A teak deck is a wearing surface. Every scrub and sanding takes some wood off, until the plugs over the screws are too thin to stay in and the seams too shallow to hold their compound. On a screwed deck, every lost plug is then a hole into the deck below.
How a surveyor checks it
Plugs. Count missing and loose ones. Argo Yachting says plugs popping out mean the teak has worn down to the fastenings. Screw heads showing settle it.
Seams. Argo says black, split or lifting seams let water in. Compound standing proud of the wood means the planks have worn down around it.
Thickness. What's left above the fastenings decides the deck's life. Lift a plug where one is already loose, or measure at a hatch or the deck edge. PBO's shipwright describes worn decks where the best plug was only 3 mm deep.
The deck below. Where plugs or seams have failed, take moisture readings and sound the deck with a hammer, and feel for soft patches underfoot. Argo notes that soft or springy patches suggest water has reached the core. Bonded teak decks, glued down without screws, avoid the fastening holes; check the bond instead by tapping for hollow areas.
What decides the grade
The deck under the teak. Wet core under failed teak is an A, because the structure is now affected. Teak worn to the fastenings or many plugs missing is a B: water is getting in, and the deck is near the end of its life. Proud or tired seams on thick wood are a C.