not a single number.
• Outline the high readings with chalk. This map is what a repair person needs.
• Don't panic over one "wet" reading on a dewy morning. Retest when the deck is dry.

A meter does not tell you whether the core is rotten or just damp. Only a tap test, a test hole or a surveyor's judgment adds that.

The wet area almost always surrounds a fitting. Note which hardware sits in or at the edge of each wet zone. If you can't tell, run a garden hose gently over one fitting at a time while a helper watches below (see /guides/deck-hardware-rebedding). Water can travel inside the core before it shows, so the drip below may be some distance from the leak.

Whatever you decide about the core, the fitting that let the water in has to be removed and rebedded properly, or the problem keeps growing. BoatUS's method is to remove the fitting, clean off all old sealant, dig the core back from each hole, fill the hole with thickened epoxy, then redrill and rebed so the bolts pass through solid epoxy instead of bare wood. The full how-to is in /guides/deck-hardware-rebedding. Until you can do it, tape over the fitting with good tape as a temporary cover.

Use your chalk outlines to decide what you have:

• A ring of an inch or two around one fitting: usually handled while rebedding (step 6).
• A small soft patch, roughly one or two square feet: Practical Sailor treats this as a reasonable DIY injection repair.
• Larger areas, several fittings in a row, or a deck that bounces: this means recoring, usually a pro job.
• Anything at the mast step, chainplates or a bulkhead: structural; call a surveyor or rigger.

Sometimes the core is dry but the top skin has come unstuck from it (delamination). In one Epoxyworks repair, drilling a few small test holes showed dry balsa with an air gap under the skin, which changed the whole plan. Drill a small hole from the top (or better, from underneath where the hole won't show) into the middle of the soft area. Dark, wet, mushy or smelly wood on the drill bit means wet core. Dry, light-colored wood with a gap means delamination.

For a small area, Practical Sailor's approach is:

• Drill small holes through the top skin only, spaced a few inches apart, including at the highest and lowest points of the soft area.
• Dry the core completely before adding resin. Epoxy will not bond to wet core, and Practical Sailor says regluing a damp core is "doomed to failure." Gentle heat (a hair dryer, never a torch) until no more moisture comes out, and acetone injected to help carry water out, are their tools.
• Inject epoxy into the high holes and work it down until it shows at the low holes, then tape the low holes.
• Put light weight on the area (with plastic underneath) while it cures.
• Seal the holes with gelcoat or deck paint. Bare epoxy breaks down in sunlight within months.

Owners often find the core won't fully dry with this method. If it stays wet, stop: the core needs to come out.

SAIL's Don Casey describes this for a wet patch around a single opening (like the mast partners): dig out the wet core back from the hole as far as you can reach, vacuum the debris, rough up the inside of both skins, wipe with acetone, and fill the cavity with epoxy thickened with colloidal silica, a little at a time so it doesn't overheat while curing. This only works where you have an existing hole to reach in from.

For large areas, Casey says it's usually less work and a better result to remove one skin, replace the core and relaminate. BoatUS calls extensive core repair beyond most DIYers. Get quotes from a fiberglass shop, and ask whether they will cut from the top (needs new nonskid) or from inside (needs headliner removal). Prices vary widely with size and access.

Photograph your chalk map and meter readings, note which fittings you rebedded, and store it in your BoatMend boat log. Re-tap the same spots each spring. A soft spot that is growing means water is still getting in.