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A New Roof Washer

March 17, 2010
We installed the rain water harvesting system about 2 years ago, and made a first-flush diverter from a 55-gallon pickle barrel. The purpose of the first-flush diverter is to discard the first water that comes off the roof during a storm, because this carries most of the dirt washed from the roof. For this reason it's also called a "roof washer". Unfortunately the pickle barrel was a bit old when we got it, and the plastic has become brittle and cracked. In the first photo below you can see the large crack in the side of the barrel. I tried welding it with our Plastic Welder and it held for a while but it eventually cracked again; the plastic is just too brittle and it needs to be replaced. The second photo shows the two holes I drilled in it to allow air to escape as it starts filling during a storm. This was an inelegant solution to venting it and I want to improve that as long as I'm replacing the barrel.

Here's a view looking down inside the barrel, and you can see on the bottom that it collects a fair amount of dirt from the roof. It also supports a significant growth of algae because the translucent walls admit enough light for algae to grow. This isn't really harmful but we'd rather avoid it. You can also see at the top of this photo the fitting where the water enters the drum, and the second photo shows the large PVC union that lets us disconnect the barrel.

I ordered a blue 55-gallon Open-Head Plastic Drum from The price was reasonable and it comes with a lid that has a 2-inch threaded opening plus a 3/4-inch threaded opening, so there's one opening for the water inlet and another for an air vent. The PVC inlet piping will need to change a bit since it won't enter at the same angle as with the pickle barrel.

With just a bit of rework on the PVC piping, the inlet was routed to the 2-inch threaded opening in the lid. In this photo you can also see that I fit a length of 1/2-inch tubing to the 3/4-inch threaded inlet in the lid, to serve as an air vent.

Below is a view of the entire assembly. During normal operation, when it starts to rain the rain water will flow from the gutters into the two inlets at the top of the photo, and down into the barrel. As the barrel fills up, the displaced air is vented through the clear 1/2" tubing. Originally I thought the air would vent back up through the inlet pipe and it does, but not well enough when it rains hard so the new vent tube should let the barrel fill completely even in a hard rain. Once the barrel is full the water backs up to the float valve, which is the bulge just above the barrel. See the rain water harvesting system page for details of how I made the float valve. With the float valve closed, water backs up and flows down the cistern inlet pipe to the left of the barrel. In a really hard rain, especially if the cistern is fairly full, water will back up and out the overflow pipe on the far left. The 1/2" vent tubing is fastened up over the top of the overflow pipe. Finally, the valve at the bottom of the barrel is left open just a little, so that the barrel drains automatically in between rain storms.