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Posted: Sun Feb 03, 2013 5:00 pm
by samselbygarden
Hiya,
Im installing a patio coming out from a new extension this week.
The client wants linear drainage channels put all around the perimeter of the extension to move rainwater from the roof.
The patio will be sloped away from the building and i'm slightly concerned i'm not going to be able to get enough run off whilst keeping it matching to the clean horizontal lines of the new extension. The extension is 5x3m approx so with an adequate fall (1:40?), there is going to be a significant height difference at either end. And thats withoutmatching the patio to it.
My question is do i actually need to put these drains in? Or will a sufficient fall on the patio away from the house be enough to carry unwanted water away?
Many thanks in advance.
Posted: Sun Feb 03, 2013 8:03 pm
by Carberry
Which slabs are you laying? The fall you need depends on if they are smooth or riven
Where will the water run to if you don't put a drain down? is it going straight in to clay? is it going on to turf which is level with the patio? flower beds?
Posted: Sun Feb 03, 2013 8:14 pm
by r896neo
The client wants linear drainage channels put all around the perimeter of the extension to move rainwater from the roof.
What does this mean? Are they not having guttering of some sort? In which case bottle gulleys or similar is more appropriate.
Being 150mm below dpc and falling away from the building is more than satisfactory to keep the building dry but dealing with the run off is a seperate issue? How big is the patio and as Carberry asked what are the surrounding levels and ground type.
If the extension is ongoing or not yet signed off then building control might have something to say on the matter
Posted: Sun Feb 03, 2013 9:17 pm
by samselbygarden
Hi, thanks for the feedback guys.
Carberry - We're laying Black Limestone slabs, so a smooth finish.
The patio will come out 1.5m in front of the extension and approx 3.7m out to the left side. It will be raised 450mm above garden level to match the extension height and will have a step or 2 down to the garden .
So with a fall away from the house, the water can potentially drain towards the garden and away from the house. (garden level is just lawn but potential for beds too.
The reason for the drainage between the patio and the extension is because he has a down pipe a long way from the nearest drain. His thinking is to carry the water away in said drainage channels.
But thinking about it, it's going to be more beneficial to direct that water using guttering as opposed to putting in drains with an adequate fall right next to the house?
Am i answering my own questions here?!
Basically he's worried water from the roof will sit on the patio so he's hoping to divert it with the drainage to the nearest drain, but if there's sufficient run off on the patio is this necessary? A lot of extra time/money/fiddling???
If he wants to keep his down pipe in the same place what other options are there? Could we run a pipe under the patio to drain into a soak away in the garden? Water butt?!
Posted: Sun Feb 03, 2013 9:17 pm
by lutonlagerlout
you need underground drainage for rainwater pipes
when it rains hard those downpipes pass a lot of water
also 1:40 is close to a slope
1:60 is more appropriate for a patio
LLL
Posted: Sun Feb 03, 2013 9:42 pm
by samselbygarden
Ok so i'm going to be better off diverting this water with guttering, or if the down pipe is staying - with underground drainage?
Posted: Sun Feb 03, 2013 10:33 pm
by r896neo
As luton is suggesting you are perhaps underestimating the volume of water that comes off a roof in heavy rain. Letting run off from a small patio drain onto lawns or flowerbeds in fine but not from a roof of any size.
Who built the extension and didn't consider drainage or put in any gullies? Where does the existing downpipe discharge to? Joining into that system may be the easiest way to go but you would probably need a new IC. This sort of work would be notifiable to BC and if your unsure of how to go about it perhaps you could sub out the drainage element to the builder? A soakaway might be a better option but that would depend on the ground and size of roof etc.
Posted: Sun Feb 03, 2013 11:05 pm
by samselbygarden
Ok sounding good - So would a bottle gully be suitable at the bottom of the downpipe? I would then connect this to piping which can feed into the main IC ? This is round 2 corners but we have enough height to create a good fall on the pipes.
Alternatively, the piping could go to a soak-away where the patio meets the lawn?
Posted: Mon Feb 04, 2013 6:49 am
by samselbygarden
r896neo - the extension roof is approx 5x3m square with a flat roof.
Are you suggesting putting in a soak-away at the bottom of the down pipe as an option? Haven't tested soil yet but if not too much clay - could be a good option....
Posted: Mon Feb 04, 2013 8:02 am
by lutonlagerlout
sam any soakaway needs to be 5m away from the building
the BCO should have advised this when the extension was built
cheers LLL
Posted: Mon Feb 04, 2013 10:12 am
by Carberry
r896neo wrote:As luton is suggesting you are perhaps underestimating the volume of water that comes off a roof in heavy rain. Letting run off from a small patio drain onto lawns or flowerbeds in fine but not from a roof of any size.
Quick calculation based on average 724mm of rain a year, taken from http://en.wikipedia.org/wiki/Bath,_Somerset
For a 15m2 roof you're dealing with 11,000L of water a year.
If you have 1 inch of rain in a day, you're having to deal with 380L running off the roof. Guttering is the best option.
Smooth stone means you don't need to have such a big fall, as LLL says 1:60 is about standard, that's 83mm over 5m, roughly 1 brick height including pointing, which won't reall be noticeable.
Posted: Mon Feb 04, 2013 4:21 pm
by samselbygarden
Thanks for all the help chaps - we're going for linear drains as first planned, draining off into main system and a soak away in the garden.
Many thanks for all your input
Posted: Mon Feb 04, 2013 5:47 pm
by r896neo
Not having guttering on a roof is a bad idea, it will leave you with nboticable staining and premature degredation of the slabs that the water fall off onto as it will always drip off to the same line if that makes sense.