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2x4 for purlins seems massive! Do you have a link to the install specs?
If you have rafters & osb, I'm not sure why you'd need purlins at all - just battens for attaching the cladding.
Cross-battening for channel cladding would be to allow airflow, rather than water flow. If your cladding is corrugated, though, I don't see any need.
NotThamesWater
hoefla
I am midway through something similar and am poised with a quote from Cladco for roofing, but am unsure about a couple of details, then I stumbled across this last night, can I tap into your experience and ask you for some insights? My roof so far is the same as yours: rafter -> 11mm OSB -> membrane.
How big were your purlins? I have to keep under the 2.5m height and it is getting tight. Installation guides for metal roofing often seem to use 2"x4" purlins but I am wondering about 2"x1" to save some height. Yours look like they might be on the smaller side too?
Did you think about counter battening under the purlins for the drainage channels down the roof? I can see the logic of this, but it adds height, whereas surely there is not that much water going to get under the roof such that it'll pool up against a purlin.
When a Cladco fixing screw says it is 45mm is that the total length of the screw, or the length of the thread under the washer & head? Like you I am trying to work out how to make sure I don't screw through into the interior, I can see why using massive purlins in appealing here for a big margin for error.
Great write up, I enjoyed the details, lots to learn from, and it is a really slick result.