Hello,
Through mortices and drainage holes 'n all address the water problems to a point but by sealing the ends with wax, for example, the natural ability of the wood, once is has gotten wet, to dry out is reduced. It's the trapped moisture rather than flowing water that causes the eventual damage as can be seen here.
First the water gets in there, the microbes and the vermin do the rest 'till the whole construction starts to sag, funneling even more water through the low spots where it stands and just wrecks the whole thing and you have to spend weeks repairing it all.
Greetings,
Don Wagstaff
