To French Drain or to not French Drain?

[LukesLog]

Update on this. The downpipes/gullys at the front of the property discharge less than 2 feet away, into clay soil. The recent storm showed up the deficiencies, with the exterior of the house sitting in 4 inch deep water for hours.

My intention is to build a 1000l soakaway with crates, 5 metres away from the house, with the 3 gullys piped into it.

 

[a twig]

There are various soakaway size calculators available online to convert the area of your roof into what soakaway volume you need

 

[a twig]

I’d suggest doing that then adding 15% - particularly if you are in clay soil. We had quite a large roof area for 3 down pipes and that plus 20% ended up being 3m cubed or 3000l. So worth double checking before you go to all the effort and end up with something undersized

EDIT: I’d personally recommend having a trap with inspection access just before your soakaway as well - yes it becomes another annual maintenance task to clear it out but it will drastically prolong the life of your soakaway

 

[LukesLog]

I’d suggest doing that then adding 15% - particularly if you are in clay soil. We had quite a large roof area for 3 down pipes and that plus 20% ended up being 3m cubed or 3000l. So worth double checking before you go to all the effort and end up with something undersized

Thank you that's a great call, will do the maths on it this evening.

Once that is tackled, my remaining issue will be this concrete slab. Coincidentally we've had a new boiler fitted today, and when drilling for the condensate pipe, the contractor noted his drill bits coming out wet, with the brick(inside this concrete render just above floor level) being 'sodden'.

Concrete slab details in my other thread here (found after this thread was started)....

L

Thread 'Concrete drainage slab confusion.'

Sep 28, 2025

Hello,

As part of my war on damp in our 1850s solid brick cottage, I've spent the last 2 days pulling up crazy paving patio stones in the front garden, as the ground level is a couple of inches hire externally compared to the interior.

Ive got to the stones against the house this evening, to find what seems to be some form of concrete or stone slab against the house, coming out around 1m or so from the wall.

This wouldn't have been a problem, as id still be lowering the exterior a good 15cm all round even at the level of the concrete, but it seems it raises up on either side, clearly...

• LukesLog
• Replies: 7
• Forum: Period Property Discussion Forum