Venting an enclosed roof space for an unheated extension

[retrocoder]

I have a slightly sloped flat roof that forms an extension between my back door and garage. This space is not heated. The roof is just tongue and groove with a felt top. This is sitting on joists that are strong enough to hold a person. What I would like to do is clad the underside of this roof with uPVC shiplap boards attached to the bottom of the joists. This will leave a space between the roof boards and the uPVC boards of about 6 inches. I’m not bothered about having this extension insulated so was going to leave the roof space empty. Should I put vents in the uPVC boards to let the air circulate? If so, should the vents be at both top and bottom of the slope or just at the highest point?