The cabin top rot is just aft of the mast, mainly on the port side, caused, I believe, by leaks in around the stand up blocks mounted on the cabin roof.
The roof is constructed from a layer of 1/4" balsa plywood (I think) to the inside followed by two layers of 1/4" plywood.
As you can see I have removed the mast collar since the rot extends under it. The collar was glued to the deck with a very tough adhesive (perhaps GM 5200) which is near impossible to break free. So, I used a heat gun to warm the collar, probably to a few hundred degees, and used a pry bar with a very wide flat base to apply some upward force. It took close to an hour of continuously moving the heat gun around to collar to break it free.
I've used a sharp chisel to remove the rotten material on each of the three layers. I've left at least 1 1/2" so subsquent layers of patch material overlap the layer below. I'll leave this area open for a week with a heat lamp under to make sure it is thoroughly dry before fitting the new plywood.
Here's the patch in place. It's 2 layers of 1/4" plywood on top of the 1/4" mahogany. It's all held together with screws and West System epoxy. The epoxy was thickened with fiberglas to fill gaps.
Here's the patch in place. It's 2 layers of 1/4" plywood on top of the 1/4" mahogany. It's all held together with screws and West System epoxy. The epoxy was thickened with fiberglas to fill gaps.
I've finished the hole repair at the base of the mast by course sanding the plywood to about 1/32" below the finished cabin roof. I mixed up a batch of West System Epoxy and added some micro balloon filler and troweled it over the patch. It's now ready for the final sanding which I will do when the weather is warmer and I can tackle refinishing the entire cabin roof.
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