Expansion on a T&G clad gate

[Deansplit]

I have a 12ft electric gate, a box steel frame, 2m high. I have 19mm redwood treated tongue & groove timber to clad it, 145mm wide incl the lip. The boards will be fixed to inner lips on the frame, but do I fit the dry timber with a slight gap (which the tongue will hide) to allow for expansion when wet, or wet it and fit it tight, or fit it dry but tight and not allow for expansion ie. will it buckle at all?

Thx for any help.

 

[big-all]

blind nailing wont work you need to screw/nail into the face or through the frame into the back for hidden fixings
you are likley to get around 6mm movement between boards from the wettest day to the hottest days

 

[Deansplit]

Hi - thx. I was planning on using coach bolts to the frame, but regardless of how it's fixed, is 6mm how much a 145mm piece of timber expands then?

 

[big-all]

yep 5 or 6mm average obviously smaller boards proportionatly less

i built a 30" wide gate from 6" txg flooring the gaps at the min are 4mm ish after several wet days
it was built with 2mm gaps during the heat off the summer to close up tight in the winter
i left 6mm clearence either side by the end off the winter i had to remove 12mm from each side for the gate to close so the boards have spaced themselves out a few mm more

now if you secure board by the tounge they will pop appart with movement

 

[Deansplit]

Hi - I've had an offcut of a dry piece submerged in water for 3 days now - has only expanded 2mm and seems to have stopped - does this mean it's all it will expand?