Hope someone can advise me on a design consideration with my patio project, upon which I'm embarking mostly with the awesome information on this site.
I'm laying a "proper" patio on a ~10m^2 area where before there was only a loose collection of concrete flags and patches of bare top-soil.
The new patio will be riven sandstone flags and the whole area will be edged with setts.
I've included a diagram to illustrate the factors involved rather than attempt to explain everything. But I have arrived at the conclusion that patio needs to fall towards the house, due to the combination of the natural slope of the garden down to the house, the level of the concrete path (A) which I do not want to demolish, and the presence of old brick soakaway (B) which tops out relatively near the surface, all hampering efforts to direct the fall elsewhere.
I've thought of numerous drainage solutions and am considering the following options:
1) "French" / infiltration drain between path (A) and patio (as per http://www.pavingexpert.com/drain14.htm)
This would be easiest to install, probably cheapest and probably look quite aesthetically pleasing.
Also sounds like a lot of fun to build!
2a) Linear / channel drain between path (A) and patio, leading to new attenuator, probably at point ( C ).
By far the most involved to install, almost certainly the most expensive.
Would like to source something discrete like a slot-drain but finding this hard as a lowly DIY-er.
This would have the advantage that if soakaway (B) ever ceases to function it would presumably be relatively easy to redirect gully (D) to attenuator ( C ), assuming it has capacity. Probably never going to be an issue.
2b) Same idea as 2a but somehow hook up plastic outflow from channel drain to clayware pipe feeding soakaway (B).
Probably not a good idea as who knows what the performance of this old brick soakaway is.
3) Create a channel within the paving (setts) falling to a square gully in the top left corner, again to attenuator at ( C ).
Working with minimal crossfall due to level of concrete path may hamper this.
Can imagine it being fiddly to lay.
If I did it right would probably look the best though.
4) Reconsider allowing the patio to fall away from the house.
This would mean excavating an additional 100mm from the back edge, including dropping the path (which fortunately has not yet been paved) even further than it currently slopes.
Disposing of the additional spoil would almost certainly tip me over the edge of my current "hippo" allowance, thus costing another £100 give or take, which would sting a little.
Questions:
- I really do like the linear filter drain concept, but is this too close to the house and might the new water here create a problem for the property? Is an attenuator solution away from the property much preferred?
What about things like wooden fence posts near to the drain - might they deteriorate quicker?
- Would option 3 be workable given the limited cross-fall? Of course I can imagine how to build a slight fall into the in-paving channel, but not sure how to go about calculating how deep to make it, etc.
- Drainage boffins, what would you do in this situation?
Many thanks in advance for any suggestions,
Surreywill.
(EDIT: Worth noting, neighbouring properties on either side of patio)
really you want a fall of 1:80 for a domestic patio will
i tend to avoid falling towards the house unless completely necessary
try and get a 45mm fall away from the house then all your drainage problems are solved
LLL
I think I picked up on a 1:60 figure for patios but if 1:80 will do then that does change things somewhat.
The flags are riven which as I understood may suggest a steeper fall required so I was erring on the side of caution.
I'll recalculate
But, just to confirm, I do still need some water dispersal method (e.g. infiltration drain) even if I can get the water to the correct side of the patio - right?
Just FYI the old brick soakway is under where the garden fork is, and the row of bricks marks where the patio proper will end. The small area underneath where the little hippo bag is will be paved also, but in a separate style.
Many thanks LLL, will do.
But again, just so I'm 100% clear - are you saying I don't need *any* type of drain? Wouldn't the water collect at the base of my raised beds with heavy rain?
the water will drain into gods special drain
the ground
without seeing the site and surveying it ,i cannot be 100 % but generally if you can get it falling out its a better job all round
LLL
Okay, I went out there today and started digging again, but immediately ran into the old soakaway which is literally 1cm underneath my original sub-grade dig level. Reversing my original 6cm fall towards the house plus adding the requisite 4cm for the reverse fall is seeming like an enormous drop when I look at it - I'd need to drop my raised beds and the path by 10cm plus fooling around with that soakaway.
So I feel like I'm back to my original question: would an infiltration drain be sufficient to prevent water causing problems for the house, or do I need to do something more sophisticated?
Don't mean to question your advice LLL - I'd seriously love to be able to reverse the fall and negate the problem altogether but it just seems like a massive undertaking, even compared to digging drains. Maybe I'm just being a wuss about it!
EDIT: Another option would be to build some cross-fall in and have the patio more-or-less level lengthwise. I guess I'd need to make sure there was no run-off onto the neighbouring garden still, but at least the rain wouldn't be headed directly for the house this way...
EDIT2: Buggrit, I'll have another go at it. Agree draining away from house is best. Hopefully the top of the old soakaway can be dropped without too much hassle and causing it to fill with mud!
From your photo it looks a really constrained location. It is always best to maintain the ground with a similar permeability to that existing. If you make it impermeable you risk drying out the soil or with a soakaway too close saturating it. Either can have impact on nearby foundations including your neighbours.
As with any surface near a building you should never fall back. Putting in linear drains sounds simple, but they often (just during the worst kind of storm) become blocked with leaves and silt. Falling away even if it costs more is lower risk in the long term.
Even when you fall away, its best to fall to a soakaway.
Stephen Gibson
Civil Engineering Consultant
Opinion provided in personal capacity.
Please don't hesitate to contact if you require any professional advice independent to contractors / manufacturer. http://www.wilsham.co.uk
