Coosa Board or Ply - Spanning 10 foot for a deck

Discussion in 'Fiberglass and Composite Boat Building' started by member78997, Nov 20, 2023.

  1. member78997
    Joined: Nov 2023
    Posts: 1
    Likes: 0, Points: 1
    Location: Mobile, AL

    member78997 New Member

    Hi all,
    My Bay boat has a large hole in the roof that has been repeatedly repaired, which has led to further damage throughout the roof, and it is time for me to simply replace the whole roof.

    There seems to be no cost effective simple solution that I can find for the size ( 24’ x 12’). Plan is to “sardine can” the old roof, add 6” of height, and then bolt a new roof to the top, attaching to the “sardine can” edge of the old roof.

    Ideally, I’d like to simply box out six 12x4 sheets, with ply support joists, reinforce the whole thing with resin/mat underneath. Install one at a time, then seam it all together once it is on by laying a few more layers of resin / woven mat. IMG_9244.jpeg

    Trying to determine if I should go with Coosa Board or Marine Ply. Neither seem to come in a length long enough to span the entire roof unfortunately. Coosa is double the price, but seems to be unlikely to fail on me again.

    I’m replacing 1/2” fiberglass.

    This is the temp fix I did while I was gone for the summer.

    Appreciate your thoughts.
  2. Rumars
    Joined: Mar 2013
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    Location: Germany

    Rumars Senior Member

    Sorry but I don't understand your problem. Questions:
    1. Why is there a hole there, was it a soft patch?
    2. Why did the old repairs fail?
    3. What is the old deck made of? I doubt any repairs could lead to damage if it was indeed 1/2" solid fiberglass.

    As for replacement you have 3 options:
    1. Solid fiberglass. Buy or make a sheet, install. Scarf if necessary. It's heavy, so only do it if the original was indeed monolithic glass.
    2. Plywood with glass on top. Plywood can be scarfed to any dimension you like, on or off the boat.
    3. Cored. The core can be wood (balsa, cedar, etc.), plywood or foam. Coosa isn't necessary, you can use PVC or PET foam, it's cheaper.

    As for your "sardine can" ideea, why not do it the proper way? If you need more stiffness just increase the core thickness.
    rwatson, BlueBell and bajansailor like this.
  3. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    Wet carpet is not a good roof covering, that may have promoted the rot situation, especially if there is no crown.
    How about showing more pictures from other angles?
    rwatson and BlueBell like this.
  4. Darkzillicon
    Joined: Jan 2023
    Posts: 143
    Likes: 50, Points: 28
    Location: Palm Beach

    Darkzillicon Senior Member

    Coosa is great but the stiffness isn't quite as good as plywood but if you use it make sure to use the "bluewater26" that's the stiffest one to span the distances that you are talking about. the heavier (weight) that you make the deck the more it will want to flex underway so that's another issue. cored foams like airex and balsa are good options but it depends how many people you want to put up there on top of it. I would probably try to build some crown into the roof and just use standard marine ply as a core between sufficient layers of fiberglass. This will give you a nice stiff deck with some crown to let the water off.

  5. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    There is no simple solution. You are seeking one and that is the first mistake.

    A 12' wide roof needs structural support and that can be achieved a few ways. Might be there now..

    Without knowing the structure below; we would all be guessing at advices.

    Also, is the roof used by large groups?

    Coosa is rather heavy for the strength returned. Even 20 pound density coosa is not my favorite here.

    I like a built up roof because it is simple to execute. Remove the old. Epoxy glue and screw 1/4" plywood over the ribs, 2 or more layers for more than one person walking above, epoxy glue marine foam above; then glass with something like 1708 csm out.

    Some tweaking of my guidance would be needed for structural deficits below it. I like the plywood because you don't need to glass the bottom side..and it conforms to most crown.

    A proper execution and water would never migrate to the plywood layers..
    Darkzillicon likes this.
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