Rainwater drainage

[JGR]

Hi,
A big fan of this site.
I have been looking at the drainage section and cannot seem to find an answer.
I have just built an extension on the house and I will be installing a new soakaway to connect the new gutter down pipes to.
This will be completely separate from the foul water system.
I have got cast iron effect down pipes and shoes so I was going to use a grill underneath each shoe. My question is should I use a bottle gully or some other kind of trap? I have read you should use it on a combined system but are there other reasons for using it in my case.
Thanks

[lutonlagerlout]

Hi james
yes a roddable back inlet gully is ideal or a P-trap
the reason I like them is it stops detritus from gutters being introduced to the soakaway
cheers LLL

[JGR]

Thank you, that makes sense.

[JGR]

Just another question if you do not mind.
My pipe layout will be fairly simple.
The first down pipe and the second are in line about 3 m apart. On the same straight line I was planning to put an inspection chamber in to join in a third down pipe at 45 degrees and then on from the IC to the soakaway. Would using the roddable access drains mean I do not need to put another IC where the second down pipe joins?
Not sure if this makes sense I could draw it if necessary

[Mikey_C]

for the reasons stated by LLL you would still put a roddable back inlet gully or a P-trap, on the second downpipe.

[JGR]

Understood, thank you.