AJJewsbury:
1mm dia. seems a bit small to me - you'd likely get a pool of water inside the fitting as surface tension will almost certainly hold the water back against the small force of gravity - there being no water pressure behind it. I think the recommendation used to be for a drainage hole in conduits systems of at least 3mm - and I'm pretty sure MK recommend 5mm for their masterseal range.
- Andy.
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