Oil furnace flue

[Mike in Ohio]

I have a fuel oil furnace to put in my shop, we are moving it from my dads shop. He has cement block walls so the flue was not a problem. My first question is which is better straight up through the roof or out a wall. If I go through the ceiling it wll be 10 -12 feet to the roof from the top of the furnace. The flue on it now is 8" I think, so this will leave it 7" from the bottem cords of the trusses as it goes between them. Will the double wall pipe be enough that far away from the furnace? And how big of a gap between the pipe and the wood/shingles do I need?

Also would you use the double walled pipe all the way up or just near the roof?

Thanks for your help,
Mike