If there's an actual hole, try to cover it with anything fairly rigid and tack it down, then cover with the tarp and strips. If you just stretch a piece of tarp over the hole, rain will tend to back up behind the lowest strip of wood and gather in the tarp 'dip'. If it's large enough, the weight of the water can either pull a piece of tarp loose, or a small piece of windblown debris can pucture it and it will split, allowing water to ooze through.

sue