Discharging rain water onto flat roof.

[paro]

Hi All - Having a bit of a challenge getting the rain water from the main house roof away from the septic tank. At the moment all of the water landing on the main house is diverted into the sewage system, a septic tank with drainage field. It does not leave the property though (this was a whole thing when buying the house).  I was hoping to divert this to a new dedicated drainage run that I've dug for the new extension but due to the slope I'm on and a bunch of other drainage pipe in the way I can't get across to it. 

 

Short of digging a whole new trench which would have to be way longer (80m which is too much) the easier solution would be to allow the water to drain off of the house onto the new flat roof. I am assuming this is just an automatic no but thought I'd ask anyway. 

 

Cheers, Paul

[Nickfromwales]

3 hours ago, paro said:

the easier solution would be to allow the water to drain off of the house onto the new flat roof

and exit it how/where?

[Conor]

In practical terms, no issue if designed correctly. 50% of our main roof drainage goes via our balcony (basically a paved flat roof).

 

Are you worried more in terms of planning / building control in regards to keeping the systems separate? 

[saveasteading]

10 hours ago, paro said:

onto the new flat roof. 

And then where?

[paro]

So mostly from a planning/BR perspective.  I mean I can do this and no one will ever really know (unless the calcs are off) but equally I am trying to do things the right way.

 

The new build extension I am working on at the moment is circa 55m2. I have two drain outlets (65x100) through the parapet  that are going into a new dedicated run of 110mm to a ditch.   The area of the roof I would be adding is 2/3 of the main house so roughly another 30m2. I have a straight run with a good fall (1:60) and no obstacles to reach the outlet on the flat roof. I need to find a calculator but wanted to see if it was in principle something I could do without being a thing. 

 

 

[saveasteading]

Is the question

1. Can my ditch take this extra water?

2. Can the extension  gutters and pipes take the water?

3. Is it ok to drop this water from existing roof onto the new roof? Why do you think "no"?

4. Other.

[paro]

#3. I'm pretty confident that the gutter/drain can take it and the ditch is no issue.  I do need to figure out how the calcs however.  it is really more the BR/planning side.

 

I found this under Part H.

1.9 Where a rainwater pipe discharges onto
a lower roof or paved area, a pipe shoe should
be fitted to divert water away from the building.
Where rainwater from a roof with an effective
area greater than 25m² discharges through a
single downpipe onto a lower roof, a distributor
pipe should be fitted to the shoe to ensure that
the flow width at the receiving gutter is sufficient
so that it does not over-top the gutter.

 

This kind would indicate a yes I think. 

 

 

[saveasteading]

OK. So the existing roof downpipe is cut off by the new extension?

All the water streams across at one point, oversailing the new gutter, hence the regs telling you to spread the flow.

Another way is to add an extra dp to the existing.

 

The weak point in the flow  of a gutter is the outlet. So you could add another or fit a hopper.

 

[Nickfromwales]

I’d look at fitting another outlet, and instead of the water running undirected, you’d contain it within a run of down pipe until it’s out and off the flat roof, and only opens to a hopper when below that level (matching the way the existing outlets discharge off the flat roof; only if you’d rather not have untamed water from the original roof adding to what goes across the flat one?

[paro]

This is what I've done for now. I've left the other drain in place just in case this doesn't work out. I've gone through a couple of calculators and I think the two outlets can handle it but will try and be sure. There have been a couple of mega downpours lately so next time I'll go up and watch how it works.