Suggested Falls on a Valley

[McCracken]

I'm currently building a valley against a parapet to deal with rain water drainage, could anybody give me an idea on what fall I should have on a 10 metre valley coming off a pitched roof? I have been using 1/80, but even that looks excessive.

 

[McCracken]

I should say I'm trying the least fall possible as I have roof windows to fit on a hip without much room for manoeuvre 

 

[markc]

You can get away with almost zero fall as long as the valley/gutter is large enough to handle the amount of water without overflowing.

steeper gradients get rid of the water faster and therefore require less volume

[McCracken]

Thanks Mark

 

[tonyshouse]

For me if you can’t design it out 1:40 and please add sluices through the parapet 

[Dave Jones]

21 hours ago, McCracken said:

I'm currently building a valley against a parapet to deal with rain water drainage, could anybody give me an idea on what fall I should have on a 10 metre valley coming off a pitched roof? I have been using 1/80, but even that looks excessive.

 

 

The required fall depends on your roof construction to an extent. NHBC covers it in detail:

 

https://nhbc-standards.co.uk/7-roofs/7-1-flat-roofs-and-balconies/7-1-5-drainage/