Thank you for your comments guys sorry I was busy yesterday. 
 
	 
 
	It looks like I am best probably adding 10% to the total floor area to be safe. 
 
	 
 
	I couldn't find the images I was going to post but will if I can find again. I added a quick rough phone sketch of the rough roof type. 
 
	 
 
	Thanks for all your help guys so far.