Forming Valley - Original roof to extension

[jono_h]

I'm pretty soon going to be at the point of forming the valley between my extension and the original house.

The original house is straight felt, batten, tiled; the extension will be felt, counter-batten, batten, tiled - so I've got an extra 25mm of height to deal with.
Will a standard GRP valley cope with this? (I know they can cope with difference in pitches)

I'm struggling to see how it will work out with all the compound angles and heights.

Thanks, Jon.

 

[robinbanks]

Pic or sketch would help

 

[^woody^]

The two roof planes meet as normal, so you would effectively build the valley up on the counter-battened side so that each side of the valley is the same depth from the top/tile surface.