Counter batten or not?

[DIYer Damo]

I'm trying to determine if i need to add counter battens to my roof build (new build ground floor extension). I've looked through older posts and on the net but can't seem to get a conclusive answer?

Roof is mono pitched @ 21 degress approx, vaulted ceiling, insulated. Rafters are 150mm deep x 3.4m approx total length and 400mm centres. Tiles will be Sandtoft 20/20 interlocking clay.

I've learnt that counter battens are required to prevent pooling of water behind battens which might find its way through the tiles, especially important if the sarking is flat, say osb board with felt on top like in Scotland. If i'm using some kind of membrane though, which can have the 10mm sag, do i need counter battens at all?
What situations would you NOT use counter battens is probably a better question?

Given the roof isn't that big its no big deal in terms of price if i do / don't counter but thought i'd ask anyway.
Sandtoft site (tile manuf) talks of using nail tape with or without counter battens since modern day breathable membranes don't squeeze as well as felt around nails.

PS - i've not yet chosen the insulation for between rafters but suspect it'll be wool with PIR across rafters at ceiling side - maybe this influences the decision based on air gap or something???

Any thoughts anyone.....

 

[John D v2.0]

Only need counter battens if your membrane can't sag between rappers. Otherwise it might soak through. You might also need them to make up to a level eg semi detached. All other situations no need, in fact better not as the sag keeps the water away from the nail holes etc