Flexible sealant for seam in fiberglass roof ???

[nopEda]

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

 

[kcassells]

Try and post a pic.

 

[Scott Danforth]

my guess is you didnt grind away enough of the old to give the new resin and glass a bite

post a pic

 

[nopEda]

my guess is you didnt grind away enough of the old to give the new resin and glass a bite

post a pic

 

[Scott Danforth]

looks like you should have had a few more layers of fiberglass covering the seam and the wood is moving.

how thin is the sheet of plywood your walking on? should be 1/2" minimum with about 1 layer of CSM and about 3 layers of 1708 over it just like a deck in a boat

Hey Woodonglass your thoughts?

 

[kcassells]

Yea yea what what the Grump said.....yea.
That area will def need a glass over of @ 2-3" ea. side over seam. Yikes... How long has it been that way and there may be some chop chop to get back to the dry good stuff.

 

[GA_Boater]

Is the joint over a rafter or between rafters?

Seeing the screws leads me to think the new plywood is butted against the old and the edges meet on the side of rafter not over the rafter. If that's the case, the easiest way to fix the flex is lift the deck again and sister another rafter to the old one to support the new deck, then reglass.

Also, is the new plywood the same thickness as the old? If it is the old glass over the existing plywood should be ground down almost to the old plywood so you are starting with the old and new at the same level. That gets rid of the 1/16" height difference.

 

[Woodonglass]

I need a few more details. What Kind of Marine resin did you use. Epoxy or Polyester? What kind of fiberglass fabric/cloth did you use? What kind of prep did you do to the new plywood and the old plywood surfaces? Why is the NEW plywood flexing at the junction between old and new? This info would really help to determine the issues and the solutions.