Warm roof conversion

[Space Race]

Looking to remove the old asphalt roof tiles from our log cabin and create a warm roof off the existing 18x94mm tongue & groove sarking. Roof pitch is 17 deg. Initially I’d fix a 2x4 timber ontop of the sarking all around the edge to create an upstand lip, I’d screw both materials but also use galv angle brackets along its length. The 2x4 is shown in green on my pic attached.

 

Starting from the outside

 

1) Box profile metal roof sheets

2) Roof sheet batten (ORANGE IN PIC)

3) Counterbatten fixed through 90mm PIR into 18mm sarking (BLUE IN PIC)

4) Breather membrane

5) 90mm PIR (RED IN PIC)

6) VCL

7) Existing 18x94mm tongue & groove sarking

 

If I’ve missed anything blindingly obvious let me know but my main concern is fixing the counter batten/PIR to the 18mm sarking. What would be the best fixing for this and would it be enough? The way this log building is constructed, The roof is supported by 2x8 purlins rather than rafters.

 

[Mr Punter]

It would give a much better fixing if you could fix the counter battens through to the purlins.  Unless you go through the t & g boards I don't think it will be strong enough.

[Space Race]

I could certainly get a some fixings into the purlins but the majority would be in the 18mm t&g only and that's my worry 

[Space Race]

Another option, I could add some ply board from the underside of sarking (between purlins) just to give the fixings more purchase

[Space Race]

Or rather than ply sheet, Add some 4x2 at say 500/600mm centers with the timbers laid so their widest face 4" was up against the sarking giving me 18mm t&g + 45mm timber to go at. 

[Russell griffiths]

What is this building going to be used for ?

why not insulate it from underneath ?

you are going to end up with some fairly hefty looking facia and barge boards.