Building regs H (Plumbing) question

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Malc

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Building regs H 2015 says WC branch pipe can have a slope of 18-90mm/m (page 9) which works out as ~1 deg to ~5 deg. Floplast, for example, do stack connections at 87.5 deg (-2.5deg) and 67.5 deg (-22.5) and 45 (-45 deg). Only the first of those meets the regs H so what am I missing? Or can I only legally use the 87.5deg part?

On a similar note, floplast have an external air admittance valve but regs H does not allow external air valves. Is there some addendum to regs H I've missed?
 
Parts may not count as the branch pipe or be going round a horizontal corner, eg when it turns from the pan connector to the branch pipe.
Also the dry part of the stack has fewer restrictions eg getting around the eaves
 
This isn't the dry part of the stack. Basically I have 2 soil pipes exiting a wall at the same level with the stack in between them. I could use a double 87.5 deg branch so both pipes have a -2.5 deg angle. Or I could use a 67.5 deg with a a 87.5 deg on top. One pipe would have -2.5deg angle and the other 22.5deg. The second is easier to build (and take apart), especially as I want to glue the bend as the pipe comes out of the wall to be sure that is water tight. I don't fancy a soil pipe leak which could go down the cavity.
 
I'm not sure you can have two branches going in opposite each other, to avoid cross flow. The approved documents are just guidance anyway, if the length of the pipe Is short then steeper should be OK, but ask the building inspector if you're using one. The gradients etc are to avoid syphoning of the trap amongst other things so if you're worried then adding an air admittance valve would help.
 
H allows opposite branches if they are swept which a branch fitting is so I reckon that is compliant. With soil pipes there is the danger with too much slope of leaving the solids behind, hence the rule of thumb I believe of 1:40 for 4in pipe, 1:60 for a 6in.
 
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