Reducing diameter of rainwater drain pipe ?

MDDIY2022 · Jan 24, 2023

Hi everyone, I have an outdoor open drain which runs along the side of our house and then into an underground 5" pipe which takes the water away to the sewer. I want to pipe the open drain so that the water isn't exposed when it runs along the side of the house.

Please see my attached diagram. The pipe in red doesn't currently exist ... the water just runs into the blue 5".

Is there any point in the red pipe being 9" pipe along the side of the house if it is then going to be reduced to 5" when going underground? Or, should I just make the whole pipe 5" from the culvert. The idea is that majority of debris will get captured at the culvert but still have pipe access at the other end to clear any debris that comes down the pipe.

Appreciate any advice on this!

gmartine · Jan 25, 2023

Maybe your situation requires 9" twin wall (perforated) so that it can also act as a more effective land drain next to your property.

Perforated Twinwall Pipes - Filter Drains - JDP

Filter drain pipes, also known as perforated twinwall pipe, is used for surface water drainage in a range of applications.

store.jdpipes.co.uk

MDDIY2022 · Jan 25, 2023

Thanks for the reply. Currently the open drain is concrete so any drainage from the perforated pipe would remain in the gravel and not soak away. Unless we dig up the concrete so that the drainage from pipe could soak away into the soil ...?

gmartine · Jan 25, 2023

Use 9" pipe or larger, don't forget although the underground pipe is only 5" it starts in the vertical and therefore utilises the full effect of gravity to remove water. You're lay pipe will sit on whatever gradient or fall you can include.