I want to pitch the flat roof on my porch.  It is concrete with some sort of membrane coating.  The problem is that the pitch will cross the bottom of the roof coming down at 90 degrees from the right hand side, this is the next door house (semi). The top of the pitch on my porch would finish approx 400mm above where the other roof finishes. How should the section where the two roofs meet be constructed?

not shure iff i understand this

the way you should do this
ridge on the porch goes back and ties into the origional roof
you then have a valey between old and new

http://images.google.co.uk/imgres?imgurl=http://www.tpub.com/content/constructio...

Didn't say anything before because I think the whole structure is going to look odd.

I've done similar jobs where BOTH parties have had their shared porch re-roofed to keep the symmetry. Given that you're talking about such a small area the extra cost of doing both will be marginal and would make far more sense, improving the look of both properties and, arguably, adding something to the perceived value in the eyes of future prospective buyers.