On Thu, 17 Jun 2010 13:10:22 +0100, Fred wrote:

I know you use a Z shape brace on the back to hold it all together but
do you just butt the planks together or is it better to use tongued
and grooved wood?

Without T&G gaps will open between the boards in summer when it dries
out.

Would you glue them or would nails/bolts be enough?

Hot dip galvanised nails or brass/stainless screws. No need to glue,
indeed as others have pointed out the movement will nagger any glued
joint.

I know 18mm T&G is readily available and often used as floorboards; is
T&G available in thicker sizes?

Thicker! Is this just a deterent against access or for real security?
I'd use 4 x 1 PAR for the ledges and braces faces with 10mm T&G
cladding for just a gate to block the way.

What's the best way to mount it? Onto a stout fence post?

At least a 3" square preferably 4" with the best part of 2' in the
ground. You say "across the passage" that implies to me a wall on at
least one side in which case 2" sq fixed to the wall and the gate
fixed to that will do.

What is the best way to lock it shut? Would you use a padlocked bolt
or could you somehow incorporate a mortice lock?

Padlocked bolt with bolts top and bottom.

--
Cheers
Dave.