I don't know which book you got this from but I would chuck it in the bin if I were you because it is incorrect. A warm roof is not vented and cannot have insulation between rafters. That's the point of it. The insulation sits above the joists or rafters and therefore the roof structure remains warm at all times and therefore is not at risk of condensation and does not need ventilation. The OPs question was about whether the insulation can sit on the joists or if it needs a ply base. The Op obviously understands how a warm roof works and his question had nothing to do with insulating between or under joists.
It's a fair comment. noseall more or less answered it in the third post and then posted a link. The broader answer is that it depends on the insulation. The Kingspan noseall linked to sits on the joists and has a bonded ply upper layer to take the waterproofing layers. In other words, no sandwich necessary. Others have different requirements.
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