"Steve" wrote in message
...

First of all the grain of the planking should run parallel with the plank
edges, so a 7ply plywood plank would have only 5 plys parallel and 3 plys
cross grain. Hence you have reduced the strength to ~ 4/7th what it should
be..


Opps! Should read: ".. 4 plys parallel and 3 plys cross grain.... "

Sorry..

Steve
s/v Good Intentions