i go bottom up
dependant on roof you will have to leave 8mm gap at the top to get the last plank over the tounge
so i work up from the bottom and remove the back off the grove on the last plank and screw an off cut strip in its place
i also screw in case off shrinkage requires the boards to be closed up
top down is fine just remember when you get to the bottom the wood should be separated from the ground and any concrete/brick
if its extremly low moisture content then leaving a 2mm gap is advisable and only one fixing per board in the top or bottom 20% off the board to stop cracking by allowing expansion and contraction
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