Hi. I'm wanting to completely pave the back garden with council flags 50mm thick. Critically the paving area will be entirely enclosed by walls on all four sides and I want to lay the paving dead flat.
It appears that slot drainage might afford a design solution.
I provide an image showing the essentials of a design I have come up with. I hope the image is understandable. There are 4 slot drainage channels that all take rainwater into a main channel that feeds it towards a drain.
I have identified a slot drainage product: ACO Hexdrain Brickslot. The problem is this product does not, in it's channel arrangement, have a fall profile in it's form, so that rainwater will flow. so I wonder if this product is suitable - unless a pump added to the arrangement would ensure acceptable flow.
Comments appreciated as to whether this arrangement would work. Thank you.
A,B,C,D,E are flags already laid that abut the wall of the house. You see the slots and the main channel taking the water to the drain are one flag distance away from the wall of the house.
Can't see why you would use 4 slot drains. You could get away with one with a fall towards it. The water will flow regardless of no fall to the aco drain, hydrostatic gradient.
Let me just explain my thinking as to why currently my preferred idea is to try to drain an enclosed area of paving laid perfectly level. That is with walls around every border.
I envisage putting up quite a large shed on the paving. Also, I was wanting to put a workbench or two under a gazebo. Because if these things I feel as if I'm driven to try to lay the flags perfectly level. Of course, the alternative is to simply arrange for a fall, which would mean in my case the far end away from the house would be about 100mm higher for a fall of 1 in 80. With a fall, I'd only need one slot near the house and parallel to the house wall. Of course that would mean making special arrangements for the shed base as it would have to counter the fall in order for the shed to be level.
"The water will flow regardless of no fall to the aco drain, hydrostatic gradient." I will look into that. Flow with no fall.
Flat paving: good luck trying to achieve that. It's virtually impossible, practically nonsensical, and it causes nowt but problems in the medium-long term.
Sean is right: if you install a linear channel dead flat, the build-up of water, known as a hydrostatic gradient, will effectively 'push' the collected water to an outfall point. This should only ever be done over relatively short distances of a few metres. Over runs of, say, 10m or more, it just allows detritus to sit there and slowly clog the channel. We prefer all drainage to have a fall and ensure the water passes through at what is known as a 'self-cleansing velocity' so that any crap, litter, etc., is swept away and the channel/pipe is kept clear.
Flat surface are always problematic, and with paving, the biggest problem is that of surface water lingering, remaining on the surface, for longer than is either necessary or desirable. This creates a damp surface which encourages mosses, lichens, algae and all sorts of blacky greeny goo. Stone and concrete will discolour as dirt isn't washed away; the vegetation is given ample time to get a foothold; and in winter, it becomes an ice rink.
On the one popular form of modular paving that we do install flat, namely concrete block permable paving, we use relatively small paving units so that the risk of water lingering is kept to a bare minimum.
If you installed your paving at a shallow fall of, say, 1:80 or even 1:100, it would look flat and level to all but the most experienced eye, and it would avoid all the problems which blight flat surfaces.
There are very good and proven reasons why we lay paving (conventional paving that is) to falls. Over thousands of years of building pavements, thgis is the most important truth that we have learned. Don't imagine you know better. You *will* be proved wrong!
You would shim the shed base, work bench etc; up to make it level. As Tony recommended go for a fall in the aco if possible. give it a bit more thought.
Yep. It kind of goes against my sensibilities to place things: sheds, benches, pallets etc on arrangements to make them standing on a level, but this is deemed the better arrangement, considering the advantages of laying the paves with a fall.
Given that I've accepted the advice to provide a fall for the paving, I now need to select suitable drainage components. I'm looking at something called ACO Hexdrain B 125. It's plastic and for "Driveways and small car parks". The area to drain is about 52 square meters. I hope one channel will take it when it rains heavy.
B125 is the load rating and that's fine for what you plan - see the Linear Drainage page.
When it comes to linear channels. it's not the width, depth or length of channel that governs how much surface water it can handle: it's the size of the outlet. YOu can put a huge F900 channel in, but if it's outfalling via a 100mm residential pipe, then it can only handle the same maximum volume of water as a DIY Shed own-brand A15 channel.
As a very, very rough rule of thumb, a 100mm pipe outlet can manage up to about 300m² of surface drainage. Don't forget to include any roof discharge when calculating this value.
