Installing a Drainage Chimney

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Many, many years ago when we put the first tanks on the back of the garage, the council inspector had said that I needed to put in a rubble drain to take the stormwater from the roof of the garage. This would have taken out most of my veggie patch at the time and fortunately he said putting in a couple of tanks instead was fine. However once the tanks were full, I still needed a place for the overflow to go. If I just let it go onto the ground the water would flow into the next door neighbours place, this crapped them off and meant the water was lost to our property.

The Tanks

This was a problem I have wrestled with for some years and had not found a suitable answer. I had considered putting in a dry well, which I had read about in some of the American books on sustainable living. It is basically a hole lined with bricks and filled with rocks, which allows rainwater to filter back into your soil. It does, however, require a fair bit of digging, and I am not getting any younger! So when I came across the concept of a drainage chimney, I knew I had the answer, and this is how I built it.


The key to building a drainage chimney is having access to a post-hole digger, one of those hand turned earth augurs that allows you to dig straight down into the earth without killing yourself. In the area where I needed to make the chimney it was quite shaded and still fairly moist after some recent rains, so it was easy to bore the hole. If this is not the case for you, start the hole off and get down about 20 or 30cm if you can, otherwise as deep as you can do it easily. Fill the resultant hole with water, let it drain, then fill with water again and leave overnight for the soil to soften. Next morning the digging will be much easier.

I wanted to dig my chimney down a metre (a nice round number!), but in the event I was only able to get down to 800mm before the handles started to run into obstructions and I had to call a halt. The digging was comparatively easy once I made it through a band of gravel some 50mm – 80mm thick just below the soil surface. The spoil went into a couple of old chook feed bags and the stuff from below the gravel layer wound up being incorporated into a garden bed which we were expanding.

Once rainwater started going into the hole the sides would collapse and it would eventually fill with dirt if that was all there was to the design, so I planned to fill the chimney with pebbles. I had access to some 30mm – 40mm rounded pebbles from my daughters place which would hold up the sides but still contain enough voids between the pebbles to hold a substantial amount of water as it moved into the soil. Even with the pebbles though, the spaces in between them was likely to fill up with soil eventually. The answer was to make a “sock” out of drain matting which would hold the pebbles together and keep out small soil particles but still allow water to infiltrate through the sides.

The Whole Hole!

It was my intention to make a sock deep enough to go from the top of the chimney to the bottom, then work out some kind of covering for the top which would let in the rainwater. I found the EASYdrain rainwater pit case in the local hardware, which is a box 260mm square and 280mm deep, which can have a drainage grate fitted into the top. As it was, there were no holes in it to let the water out so I needed to provide a drainage hole. With the pit case going down 280mm into the drain chimney, the sock did not need to be so long, which in the end turned out well.

The Sock

The drain matting I buy is 600mm wide, by 6m long and to make a sock 250mm in diameter I needed the fabric to be roughly 750mm wide. By cutting 750mm from the long side it gave me a sock 600mm deep, which was a bit short for the 800mm deep hole I needed, but with the 280mm contributed by the pit case it gave me 80mm overlap between the top of the sock and the bottom of the pit case.

Stormwater Pit Case

To fit the 260mm square pit case into a 250mm diameter round drainage chimney hole I had to dig away the sides of the hole a bit, and remove enough soil to make corners to fit the square profile of the pit case. This was basically trial and error – dig out the soil, fit the pit case and see how far down it would fit, remove and repeat – until it fitted all the way into the drainage chimney.

To make the hole in the bottom of the pit case to allow the water to drain through was a simple matter of applying a 75mm hole saw to the centre of the bottom of it and to attach the drain matting sock I used that marvel of modern engineering – duct tape. This proved to be as easy as fitting the open end of the sock over the bottom of the pit case, allowing a 50mm – 80mm overlap, and then securing the sock with a 150mm of tape up each side of the pit case. A final run of tape around the top edge of the sock and it was bound to the pit case (I hope) permanently.

To allow me to fit the sock/pit case into the hole I dropped a few pebbles through the hole and into the sock, keeping the sock vertical and stopping the side of it fouling on the sides of the hole. With the drain case and sock in place it was a simple matter to fill the sock the rest of the way to the top with pebbles through the central hole in the bottom of the pit case. I figured the pit case would stop the hole collapsing at the top level so I did not will it with pebbles, thus leaving more volume for rainwater to fill during a deluge. With the fitting of the removable grate in the top of the pit case, that part of the project was completed.

Directing the outflow

The outflow from the tanks goes into a black 200litre plastic drum and from there just overflows onto the ground, so I had to fabricate a 50mm overflow pipe from the side of the drum into the drainage chimney. This was accomplished with some 50mm pipe, a 90° elbow, a 50mm floor flange my perennial friend, silicon sealant. I just drilled a 75mm hole in the side of the 200 litre drum, inserted the floor flange, sealed with silicon sealant and screwue4d into place, then fitted the horizontal pipe section and also screwed it into place, sealed with silicon. The 90° elbow was then placed on the end of the pipe and then downpipe inserted into the other opening of the elbow. Done!

We have had one downpour since and the system seemed to work pretty well, we’ll see how it goes in the future.



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