Warm roof over new felt - questions and advice

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Hi all,
After happening across a cracking deal for 150mm celotex-type boards, I purchased a sufficient quantity to insulate the roof in the concrete block built garage the previous owners had partially built.

The current roof construction is 6x2 joists which was overboarded and felted.

Initially I was going to insulate inside (cold roof) but on second thoughts, would prefer to warm roof it with the PIR boards and an EPDM roof over the top.

My intention at the moment is to build a frame using 6x2 around the perimeter of the roof and glue the insulation boards directly onto the felt roof, effectively using the felt roof as a vapour control layer. Use the insulation board fixings to secure the insulation to the joists underneath at strategic points and then board over the top with some 10/12mm ply or OSB before laying the EPDM membrane.

I would tape the inside corner of the framing timbers and seal it to the felt as best as possible

The rubber EPDM would extend over the edges all the way to the bottom of the existing roof across all 4 sides, excess would be overclad with upvc board.

The framing timbers would be primed/coated with a waterproof coating of some sort - yet to sort.

Insulation would be bonded to the felt roof using Insta-stik (first product I found online,if there's other suggestions, please let me know!)

Have i overlooked anything?

My only worry at the moment is, how do i secure the plywood deck on top of the insulation to the joists beneath the 150mm of PIR?

If I screwed all the way through with a 200mm screw, it would go through the top deck, through the insulation, through the felt and into the joist - would this cause potential for leaks (air and water) or am I overthinking?

Should i use tongue and groove decking for over the top so i only need to then screw it into the framing timbers?

On the inside of the building, i guess I can just plasterboard and skim it straight over the joists without worrying about vapour control/breather membranes? the roof does have some degree of ventilation at the moment.

Thank you for your time
 

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TBH, that post is a bit long and I'm struggling with what the actual question is.

But if its about adding at least 150mm on top of the roof, then isn't that exceeding the permitted development eaves height?
 
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I guess it was more for a sense check that I was heading in the right direction

Combined height won't fall foul of building control in our area
 
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Your on the right track, but yes you are over thinking it! definitely use 18mm t&g and buy some 225mm long screws.
 
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