As far as I know, a big figure can target and be targeted from the ground wether or not it is on the border square of the roof. Hence my question.
Am I wrong on this ?

LoF between Sentinel and an elevated figure is always as if both are elevated. LoF between Sentinel and a soaring figure is always as if both are soaring.

LoF otherwise is normal, other than the Sentinel's ignoring of intervening figures. So if Sentinel is on the ground, LoF between Sentinel and a grounded figure is like LoF between two grounded figures. If Sentinel is elevated, LoF between Sentinel and a grounded figure is like LoF between a grounded figure and an elevated one.

In other words, if Sentinel's on the roof, he has to be on the edge to shoot down.

So in other words, a grounded Sentinel sees and is seen by elevated figs that are not necessarily on the border square of the roof ?
But on the other hand, an elevated Sentinel does not see and is not seen by grounded figs unless it's on the border square of the roof ?

If it's really like this.. I've just played it wrong for two tourneys in a row and nobody noticed... the worst thing is that I've won both and therefore wronged many of my opponents.

You may have misused the rule, but you didn't wrong them. Everybody makes mistakes.

But, yeah, you have it right now.

IMO, you didn't wrong them because you didn't fully understand the rule.

Now, if you do it again, knowing the correct rule, you will wrong them.

There is a difference between ignorance of the rule, and blatently ignoring the rule....