Roofing

Hey, I'm building I tiny house for myself and I have the rafters up. I'm now thinking of sheathing for metal. My insulation is 8.5 inches thick and my rafters are 9" 1/4, is that 3/4 of an inch enough air space in a vented roof? I’m in zone 5, The pitch is 6/12.
I'm thinking I might put vertical board above the rafters to get an extra half inch or inch but I wouldn’t if I don’t need to. Also I'm curious if I can just use purlins to attach my metal roofing to instead of buying plywood? I have some good waterproof underlayment.
I would really appreciate appreciate any input!
I have seen a video where the guy hangs his underlayment down a bit under his purlins between rafters so when water condensates from the roof it has a place under the horizontal purlins to flow down the roof and not get stuck behind the purlins.
I like this idea but I’m worried that if I did that without adding more wood on my rafters that might not be enough air space 🤷🏽‍♂️

Have you looked at drawings detailing what you ask?
I think the answer to help you is to try what we call builders blanket in Australia.
Its is rockwool style of insulation bonded to a reflective 'sisalation' and layed over the roof purlins
from the high point to the bottom of the roof.
It prevents condensation forming on the underside of the metal roof.
I highly recommend the use of metal roofing.
I use 'purlin' screws which are now available to hold the purlin to the roof joist.
And use Hex headed roofing screws at least 65mm long to attach the metal roof to the roof purlin.

Angus -

You might have a look at the US Army's Cold Regions Research and Engineering Labs report on attic insulation and ventilation:
https://www.poa.usace.army.mil/Portals/34/docs/engineering/MP-02-5778,%20Guidelines%20for%20Ventilating%20Attics%20and%20Cathedral%20Ceilings%20to%20Avoid%20Icings%20at%20Their%20Eaves.pdf
I suspect the "Cathedral" section will apply, perhaps Figure 8 in particular seems close to your specs.

I am in the process of building a small gambrel roof shed (10 X 16 feet outside dimensions, 2x6 nominal framing lumber).  I intend to use metal roofing over light 1X nominal (3/4" or 19mm actual thickness) purlins on 2X (1-1/2" or 38mm actual) sleepers over the plywood roof deck.  The sleepers will be screwed down over the rafters, and I'll put down synthetic roofing membrane (the modern equivalent of tar paper) over the roof sheathing plywood before the sleepers are screwed down.  This will provide an air gap between the structural roof deck and the metal, and will allow me to apply 6" of rolled fiberglass between the 2X6 rafters under the roof deck.  The 1-1/2" air gap venting to the ridge is slightly excessive, but will hurt very little.  The synthetic roofing membrane will shed water, including condensation, if necessary.

I don't know whether any of this is directly applicable for you, or if it is advisable for your particular situation.

Kevin