A Medium Sized Rain Water Collection System -- Collection Plumbing

This page goes through the major part of the job, which is positioning the rain water tank, installing the collection gutters, and installing the collection plumbing including the first flow diverter.

building a rain water collection system

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Picking and Placing the tank

We looked at several tank types, and ended up choosing a polyethylene tank.  This is probably the least expensive option, and with sun protection, this kind of tank should last many years.  We chose a dark green (opaque) tank to discourage growth in the tank.  Its important when you buy the tank to specify which fittings you want installed where -- or, at least, make sure the ones it comes with are OK for your use.   You can look in the references section for the low down on all the different tank options.  See the Sizing Section for the logic on tank size.

I thought that getting the tank off the truck and over into place was going to be a real adventure.  Before the truck arrived, I made some extensions to fit on my tractor bucket so that I would be able to pick up the tank from the truck bed and carry it over under the lean-to along the building where it was going to live. 

unloading rain water collection tank
Getting the tank off the truck and onto
the tractor.
tank on tractor
Sitting on the bucket extensions -- works
pretty well.
tank ready to tip up
In position and ready to be tipped up.

As it turned out, the tank is light enough that two or three people could probably have let it down from the truck without the tractor.  Once on the ground and sitting on its side, the tank rolls very nicely to where you want it.  A couple of people can then tip it up into place.

All of this was much easier than expected just do to the light weight of the tank.

Before the tank arrived, I spread some 3/4 washed smooth gravel out in a level pad for the tank to sit on.  I think the washed gravel or something light that is fine, but larger angular and sharp edged gravel might be too hard on the tank bottom.  The pad for the tank should be level, but not painstakingly so.

The tank came with a "manhole" and with bulkhead fittings for the inlet and for a drain fitting.

tank fittiings
The manhole (more like a small boy hole), and
the inlet fitting.
outlet fitting
The outlet or drain fitting on the tank.

The gravel pad that the tank rests on is more of the same material that you can see in the two pictures above.

Note that the manhole cover is large enough for kids to get into and could be a safety hazard once the tank has water in it.  So, if you have kids, work out some way to lock the cover in place.


We decided to have the gutters installed by "Gus the Gutter Man".   He does very nice aluminum one piece gutters with no seams and a good hanger system.  This is the type of gutter that starts in his truck as a roll of painted aluminum sheet.  He feeds the end of the roll into his magic gutter bender gadget, and out comes a very nicely formed gutter of just the right length.

rain water collection gutters
The installed gutter.  Very solid.
I added the screen basket to keep any large
debris out of the pipes.
It would probably be a good idea to
add some screening fine enough to keep
insects from going down the collection
gutters rain collection gutter

We chose these gutters because they were not a whole lot more expensive than buying the 10 ft lengths at the hardware store and splicing them together ourselves.  The end quality and sturdiness seems much better.

I guess its obvious, but the gutters need to slope down toward the end where the collection pipe picks up the water. 

If you live in an area with lots of trees and a lot of leaf accumulation, then you will have to deal with keeping as much of that as you can out of the collection system plumbing.   One of the screen products that send the leaves out over the gutter edge may be in order.

Collection Plumbing

Here is an overview picture of the whole plumbing connection system:

overview of rain water collection system plumbing

This collection plumbing is the kind of system shown in the rain water collection system books, but tailored and simplified to our situation to some degree.  You will want to read over the systems described in some of the references before deciding on exactly what design you want to use.

Collection Pipes

The collection pipes that extend from the gutter to the tank are 3 inch schedule 40 PVC pipe.   The fittings are standard PVC solvent weld fittings.  I was not able to find any good design information on what size pipe should be used to support a given size of roof, but the 3 inch seems like plenty for our relatively modest roof areas and for the normally not to heavy rain falls we get.  If you are in an area (like some desert areas) where a good fraction of the yearly rain fall can come in a small number of very intense thunderstorms with heavy rain fall, then it would be good to investigate the size of the collection pipes more thoroughly and make sure that you don't miss a lot collection opportunity because your collection pipes don't have enough capacity for a heavy rain.  The Reynolds book "Water From the Sky" discusses this issue.

  Note: Neal has provided some information on sizing collection pipes and gutters using the Manning channel flow equation ... (pdf)    Thanks very much to Neal!

The collection pipes are sloped down toward the tank.  This is important to keep them dry and prevent any kind of growth and to prevent ice damage.  I used about 1/8 inch per foot of slope on the longer pipe. 

I used 2 inch PVC for the last 6 inches going into the tank.  The bulkhead fitting that came with the tank for the inlet was smaller than this, so, I just took it out and ran the 2 inch pipe through the same hole, and caulked around it.  I plan to get a larger bulkhead fitting for the 2 inch pipe later.  My feeling is that its a good idea to make sure that all the openings into the tank are sealed up to keep bugs out.

rainwater system -- collection plumbing
Putting in the collection pipe to the
west gutter.  Not temporary blocks
to establish slope.
The glazed thing is the solar heating collector
for the shop.
pipe clamp
When joining the 3 inch diameter PVC pipes
its handy to have something clamped
to the pipe to hold onto when pushing the
next pipe in.

The tank end of the west gutter
collection pipe.


rain water system collection plumbing
Plumbing the area where the collection pipes from
the east and west gutters drop down
to the tank water intake.
The first flow diverter hooks on to the
bottom just below the inlet pipe to the tank.
rain water collection plumbing
Completed collection plumbing with the
first flow diverter in place.
rain water collection system plumbing
Showing the support for the first flow diverter
and the tank outlet connection.

If you have never done the PVC glued pipe fittings, its straight forward. 

- Cut the fittings/pipes to length with an ordinary wood cutting saw and clean up the burrs with a knife. 

- Dry fit the pipes together. 
If its a somewhat complicated collection of fittings you are glueing, then put witness marks across the fittings with a marker pencil when they are in the dry fit position.  These will allow you to quickly get the same alignment when you are doing the actual glue run. 

- Take the fittings apart, and apply the purple primer to all the pipes and fittings --you can do them all at once.

- Apply the PVC glue to the fittings and push them together until they fully seat -- use a slight rotation as you push them into place.  Hold them for a 5 count. 
You only have a few seconds from applying the glue to get the pieces pushed together, so if you are new to this, I'd stick with one joint at a time.  As you get more experienced, feel free to go for several at a time (warning -- its easy to get yourself in trouble trying too many).

- Again, if you have not done this before, its best to start with some of the smaller diameter and easy fittings and work your way up to the large ones.  The larger ones can be a bit of a challenge to fully seat if they are not lined up well.

I did not find any really good clamps to attach the 3 inch PVC to the wall, and ended up making some of the supports from scraps of MDO plywood.  MDO is a very water resistant plywood used for tanks and concrete forms -- it should last as long time, but it would be nice to find some good weather proof clamps.

rain water system -- pipe supports
Clamps made from MDO scraps.
Note plastic strapping over the top of
the pipe.
rain water collection -- pipe clamps
Clamps to hold the pipe in place and on slope where
it crosses over the top of the solar collector.
plastic pipe clamp
A somewhat Mickey-Mouse commercial
plastic pipe support.


Connection to the Gutters

I'm sure there are lots of ways to do this, but the scheme we worked out is shown just below.

rainwater collection -- plumbing
West gutter to collection pipe
connection.  The plug comes out
for the winter.

Another view of the west gutter
rain water collection -- plumbing
East gutter connection.

All of the fittings are standard hardware store PVC fittings glued together in the usual way. 

The 3 inch elbow fits nicely over the gutter downspout stub and makes a nice easy connection.

The branch that takes off from the middle of the elbow is intended to capture any junk that comes off the roof -- like the little rock granules that fall off the shingles.  It has a standard pipe plug at the bottom that can be unscrewed to empty it. 

During the winter, I plan to take the plugs out and just let the gutter water drain out where the plug socket is.  This should keep the system mostly free of water and ice.   I angled the stubs out to try and get the winter water a ways away from the building.

After several months of operation through the spring rains, and with the tank full, I took the plugs out just to see what had accumulated.  There was about 1/4 cup in each stub of the shingle granules and that was all.

First Flow Diverter

The first flow diverter diverts the first few gallons of water away from the collection tank so that any dust and dirt etc. accumulated on the roof does not end up in the tank.  There are some very fancy designs for these system in the references, and you should look them over and see what works out the best for you.

We decided to do a very simple one.  The larger diameter section of pipe just below the 2 inch tank inlet is our first flow diverter.  At the start of a rain, this 3.5 gallon capacity section of pipe is empty, and all of the first rain flows into it.  After it fills up, the water then flows through the 2 inch inlet that goes into the tank.  Sometime before the next rain, the small faucet on the bottom of the diverter is opened and the accumulated water is allowed to drain.  Then the faucet is closed and the diverter is ready for the next rain. 

This is the bottom of the scale as far as first first flow diverters go:  1) its on the small side, 2) if you don't remember to empty it between rains, it does not do anything, and 3) if you forget to drain it for the winter and leave the drain open, it could be damaged by freezing.  We decided we were OK with the limitations and just decided to keep it simple -- if we screwed up its not so hard to fix later.   One thing in our favor is that the roof stays pretty clean -- there are no trees close enough to be a problem. 

Here are the details on building the diverter.

The diverter is made out of the larger diameter 4 inch PVC pipe in order to get is capacity up to a useful level.  As it is, the capacity is about 3.5 gallons, which is on the small side for our size roof.  One could go up to 6 inch diameter, but the pipe and fittings get heavier and more expensive. 

top of first flow diverter
Fittings that adapt the top of the 4 inch
first flow diverter pipe to the 3 inch
collection pipe.
The purple is the primer, which cleans the surface.
This is followed by the PVC glue that is applied
just before pushing the fittings together.
bottom of first flow diverter
Pipe cap that makes the bottom of the first flow
diverter vessel.
first flow diverter ready to go
The finished diverter ready to install.

Installing the drain valve in the first flow diverter.  This is the valve that allows you to drain the diverter after each rain and for the winter.

measuring threads
Measure the thread size on the faucet
and go with a hole in the PVC that is
just enough smaller for a good tight fit.
installing faucet in first flow diverter
Drilling a hole to thread the drain faucet
into.  You need to get the hole size just
right so that the faucet makes some threads
as it screws in.
installing faucet in first flow diverter
Put Teflon tape on the faucet threads and carefully
screw it into the hole.   It should be a tight fit so that the
faucet threads cut into the PVC for a water tight seal.

Go on to installing the water delivery system...

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Comments or questions...?

Da Boss
The Boss
Knows who the boss is.



Gary July 6, 2011