Flexible sealant for seam in fiberglass roof?

Discussion in 'Materials' started by nopeda, Aug 20, 2019.

  1. nopeda
    Joined: Aug 2013
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    nopeda Junior Member

    roof1.jpg I have a '68 Lazy Days houseboat with a fiberglass roof. I replaced a section that was rotting, from side to side and about 4 feet from back to front, replacing the plywood beneath and new rafters/joists then covered it with new cloth and West Marine resin. It all cured out very well and I covered the new section overlapping and also covered the old section that was still good to cover spider cracks and it all seemed great. But! After a while the section I replaced being about 1/16" or so lower and tending to flex a tiny bit caused a crack in the fiberglass between the old and new section. I applied more resin and it looked great again. But! After a while there was that crack again. So? Can you folks suggest a dependable flexible product to cover the crack with that will last for years and can be painted over? I was thinking the 3M 5200 through hull caulk might be a good choice. Or maybe White Lightning 3006 40 year caulk. Or are there better products you people can recommend that I have no idea about?

    Thank you for any help with this!

    David
    Buford, GA USA
     
    Last edited: Aug 21, 2019
  2. Deering
    Joined: Feb 2005
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    Deering Senior Member

    The roof is only going to get more spongy over time. I don’t think any flexible adhesive will last years under those conditions. If it was me I’d consider using something more substantial than fiberglass cloth to span the gap. Maybe some biaxial tape. A pic might be helpful.
     
  3. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    If I understand your description correctly you did not taper the old fiberglass at all, you just glassed over the 1/16 step. If that's the case that's your problem, fiberglass does not like sharp corners. You need to grind the step away creating a smooth transition then apply several layers of fiberglass in steps until you reach the initial thickness, then fair and paint.
    The new plywood patch is probably also not scarfed into the old ply. If the joint is not on a beam you also need fiberglass tape on the other side to stabilise the joint.
     
  4. nopeda
    Joined: Aug 2013
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    nopeda Junior Member

    What do you mean by the other side? I can't put it on the inside. Do you mean span the gap with fiberglass tape, then maybe overlap that from the original side and also the new side, then soak it all good with several coats of resin?
     
  5. nopeda
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    nopeda Junior Member

    Will the tape make it a lot stronger? This is my first fiberglass attempt and I was imagining that with a layer of cloth and 4 coats of resin it would be hard and strong since they make boats out of nothing but cloth and resin. So would the tape make it a lot stronger than just the cloth after adding several more coats of resin?
     
  6. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    I mean that if the joint between old plywood and new plywood is not a scarfed one but a butted one this joint needs to be supported on both sides, meaning inside and outside. Now under the plywood and suporting it are some beams (what you call rafters/joists). If the joint between new and old plywood falls on one of these beams you need do nothing.
    So here is the question: have you screwed, nailed or glued the edges of your plywood patch to some "rafter"? Yes or no?

    That step there in the picture needs to be ground away. I suppose that is the edge of the original fiberglass covering the original plywood. You need to grind a slope then fiberglass until you reach thickness again.

    Epoxy resin is not UV proof, it needs to be painted or it will become brittle and delaminate.
     
  7. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    You need to freshen up on your fiberglassing techniques!
    West Systems publishes a very thorough and comprehensive guide that will walk you through it.
    Not much to go on, but I suspect flexing of the structure is the root of your problem, more than glassing.
     

  8. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    Do not apply excess resin, you’ll just be making it weaker.

    The strength comes from the glass, the resin itself is comparatively weak. So if you plan to strengthen the joint, you will need to apply more glass over it.
     
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