Second opinion: are my roofers doing a good job?

[S1llyB1lly]

I'm having an overhaul of my ground floor flat. The builders I am using came via recommendation, although despite this they have done one or two things, which I haven't been happy with (fixed however once I pointed out, but had I not noticed it would have been swept under carpet).

Anyway. Back addition consists of a flat warm structure deck and pitched sides.

Warm deck structure (top down):

• GRP fibrgelass (think TuffSTuff / CureIt)
• 18mm OSB3 T&G board
• 100mm Celotex insulation
• Vapour control layer
• 18mm OSB3 T&G board screwed into rafters
• Vapour resistant ceiling plasterboard

Pitched sides structure (top down):

• Fibre cement slate
• Horizontal batten
• Counterbatten
• Breathable membrane (Tyvek Supro)
• 120mm Celotex in between rafters
• Vapour control layer
• Vapour resistant ceiling plasterboard

Images give an idea of construction / scale.

Now. I checked up on site today and noticed following - no idea if it its right or not?

1. Orange vapour control layer was laid onto 18mm OSB3. Celotex was added on top and then another layer of OSB3 on top of Celotex. Fine, but instead of using helical fixings to secure Celotex to 1st layer of OSB3, they used 150mm screws to drive through OSB3, Celotex, Vapour Control Layer, OSB3 attached to flat roof joists in one go. Fine I'm guessing as, after all, there are many ways to skin a cat?

2. The orange Vapour Control Layer will be cut horizontally, what you see in the pics is the excess flapping about. Once cut, shall we seal it with aluminium tape to OSB3 / Celotex?

3. What sort of expansion gap is necessary when butting up the OSB3 against the building? 3 to 5mm sufficient?

4. I have acquired some fibre cement slate, 600mm x 300mm. Guy doing tiling says that he needs 600mm x 600mm as well, for the starter course and can't begin without those. I thought starter course was simply 1 + 0.5 slates or is that wrong?

Any comments welcome!