Re-roofing: Rafters and Ridge boards

[ab12]

Want to re-roof 100 year semi detached property as 

1 ) non breathable felt is failing and allows leaks into the rooms below

2 ) old 3 by 2 rafters in place, appear in good condition and supported by purlins  .Purlins placed on opposite side of the room allowing for flat ceiling between the purlins in the room centre and sloped ceiling between the walls and purlins ( I have attached photograph for reference).

 

Gable end is on the side of the property and not the front.

 

There is a single 1 inch ridge board, about 9m,  from wall to wall of the 2 adjacent rooms,  Ridge board is not slotted into the brick work of either walls, its shy of the brickwork at either end by about 3 inches.

 

There is no visible scarf joints in the board and although its  old and it is in good  condition.

 

Plan:  I want to  install 8 by 2 rafters or even 9 by 2 rafters so I could maximally insulate between the rafters.

 

Questions:  1) Can the old ridge board remain or will I need a new one.?  Had chat with timber merchant, max timber length he can get is about 7.2m. How do I overcome this? Scarf joint between 2 lengths of timber to make one massive ridge board 

 

2: Which is best breather membrane which allows  both air and vapour to pass through it?

 

Roof will be finished off with Fibre Cement tiles.  Currently there some Vented fibre cement tiles  above the flat ceiling spacing between the 2 purlins. Current fibre cement tiles are probably about 15 years old (a guess). Is it worth replacing them? How can I tell if they had their and need replacing

 

I take it as I'm using a breather membrane I wont need vented tiles any longer.

 

Thank you for the advice.