Like Smoke says, it's a bridge. Any bridge, or simpler to visualize, a beam relies on tension and compression. When you stand on (load) a beam, the downward force tries to bend the beam down. That makes the bottom want to stretch and the top want to compress (get shorter). Fiberglass is is both hard (resists being compressed) and the fibers are strong (resist stretching).

Like he says, when you glass the bottom, particularly the support stringers, the reinforcing in the glass will add LOTS of strength. You do also want to adhere the plywood well to the fiberglass deck above, so that when the load makes it want to compress, it can't buckle (to get shorter) instead of staying in place and resisting compression.

Good stuff. Built a "bridge" last night and it will be plenty strong enough. Just a couple of final questions.
The plywood I removed was dozens of 6"x6" pieces. Is there a reason for this, or can I replace them with 2 or 3 larger sections? I know plywood has a tendency to bow, but wouldn't adding support stringers stop that?
Also, should the supports be attached directly to the plywood and the whole thing glassed over, or glass the plywood first, then add the stingers and another layer of glass?
Sorry for the questions, but working on my back under the deck is a PITA, and I only want to do it once.
Thanks again for all the comments and advice.

the 6x6 pieces have 2 functions(so far as I know).
First, they lay flat when they build the deck upside down vs a large sheet that might be curled.
Second, they prevent water from one screw hole wicking from one end of the deck to the other.

btw, have you ever glassed upside down before? its not easy. especially a large surface. The glass is gonna want to droop and fall off.


Thinking about it...
I would probably use one piece, glass the bottom side on a table and let it cure.
then coat the rest with resin and let cure. Coat the cleaned inside top skin with resin and cure. then use thickened resin as adhesive, slather it on the top of your piece and push it up in place, hold in place with some screws from the top(try to use the holes for hardware that have to be there anyways). Then glass the inside edges.

ARe you using boaters resin(polyester) or WEST epoxy?

Poly is cheaper but I find that I can't make more than 4oz at a time or it gels in the cup before I'm done...I have a lot of hockey pucks!!! and you have to count drops when mixing hardener.
The fumes suck too.

WEST epoxy is very expensive but much more pleasant to use if you have the pumps

I would absolutely use epoxy... I get mine from Jamestown distributors.

If a single piece of ply will fit and lay correctly I would def go that route and def prefab as much as possible.

If you can build outside the boat and simply 'glue' in place you will be way ahead of the game.... The thought of glass work over my head, face, and chest makes me want to shoot myself..... Btw I have to do the same repair soon to my 26' carver.

Thank you (and Skunkboat). A quick measuring shows that it can be done in one piece. Going to go that route and hopefully save a lot of work and time.

Be sure to let me know if an armpit full of resin is as much fun as I think it is....

An eye dropper works well to measure hardener when mixing poly resin and gelcoat. I use the leftover ones that have ml markings on them.

One piece is definitely the easiest (as big a piece as you can get into the cabin) but you need to be sure the surface above is flat enough that the plywood will stick to the whole surface. Take a straight edge, a level, aluminum bar stock, etc., and check the surface for flatness so you know what you're dealing with.

When you dry fit the plywood, make some sticks the right length from scrap lumber to wedge in under it and have them handy when you goop it up to hold it firmly up against the overhead.

Thanks for the tip. Exactly what I was mulling over. I'm going to get 3 or 4 strait pieces of lumber, long enough to reach over the entire width of the top of the deck, and put a bit of weight on each end of them. Then when I fit the new piece underneath I can temporarily shim it using your suggested wedges. As long as the top strait edges remain flat on the deck, with no high/low spots, I should be okay.

I'd brace it up so there's at least some hump in it - you don't want puddles on your deck.