What happens is two main things.
One.... the plugs either come out or get sanded/worn down in thickness and pop out or crack. This allow water to enter the screw cavity and then it wicks into the cored fiberglass deck.
Two...the caulking between the teak planks opens up due to many years of expansion and contraction. Then some water stays in over the winter and opens it up a little more when it freezes. Water then finds the screw cavity and wicks into the core.
Horizontal decks are made using "sandwhich construction" which is a plywood or balsa core sandwhiched bbetween two thin fiberglass layers. One water gets into the core it can't get out and eventually rots the wood. Add the freeze/thaw cycling and it will actually delaminate when it gets bad enough. The freeze thaw also opens up tiny cracks or seeps thru corner /edge joints and can get into the interior.
I have a power point of the work I did on my old Mainship flybridge deck that shows how deck are constructed. Send me an e mail to
jleonard@usa.norgren.com and I can send you the file. Same problem except no teak. One pic shows me squeezing the water out of the core...like a sponge.
Some/many companies use a foam core today so the WHEN water gets in it does not rot the core material. Yes it only a matter of when the water gets in. And it always gets in. Esp if the boat has a nice railing or stanchion that a "dock helper" can grab and attempt to pull the boat into the slip against a 20 knot wind. That loosens screws every time and starts the process.
OR as in my Mainship's case, the dealer assembles the flybridge after the boat is trucked in and "forgets" to use caulk for the seat mounting screws...oh yes. I don't think any of the Mainship dealers were allowed to say the word caulk. (oops sorry I digress)
I had Lockwood marine meet us to give me a price to have it done. No word yet, so I guess that we will do it ourselves. Good luck to those other poor saps with teak decks, I knew better abd bought a teaky leaky drooler anyhow... Jay AKA Mudskipper