How much water can you collect?

1 inch of rain per 1,000 square feet collects 600 gallons of water.

Rule of thumb: Plan your system to hold a 2”- 3” rain event.

Rainwater Design

Here’s some helpful tips to make it easier to design your rainwater system. No two systems are exactly alike and there are multiple ways to achieve the same goal!

Within a year, many clients could collect 30,000 or more gallons of rainwater. In Texas a 400 gallon tank typically fills 30X annually when collecting from a 500 sq. ft. structure, your garage for example.

Plants love rainwater, it picks up nitrogen as it falls to the earth and doesn’t have any harsh chemicals or salts.

How to plan your rainwater system.

Read on for useful info and answers to frequently asked questions.

Pre-Design Considerations

What are you going to use the water for?

Tank size is often determined by what you use the water for. We can use 70% of our water for irrigation!

How much can you collect?

In Central Texas, where we get about 32” of rain in a normal year, a one story 2000 sq. foot house can collect 35,000 gallons of rainwater a year. Don’t design a system that tries to direct that much rain into a 300 gallon tank!

How much space do you have?

Lots of times space is a limiting factor. Are there trees that prevent installation of large tanks? How high are the eaves on your house? Seven foot tall tanks work well for eight foot eaves. Is there a fence in the way? We’ve temporarily removed fencing to get tanks into place.

Are you applying for a rebate?

Many municipalities offer rebates from $0.50 / gallon up to $1.00 / gallon. In some instances you need to be pre-approved before you purchase and install your tank.

Are you part of an HOA or POA?

Great news! In 2013 the Texas Legislature passed a law prohibiting HOA’s from banning rainwater harvesting systems. They might limit the location, but they must allow rain-water tanks. If your HOA is unfamiliar with this law, tell them to see Texas Property Code Sec 202.007.

Designing Your System

Sizing the Tank

Rule of thumb: plan for a 2” – 3” rain event.

A 1” rain collects 600 gallons per 1000 square feet of covered space (roof). Therefore if you are collecting off of 500 square feet, that will produce
300 gallons on a 1” rain. So, a 500 gallon or larger tank would be appropriate. For larger surface areas, consider using multiple tanks.

Sizing the inlet and overflow

Rule of thumb: match the overflow size to the inlet size.

If your tank is 1000 gallons or less,you are probably only collecting from one downspout. In that case a 3” inlet is probably fine. If you have 6” gutters you might want to go with a 4” inlet. If you have multiple downspouts, you can still go with 3” PVC piping, but at some point expand to 4” and have a 4” inlet. Note: If your tank is collecting from one downspout, a basket inlet is often easiest. Simply spin the top so the basket is under the downspout then secure with four screws.

Choose an outlet size

If you are just going to attach a garden hose, go with a 3/4” bulkhead connection. That will allow you to screw in a spigot.

Pump or no pump?

Many people want to pressurize the water to be able to pump into the house or run an irrigation system. Within city limits pressurized systems need backflow protection devices. Another option is to buy a submersible sump pump.

Keeping debris out of your tank

If your gutters clog with leaves your tank will clog will leaves if you just run it directly into the tank. Gutters screens are your first line of defense.
You may also choose to use a basket on the tank. If those options aren’t doable, a first flush diverter is recommended.

Rainwater Collection Calculator

    Roof Area:
    estimate your roof area here

    Average Annual Rainfall:
    find rainfall averages for your area here

    You might collect:

    gallons per year!

    Installing Your System

    Choosing a base

    The tank needs to be installed on a solid base supporting the entire bottom. We recommend crushed granite or a similar base. Limestone block, brick, or pavers can also be used. The simplest thing to do is get some treated landscape timbers and form a square about a foot wider than your tank. Fill with 2”- 3” of crushed granite and tamp down. The tank can sit directly on the crushed granite. Of course, a concrete pad also works! Keep in mind water weighs 8.3 pounds per gallon, so a 2500 gallon tank weighs over 10 tons when full!

    Moving the tank into place

    Large tanks slide easily across 2 X 4’s, or better yet, PVC pipes. Tanks can also be tipped on a bottom edge and rolled. Do not lay a large tank on its side and roll it!

    Positioning of fittings

    Generally, don’t position the overflow above the outlet. If you decide to plumb the overflow, the pvc pipe will come down where your outlet is! The overflow should channel water away from the tank and house towards a low lying area.

    Bulkhead connectors

    Important!!! These are threaded on both the inside and outside. You will only be able to screw things into the inside of the bulkhead. The outside threads are reversed thread and only accept the locknut. Therefore a 3/4” bulkhead accepts a 3/4” spigot, for example.

    Painting the PVC

    Choose a metallic gray spray paint to make the overflow and inlet look like metal. Rustoleum™ makes a good product available at The Home Depot.

    Securing the top

    The top of your tank needs to be secured in place so it doesn’t blow off. Four self tapping sheet metal screws should be included with your tank. We recommend pre-drilling the holes about an 1 1/2” from the edge. There is a lip at the top of the tank.

    Design Inspiration