Parapet flat roof drainage

[omph]

Whats the best drainage solution for a flat roof which is sealed off with a parapet? The roof in question is 10m x 2m and I am being forced to seal it off because of the neighbour wanting to build an extension, but this leaves me in a difficult situation with regards to drainage. I will be ripping out the roof and replacing the felt with EPDM. Roof layout image attached.

 

[freddiemercurystwin]

Form a box gutter alongside the new green wall with an outlet popping through above your new drain point looks like the easiest solution, if the existing parapet heights work etc? A photo might help.

 

[omph]

Form a box gutter alongside the new green wall with an outlet popping through above your new drain point looks like the easiest solution, if the existing parapet heights work etc? A photo might help.

I will sort some pictures out.
In the meantime, what kind of box gutter am I looking at here ?
How deep will it need to be ?
I am guessing I will need to create notches in the joists ?

 

[freddiemercurystwin]

Probably about 50mm minimum, ideally the box gutter would also be to a fall but suspect that might be a bit much, you would keep it all above your existing joists, just depends what needs replacing really, if all you're doing is literally changing the roof covering that might not work out, if you're adding new firrings and a deck then it's easy to accommodate, existing parapet/roof heights etc aside. In theory you don't actually need a box gutter, it should all find it's way down to the low point anyway, depends if you want to do the absolute bare minimum or a pukka job.

 

[omph]

Probably about 50mm minimum, ideally the box gutter would also be to a fall but suspect that might be a bit much, you would keep it all above your existing joists, just depends what needs replacing really, if all you're doing is literally changing the roof covering that might not work out, if you're adding new firrings and a deck then it's easy to accommodate, existing parapet/roof heights etc aside. In theory you don't actually need a box gutter, it should all find it's way down to the low point anyway, depends if you want to do the absolute bare minimum or a pukka job.

Pukka, of course.

Fortunately, the roof already has firrings and a good slope, so I don't need to worry about that.
When you say the box gutter needs to be 50mm, is this the depth of the gutter or is it in relation to the highest point of the slope ?
Also, how strong is the box gutter ? If it's supposed to sit on the joists, what happens if someone occidentally steps on it ? Won't it dent ?

 

[freddiemercurystwin]

50mm would be a reasonable depth to aim for but if you're not changing the firrings then I can't see how you would achieve a box gutter at all. You would ideally have a fall within the gutter too but agin that doesnt seem possible here. If you did have a box gutter sat on the joists you would line it with 18mm ply or similar, precisely so that someone can step on it.

 

[omph]

50mm would be a reasonable depth to aim for but if you're not changing the firrings then I can't see how you would achieve a box gutter at all. You would ideally have a fall within the gutter too but agin that doesnt seem possible here. If you did have a box gutter sat on the joists you would line it with 18mm ply or similar, precisely so that someone can step on it.

In that case 50mm is too much. Perhaps I could compensate for the lack of height by increasing width ?
A depth of 50mm(H) x 80mm(W) has a cross-section of 4000mm^2. A depth of 20mm(H) x 200mm(W) would have the same cross-section.

Also, how would you make a 10/20mm fall over a 10m length ? Firrings create too much of a height difference, 4inch/6inch respectively - I've never seen firrings with a smaller profile.