How to drain a new patio? - On clay... :(

[Ouch]

Howdy,

I'm in the early stages of a major garden remodelling: replacing some rotten decking at the end with a patio, and replacing a badly laid patio (that's using the word generously) near the house with something a little better.

Neither decking or existing patio has any drainage, the existing patio, or collection of flagstones, is laid on mud and rubble.

My current plan involves imported sandstone (ebay bargain) so is pretty impermeable. There'll be a patio at the end of the garden, about 25sqm (8m wide x 3m) and a patio/wide path around the conservatory which sits midwayish along the rear elevation (end terrace). The house drains are combined, though our gutter actually drains into next doors downpipe as they are shared. There's a gulley at each corner of the rear elevation, however current thinking is that surface water should be disposed of elsewhere.

The soil is clay, but the back garden currently is not prone to flooding. It rises gently from the house, about 8m wide and 12m deep (from conservatory). I'd guess at the end of the garden being about 300-500mm higher. Between both paved areas will be lawn (about 57sq m). the conservatory is well constructed, but a little low, the current paving slabs are about 1 brick below the DPC on the dwarf wall.

So, enough waffle. My dilemma:
How do I adequately drain these two areas without massive expense?

Plan A: Linear drain around conservatory, discharging to a soakaway under the lawn. Second linear drain on top patio (Furthest edge, to avoid collecting lawn detritus) discharging to same soakaway. Soakaway size estimated to be 1.2 cubic metres (approx 45sqm patio in total). Both patios fall away from lawn.

Plan B: Land drain network under lawn to redistribute water flowing off patios, to avoid saturating lawn adjacent to paving edges. Both paved areas fall towards lawn.

Do either of the plans above have merit? My next task, after clearing the decking, will be a test pit to see how permeable the ground actually is, I guess that might answer this question. Assuming the worst case, what else could I do?

[lutonlagerlout]

without seeing the site its hard to say but can the patios just not fall to the grass areas?
mine has been doing this for the last 94 years with no ill effects to the grass
and yes we are on clay

however if the patios fall towards the house you need to address the drainage issue
LLL

[Ouch]

without seeing the site its hard to say but can the patios just not fall to the grass areas?
mine has been doing this for the last 94 years with no ill effects to the grass
and yes we are on clay

however if the patios fall towards the house you need to address the drainage issue
LLL

I could fall towards the grass, however the patio is more than 1.5m deep in most places, so I am worried that I'd end up flooding the lawn around the edges..

[seanandruby]

1.5m deep, wow!! That is some 'sunken patio.' ???

[Ouch]

Not 'deep' deep, but deep as in not width...

Am I making sense yet?

[lutonlagerlout]

do you mean wide rather than deep?
picture would be handy
cheers LLL

[Ouch]

No dims on this, but the top patio is about 8x3m:

[seanandruby]

Am I making sense yet?

No

[lutonlagerlout]

ouch what i am saying is that even if the patio is 100 m2 as long as they both fall onto the grass I cannot see a problem
cheers LLL

[Ouch]

ouch what i am saying is that even if the patio is 100 m2 as long as they both fall onto the grass I cannot see a problem
cheers LLL

yeah? Cool.

Was worried due to the advice on this page that suggested draining more than 1.5m onto grass could result in waterlogging, especially given the clay nature of the soil around here..

Draining more than around 1.5 metres width of hard paving onto open ground or garden usually results in waterlogging or flooding, albeit temporary. So, while the 3m wide pavement on the right could be drained to either side, as shown, draining the whole path just to one side, as shown on the left, is probably asking for trouble.

[Tony McC]

It all depends on how free-draining is the ground at the moment.

That value of 1.5m is based on experience and feedback from right across Britain and Ireland, and in most cases, any flooding is more akin to waterlogging and tends to be short-lived. Due to there being so much variation from plot to plot, what would flood one garden might be swallowed-up without a trace by a neighbouring plot.

Do a simple permeability test. How 'thirsty' is your lawned area? If you dig a pit, say, 450x450x450mm, fill it with water and it disappears overnight, you will probably be ok with any of the options so far discussed.

If you have the time and don't mind chewing-up the lawn for 2-3 months, the land drain netowrk would be best as it helps spread the hydrostatic load over a wider area; the soakaway would be second choice because it provides storage and attenuation but in a more localised setting; and simply draining onto the lawn would be all well and good as long as the ground is free-draining and you don't mind soggy grass every now and again.

[Ouch]

Thanks Tony, once I've lifted some of the existing slabs, I will dig a hole and see how it drains, and then report back if it looks like a problem.

Is there anything in particular I need to be aware of in preparation of installing a land-drain - if it is required - that I'll need to take in to consideration during site clearance and levelling? Due to access issues I'm consigned to the fact that the existing lawn may well end up getting destroyed by the plant required to level the end of the garden, so the lawn area may well end up being freshly turfed.

[Tony McC]

Well? How did it turn out?

[Ouch]

Finally got around to digging a hole over the weekend (along with filling a 8yd skip with what looked like the remains of a kitchen wall that I found under the decking).

Hole was about 2ft deep (I could have gone deeper but it was getting late by then), through some sort of fluffy black stuff (ground clinker perhaps?), nice brown, topsoil, bright yellow clay, then hit what looked like a fairly uniform layer of stone, possibly old paving*

Filled with water about 8pm, was gone the next morning, even allowing for overnight rain (I did cover the hole).
I'm assuming this is good, however I'm conscious of the fact the hole isn't very deep (mostly due to the stones I hit, they looked quite big), question is, I guess - is it deep enough, or am I just kidding myself?

*The garden/site/area has a fairly interesting history, as "during the war" it was a POW camp, it was demolished in stages, starting in the 60s, to make the village as it is today. A few years ago whilst excavating the front garden for a gas supply, we dug up a lot of flat stone that looked like it could have been used as a floor in a camp hut. I'm wondering that the quantity of rubble and random stuff mixed in with the subsoil is helping it drain.

[Tony McC]

Might be worth puncturing through that 'stone' layer just to check what's beneath.

However, I'd hazard a guess that you would be OK to install a shallow-ish land drain