Make a first flush diverter (FFD)

Like so many things there are alternatives to buying the commercial product and these are readily available on the internet. We originally picked up the design (unfortunately it was a while back so a proper reference cannot be given) and have added our own materials and ideas but essentially this was just to allow it to be produced with the materials at hand. The biggest benefit of making it yourself is cost and secondly the learning that takes place and that can be passed on with a slightly different explanation that might help somebody out there understand the process better.

After building a few with slight quirks we believe that they are relatively “foolproof” and seem to function well despite and production shortcomings.

Ours have been built with left over spares and offcuts to minimize the cost. Our biggest cost saving tip is to approach a genuine plumbing supply company and not a generalist store as it makes a big difference to overall cost. Better still if you have the bits lying around.

How it works and do you really need it?

The mechanism is really a cool bit of thinking. I think in some places it might be more relevant than in others. Certainly in the CradleArk environment where there is a lot of dust and a long period between rains for accumulation of other unpleasant collections, we think it is very useful. You don’t have to have one and can connect your tank directly but our thinking is the cleaner the water entering the tank is , the better.

The principle is relatively easy to follow. The rain that reaches the gutters off the roof is initially diverted into a chamber bypassing the tank. As this chamber fills a float will eventually be raised into a forcing cone to a large extent blockin the entrance to this chamber and diverting the subsequent water into the tank for collection. In theory all the dirt is  in the chamber and clean water in the tank. The dirty water drains slowly through the bottom of the chamber and by the time the subsequent rain follows the process is repeated.

How to make it

We made ours with a combination of 110mm drain pipe and 80mm pvc gutter down pipe. A collection of bits and pieces is shown below.

You can start anywhere but the easiest is with the 110mm pipe. You can work out exactly the volume of the diverted water which rests in the 110mm pipe but this requirement will vary depending on size and pitch of the roof, the intensity of the downpour etc.  We tend to make the length of this pipe around 1200mm. Add the removable bottom having drilled a 4mm central drainage hole.

We use a bracket from an old rain gauge as a forcing cone and a ball valve float that fits our pipes diameter.

We cut off the excess of the rain gauge bracket and smoothen this. Then we remove the thread on the ball valve float leaving a slight hole. Using a soldering iron (or silicon or whatever you like) we seal this hole so water can’t get into our float The Ball now fits very nicely into our storage chamber.

Next we take the shaped rain gauge forcing cone and attach this to a coupling to allow the forcing cone to attach to our 80mm pipe.

We take the 80mm downpipe and set the cutter size and cut out a hole for the pipe to fit through the top cap.

This is the pushed through the top cap and attached to the forcing cone (glue gun , silicon etc)


This is the  fitted into the 110mm pipe and the excess pipe trimmed.


Then we get the t piece (or nearest that we had) and add two elbows. This is then glued in place onto the 110mm  chamber

The ffd is pretty much done and just needs to be installed and connected to the tank. The picture above has the “split” vertically whereas our other tanks had the split horizontally as the”t pieces” were different .  Although we were concerned it has worked perfectly through this rainy season . We do feel that the horizontal is a bit more aesthetically pleasing!

The final picture features a home made leaf skimmer just below the downpipe. This is also easily made and this prototype has worked fine and the explanation also under the “How To” tab