Hi Lulu, my husband found these links http://www.ashireporter.org/HomeInspection/Articles/Exterior-Door-Inspection-Will-These-Doors-Leak-/2527 http://www.doityourself.com/forum/doors-windows/284792-leaking-french-doors.html#b and he bought from B&Q plastic strips you glue to bottom of the door on the outside side. these act like little skirts, diverting rain water away from bottom before it pools on the sill . Also resealed around the glass on the outside as the water hits the glass before it gets to the skirts, so any tiny hairline gap is going to let water in which will then flow inside till it gets out again- usually through the beading on the inside. Our main leak is the non moving bit and this is really hard to access without falling out the window! we didn't put the skirt on the bottom of this bit, but maybe it just needs sealing along the gap. the holes should be kept clear as they are meant to drain any water that gets in, out again. I think our builder was pretty good, but like a lot, once the job is over, its a bummer to get them back. Having said that, he is coming back but he advised us to clear out the drainage holes anyway. I think maintenance may be key. But all my googling has told me wooden inward opening doors are really difficult to make watertight. hope that helps.