Oh that'll be fun. What's a side Velux anyway?
Presumably it's a potential overlooking problem...assuming it is timber framed you'll need a 100mm or so screw and screw through the moving bit into the fixed bit at the max distance from the hinges. Of course you'll need either big ladders or scaffolding to do it from outside....
The ones in my attic roof have a hole top/bottom on one side, I have wondered what they are for - but thinking now - if I put a dowel in them then it wont be able to be opened from the outside. ???
Mattylad it there so you can reverse the widow and bolt it in position for cleaning, or bolting it open a few inches for ventilation without the wind being able blow it fully open. If BC want it screwing shut from the outside for some reason then a 70mm screw will be enough.
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