12 September 2012

A Rainwater Riddle

Question: If 1,000 square feet of roof surface generates 623 gallons of rainwater from a 1" rain, how long does it take to fill up a 2,500-gallon cistern?

Answer: Five months and counting.

Yes, we're still working on the rainwater collection system. We've had some delays due to city requirements, hunting down parts, and the fact that there's only so long I can work outside each day during the summer in Texas.

Although we're still a long way from finished, I wanted to share some of the details of the downspouts that we built to replace the regular metal ones. In our case, since the cistern isn't right next to the house, water will have to travel down the downspouts, underground across the yard, and back up this pipe into the tank:

(More about that in this post on the "first flush" system.)

Here's a good diagram of this type of configuration (borrowed from here):

This is referred to as a "wet system" because the underground pipes, as well as the downspouts, will hold water between rains. This kind of system uses gravity to push the water up the pipe into the tank. (If you're thinking "that's not right! Gravity makes things go down, not up," imagine rain draining into a downspout and filling up the underground pipe, then rising up the downspout. Since water always wants to find the lowest point, the downspout won't stay full if there is an empty pipe on the other end; it will climb halfway up that pipe so that the surface level is equal in both vertical pipes. Eventually both pipes will be full, and as long as the inlet into the tank is lower than the top of the downspout, water will begin to flow into the tank.) Because the downspouts have to actually hold water, they need to be watertight -- hence the need to replace the original metal downspouts with sealed PVC pipes.

So here's one of the new PVC downspouts standing at the ready:

Looking closer, you will see that we added a cleanout at the base of the downspout, just above ground level:

The square cap on the left screws off of the silver T fitting to access the pipe. Each downspout has one of these as a precaution in case they clog (although the gutters will have covers, it was easier to install these up front than to have to deal with an unexpected clog later with no way to get into the pipe). This happens to be the lower side of the lot, so it's also likely that we'll use this cleanout to drain the pipes before a freeze, but that wouldn't do anything about the water below the cleanout (the water in the underground part of the pipe), so we also put a cleanout underground in this irrigation box:

Although we still won't be able to empty it out fully, the slope of the lot will allow most of the water to flow out if necessary (either before a freeze or to drain any debris that might slip through the gutter screens and collect at the bottom of the underground pipe).

Most irrigation boxes have green covers (to blend into grass and other landscaping), but we used a purple one to denote reclaimed water. Rainwater isn't technically "reclaimed" water, but this relates to a city requirement for rainwater harvesting that I'll explain in another post.

Although we put the new downspout on the same side of the corner to maintain the clearance on that side of the house (since every inch was critical when we rented the bobcat to do our landscaping), we repositioned the outlet from the gutter to the downspout for aesthetic reasons. We didn't realize it when we decided where to put the downspout during construction, but the old positioning gave us this view from our bathroom:

So while we were replacing the downspout anyway, we called our awesome gutter guy (Lionel at Central Texas Custom Gutters) to move the spout and patch the hole from the old one. (Last year he helped us fix some mistakes from the initial installation -- which was done by a different company -- and we've been really happy with his work.) Now when we look out the window, this is what we see:

With all of the trenches and pipe installation underway, the view from most of the windows is still in the "it has to get worse before it gets better" phase, but at least this little upgrade is complete. Which makes the bathroom just a little more pleasant:

But back to the pipes....


  1. Just a point of clarification, 1000 square feet of roof equals 623 gallons of water in a 1" rain storm. Not 100 sf as mentioned above. Otherwise an interesting read.

    1. Oh yes, thanks for pointing out that typo! Corrected.