Downpipe Across Extension Roof

Pierrebandestande

Dear all,
First time poster, so Apologies if this is OTT or poorly put together.

I'm living in a Victorian terraced house with a shared downpipe (I can hear the groans already). Both properties have a rear extension that pre-dates either ownership. At present the existing downpipe discharges via a shoe direct onto the tiles of the extension roof, before being collected by a standard 112 half round gutter and thence into the downpipe (see image)

My neighbour has recently had their soffit and fascia replaced, and has come to us, requesting that we "sort out your guttering". Their premise for this is that the additional water from our roof is causing damage to their downpipe and also to the fascia and soffit on the extension as a result of overspill/splashback on their extension gutter. They have not suggested what they want done, it would appear that they want us to come up with a solution.

I have done some rough numbers based upon BSEN-12056-3:2000 (haven't used the National Annex) to get an idea of the situation. Using these numbers I make it that the extension gutter is undersized.

As I see it this leaves me with 2 options (to suggest to the neighbour)
a) Upsize the extension gutter, leave the main roof flow running over the extension roof
b) seperate the two flows (main roof and extension roof)

In terms of
a) A squareline or hi capacity gutter can cope with the calculated flow - see image:

In terms of b) I was looking for some advice - can I run a downpipe above the roof and discharge direct into the running outlet on the extension gutter, like this:

or am I better off suggesting that the roof flow is only added into the downpipe direct?

Which option do you beleive to represent the best solution.
Also, with 67.5 offsets, I end up having to support the downpipe with shoes off the party wall - how much tolerance is there in the joints between offset bends and standard pipe sections, i mean, could I just rest the cranked downpipe on the roof and take up the difference between the the 22.5 angle and the extension roof angle in the joints?

Many thanks in advance, I'm doing my best to be a good neighbour but falling short in terms of the practicalities of guttering!

Can you post pics of the rear elevation with both you and your neighbour's elev showing? Does this RWP drain both your properties, or only your roof?
Unless access and new falls are too difficult, you might be better off bringing the main roof outlet and RWP away from the property line, and over to the right.

However, you might also come off the bottom of the RWP at the shoe, and keeping it just above the parapet flashing, clip the RWP extension to the parapet wall, and just before the gutter ( say 12") come over with a 45 degree fitting(s) and discharge into the gutter with a downward pointing shoe. This will keep you away from your neighbour and give a splash free discharge.

If you go on your roof use a roof ladder, and tie in any access ladder.Dont work alone.

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