After some sensible advice from Steve, I am recommending to Simon that, for the stern area, we go for a 4x4 grid which provides support for all sides and ends of the plywood sheets. If there is movement / bounce / sag it risks damaging the waterproof roofing felt which will go on top of it all. For the sake of some extra 4x4 timber, we can avoid this.
So first we coat the deck in bitumen. Then we build the grid (4x4 in red), making sure that the plywood panels will fit fore/aft snugly against the salon wall and then half-way across the first lateral joist and laterally half way across the middle fore/aft joist and so on. Lap joints where joists cross and half-laps when we need to extend a length. Both glued and screwed. Are you following this?
We will get all the lengths and the joints cut beforehand and have the deck and step marked out. The plywood panels and the insulation panels will also be cut to shape before the actual building starts, including all the odd-shaped bits.
With everything prepared and ready, we can choose a dry day which hopefully will fit in with the roofers so that the whole job can be finished and the stern deck watertight before any rain arrives. It would be nice to lay the grid onto the bitumen while it is still tacky, but that might be very messy! When the insulation panels go in, we have a can of insulating foam on hand to seal any small gaps.
Next come the whole and nearly whole boards of plywood, which will be screwed down on all edges either to the 4x4, or to the gunwale step (dark grey). And we finish with the small bits at the stern and at the sides. Gutter sealant or something similar will be applied between the boards and also between the boards and the gunwale and the salon.
The finished, well half finished product will look something like this (Photoshop can only do so much!) .....
Send for the roofers.
Then comes the bow area, following the same sort of procedure. Once the bow and stern are decked and watertight, any rain will run off down the ‘sidewalks’ and out of the drain holes. (I am sure there are proper nautical terms for both of those). Finally we do the sidewalks and “t’ job’s a good un” as they say.
That’s the plan anyway. Any helpful suggestions would be welcome.