French Drain Queries.

[TudorCottage]

Good Evening

Due to pooling water in heavy rain I’m in the process of renewing the French drain system around our cottage.

The current system comprises of 2 60mm drains laid side by side with a membrane just laid across the top of the drains rather than wrapped around them. It feeds into a soak away approximately 15 metres away from the cottage, the new drains will follow the same route.

I’ve removed the old drains and now have a blank canvas to replace with new drains. Space is not tight and thought I’d gauge opinions on here.

Is there any benefit in increasing the size or the number of drains?
i.e 3 x 60mm, 2 x 80mm or even 3 x 80mm. I think 100mm is getting too big.

Also, I’ve lowered the path to the bottom of the stone plinth and after digging a further trench for the drains was going to back fill with 20mm shingle back to the top of the plinth. Is there any advantages/disadvantages of going lower. It’s soft clay so whilst not easy it’s not back breaking.

Thank you for any opinions.

 

[a twig]

Areas of circles being what they are (and thus cylindrical volumes) a single 100mm diameter gives a larger capacity than 2 x 60mm.

If you want to increase volume on the existing I’d just do it once with 100mm

If you are feeling really enthusiastic 160mm is available and would be equivalent to ~7x 60mm diameter pipes!

 

[a twig]

Are there benefits?

Well larger pipes can cope with a larger volume of water and are less likely to get blocked, can be easier to maintain as well provided you give yourself suitable access / rodding points.

Is the soakaway new / been renewed recently? Otherwise you might end up just moving the flooding somewhere else. Our old soakaway from our gutters was both too small for the area of roof and completely clogged as it was 40 years old, so we resorted to digging a suitably large new one further from the house

 

[TudorCottage]

I thought maybe covering a wider distance might capture more water. For example spaced at 50mm intervals giving a width of approximately 300mm. Path is about a metre wide and takes a hammering in heavy rain as it runs off the thatch.

I rod the pipe leading to the soak away every so often and that always seems clear. Also the drains empty into a tank that holds about 300 litres of water and the pipe leads to the soak away from there. If the soak away does clog there is some margin to play with.

 

[Cubist]

In my experience it's more about how quickly any water captured leaves the exit of the pipes. One of my drain pipes emerges above ground on a slope down to a ditch but another terminates under the top soil headed toward the same ditch 50 feet away. During heavy rain and when that soil is sodden the feed pipe can quickly fill to induce puddling against the house - that's why I installed the second drain.