Air Admittance valve on long shower waste

[mr-b]

Hi

I'm adding a shower in my loft and the waste is going to be long one, routing downstairs into the 40mm waste after the bath, and the advice seems that I'll need an AAV at the top of the run.
It'll look a bit like the attached ASCII art.

Yes I realise the vertical drops aren't ideal but there really isn't any alternative.
Normally I understand the AAV should be on a 200mm vertical stub pipe off a tee on the shower waste but that is going to be rather problematic as I'd have to build a maintenance access panel in the shower enclosure studwork. I understand that BS EN 12380 A1-rated AAVs can be installed below the flood level of connected appliances ( I'm not sure how this relates to a shower tray waste) but how far below can they go? McAlpine mentioned the spill over level - is this the same as the flood level? I was hoping to tee off in the cupboard below the loft floor and then have the AAV on a vertical stub pipe accessible from there. Or does it really have to go above the shower waste, even if it's A1 rated?
All the advice I've seen mentions basins as being the flood level but nothing about shower only wastes.

Also what determines the size of AAV in terms of air flow ? Is it just whatever fits the appropriate pipework or are there other considerations e.g. flow rate of waste water or waste pipe length? Should I be considering >40mm AAV?

Any suggestions gratefully received.
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Slightly better diagram

 

[oz-plumber]

I think the AAV will be the least of your problems.
From what I can gather, you intend to run @ 12m of 40mm pipe from your shower, tee into a 40mm pipe and then into a stack.

It might work if the people who use the shower are hairless and don't use soap.

You need to get a bigger drain pipe to as close to the shower as possible.

 

[mr-b]

Tx, I can certainly put up a notice. Just like I do with the Saniflo ... ;-)

I could run 50mm for the first 5 or possibly 8m but how would that help exactly? Is it the vertical sections that are problematic or just that it's a long run?

 

[oz-plumber]

Over that distance, I don't even think 50mm pipe would comply with regulations