How to Find Cracks in a Fiberglass Deck

Wash away dirt, salt, mildew and wax that could hide fine cracks. Allow the deck to dry completely before inspecting it.

Do not sand the area before the inspection. Sanding can remove evidence of a crack or make it harder to determine where the crack began.

Inspect one small area at a time so nothing is missed. Painter’s tape can be used to divide a large deck into manageable sections.

Pay particular attention to areas where stress commonly develops:

• Around cleats, railing supports and other deck hardware
• At scuppers and drains
• Where horizontal and vertical surfaces meet
• At corners and changes in deck level
• Around hatch openings
• Along the edges of previous repairs
• Anywhere the deck flexes underfoot
• Areas where water has previously leaked below

Hold the flashlight close to the deck and aim the beam almost parallel to the surface. The light should travel across the deck rather than shine directly down on it.

This grazing light creates tiny shadows along irregularities and makes fine cracks easier to see. Move the light slowly while viewing the area from the opposite side.

Repeat the inspection with the light coming from several directions. A crack that disappears when illuminated from one direction may become obvious when illuminated from another.

Adjust the brightness as needed. Maximum brightness is not always best because glare from a white or glossy deck can conceal small defects. A medium setting often provides better contrast.

Use the 5X magnifier to inspect every line that appears questionable. Look for:

• A narrow, sharply defined opening
• Branching or spiderweb patterns
• Dirt or discoloration trapped in the line
• Separation along the edge of an earlier repair
• A crack that continues beneath paint, caulk or hardware
• Slightly raised or separated edges

A surface scratch usually looks wider and more irregular, with visible abrasion alongside it. A crack is more likely to form a distinct line extending into the coating or laminate.

Place a short piece of painter’s tape beside each suspected crack. Number the locations and photograph them with a ruler in the frame.

Record the location, approximate length and direction of each crack. Good photographs make it easier to determine whether a crack is growing or reopening after a repair.

Apply light hand pressure around the crack and watch it closely. Do not jump on the deck or apply enough force to cause additional damage.

Movement, flexing, clicking or a crack that opens slightly under pressure may indicate a problem beneath the surface rather than a simple paint or gelcoat crack.

Lightly tap the deck around the crack with a plastic screwdriver handle or similar lightweight tool. Compare the sound with a nearby area known to be solid.

A solid laminate generally produces a sharper, more consistent sound. A noticeably dull or hollow sound may indicate a void, delamination or deteriorated core.

A tap test is comparative and does not prove that damage exists. Deck thickness, internal structure and hardware can also change the sound.

A moisture meter can help identify areas that deserve further investigation, particularly on a cored deck. First take readings from nearby areas believed to be dry, then compare those readings with measurements around the crack.

Treat the readings as relative rather than exact. Paint, surface moisture, metal fasteners and different core materials can affect the results.

A crack accompanied by elevated moisture, softness or a hollow tap-test response is more concerning than an isolated hairline surface crack.