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Sep 5, 2025

Rainwater Collection Basics

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Quick Answer: Rainwater Collection Basics

Start with gutters, barrels, and first-flush diverters. Calculate your roof area for collection potential, check local regulations, and focus on storage capacity that matches your actual water usage needs. Rule of thumb: 1 inch of rain on 1,000 sq ft roof = 600 gallons collected.

Safety First: Rainwater Is Not Pure

Never assume collected rainwater is safe to drink without purification. Rain can wash contaminants like bacteria from bird droppings, chemical residues from roofing materials, and airborne pollutants into your system. This guide focuses on collection for non-potable uses like gardening. For drinking, you must use a multi-stage filtration system.

Why Rainwater Collection Makes Sense

My water math (your results will vary):

  • Roof area: 1,200 sq ft
  • Annual rainfall: 35 inches
  • Potential collection: ~26,000 gallons per year
  • Reality: I collect about 40% of that due to storage limitations and seasonal timing

The benefits I’ve experienced:

  • Lower water bills: Saves me ~$180/year on irrigation
  • Better plant health: Plants prefer rainwater over treated tap water
  • Backup water source: Emergency water with basic filtration
  • Reduced runoff: Less flooding in my yard during heavy rains

Getting Started: The Essentials

Step 1: Assess Your Setup

What you need to check first:

  • Roof material: Asphalt shingles, metal, and tile work well. Avoid collecting from roofs with lead flashing, treated cedar shakes, or very old tar-and-gravel roofs.
  • Gutter condition: Clean, intact gutters are essential.
  • Downspout accessibility: You need to be able to redirect the flow.
  • Local regulations: Some areas restrict or regulate rainwater collection.

Pro Tip: Calculate Your Potential

Use this simple formula to estimate your collection potential:

Roof Area (sq ft) x Rainfall (inches) x 0.623 = Gallons Collected

This accounts for the volume conversion. Remember, this is your potential - actual collection will be less due to evaporation and overflow.

Step 2: First-Flush Diverters (The Game Changer)

Why I added this after my first season: The water from the first few minutes of rain is nasty - it washes dust, pollen, bird droppings, and whatever else has accumulated on your roof. I stored water without a diverter once and it was cloudy, smelled off, and clogged my garden sprinkler.

My simple diverter setup:

  • 4-inch PVC pipe with a ball valve at the bottom
  • Sized to divert the first 10 gallons of runoff.
  • When it fills, clean water overflows to my storage tank.
  • I empty the diverter after each rain event.

Commercial option: Ready-made diverters run $50-150 and work great if you don’t want to DIY.

Step 3: Storage That Actually Works

My barrel evolution (learned through trial and error):

55-gallon barrel (where I started):

Pros

  • Cheap ($25 used)
  • Easy to move
  • Fits in small spaces

Cons

  • Fills and empties very quickly
  • Prone to mosquito breeding if not sealed

275-gallon IBC totes (my current setup):

Pros

  • Excellent value ($75-150 used)
  • Large volume for dry spells
  • Stackable for vertical storage

Cons

  • Takes up significant space
  • Requires a strong, level platform
  • Can be an eyesore if not hidden

Gravity flow considerations: Each foot of height gives you about 0.43 PSI of water pressure. I elevated my tanks 3 feet for decent pressure for my soaker hoses.

Advanced Setup: What I’ve Learned

Linking Multiple Barrels

Series vs. parallel (this matters more than you’d think):

Series connection (my recommendation):

Pros

  • Water fills one barrel completely before moving to the next.
  • Easier to monitor water levels.
  • Prevents stagnation in partially filled barrels.

Cons

  • Slightly more complex initial plumbing.

Parallel connection:

Pros

  • Simpler plumbing concept.

Cons

  • All barrels fill and empty simultaneously.
  • Can lead to uneven filling if not perfectly level.
  • Harder to isolate a single barrel if it leaks.

Filtration for Different Uses

My three-tier approach:

Level 1 - Garden irrigation (basic screen filter):

  • Keeps out leaves and large debris
  • Perfect for sprinklers and soaker hoses

Level 2 - Cleaning/washing (sediment filter):

  • 5-micron filter removes fine particles
  • Good for washing cars, outdoor equipment

Level 3 - Emergency drinking (multi-stage filtration):

  • Sediment, carbon, and UV sterilization
  • Only for emergency situations - not daily drinking

Seasonal Management

Winter prep (learned this the hard way):

  • Drain or insulate: Frozen pipes are expensive lessons
  • Disconnect diverters: Ice expansion will crack PVC
  • Maintain access: Snow-covered valves are useless valves

Summer optimization:

  • Shade storage: Dark barrels in sun = algae growth
  • Circulation: Use oldest water first to prevent stagnation
  • Mosquito control: Tight-fitting lids or add mosquito dunks

Know Your Local Rules

Check Local Laws Before You Build

Rainwater collection laws vary dramatically by state, county, and even city. Some areas have restrictions on volume or use, while others offer rebates. Ignorance is not an excuse. Check with your local planning department or water authority before buying any equipment.

Common regulations (varies widely by location):

  • Collection limits: Some areas limit barrel size or total capacity
  • Use restrictions: Garden only vs. indoor use permissions
  • Health department rules: Especially if you plan any indoor use

HOA and Neighbor Considerations

Making it neighbor-friendly:

  • Screening: Planted shrubs hide my IBC totes completely
  • Color matching: Paint barrels to match your house
  • Maintenance: Clean, organized systems look intentional, not trashy

Real-World Performance

What I Actually Collect

Monthly averages (Colorado Front Range):

  • March-June: 50-150 gallons/month (spring rains)
  • July-September: 200-400 gallons/month (monsoon season)
  • October-February: 20-80 gallons/month (snow doesn’t help much)

Storage strategy: I size my system for the dry periods, not the peak collection months.

Common Problems I’ve Solved

Algae growth: Started when I used clear containers in direct sun. Solution: opaque containers or shade cloth.

Mosquitoes: Happened when barrel lids didn’t fit tightly. Solution: weather stripping around lid edges.

Low pressure: Initial setup was too low. Solution: elevated platform and gravity-feed design.

Overflow flooding: Heavy rains overwhelmed my storage. Solution: overflow directed to rain garden.

Simple DIY Setup

Basic 200-Gallon System ($150-250)

Shopping list (what I actually bought):

  • Two 100-gallon food-grade barrels ($50-75 each)
  • First-flush diverter kit ($60)
  • Spigots and fittings ($30)
  • Platform materials ($40)

Installation reality: Took me one weekend with basic tools. The hardest part was cutting into the downspout cleanly.

Maintenance Schedule

Weekly (during collection season):

  • Check and empty first-flush diverter
  • Verify all connections are tight
  • Clean debris from inlet screens

Monthly:

  • Inspect barrels for cracks or algae
  • Test spigots and valves
  • Check platform stability

Seasonally:

  • Deep clean storage tanks
  • Replace filters
  • Winterize or prepare for rain season

Bottom Line on Rainwater Collection

Start simple: One barrel, basic diverter, see how you like it Focus on garden use: Easiest permitting, biggest water bill impact Plan for mosquitoes: They will find any standing water Elevation matters: Higher storage = better pressure

Rainwater collection isn’t going to replace your municipal water supply, but it can dramatically reduce your irrigation costs and provide a satisfying backup water source. Plus, there’s something deeply satisfying about watering your garden with water that fell on your own roof.

For comprehensive water preparedness and conservation, check out these guides:

TAGGED:

Water Storage , How To Guide

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