Rainwater Harvesting Systems in Kenya: Design, Storage Tank Options, and Installation Costs

In Kenya, water security is an ongoing challenge for homeowners, developers, and property managers. Municipal water supplies from companies like the Nairobi City Water and Sewerage Company (NCWSC) are subject to weekly rationing, while drilling private boreholes can cost between KES 1.5 million and KES 2.5 million. Furthermore, borehole water in areas like Syokimau, Kitengela, Nakuru, and parts of the Rift Valley is often high in fluoride, sodium, and heavy minerals, making it corrosive to plumbing fixtures and unsafe for drinking without expensive reverse osmosis filtration systems.

Rainwater harvesting offers a highly sustainable, cost-effective alternative. Rainwater is naturally soft, low in minerals, and completely free. By capturing and storing rain from roofs, homeowners can offset their municipal bills, protect plumbing systems from hard-water scale, and ensure water security during dry spells.

This guide breaks down the core design components of a residential rainwater harvesting system, compares plastic water tank prices in Kenya KES, analyzes overall installation costs, and compares various storage options.

Design Components of a Rainwater Harvesting System

A functional rainwater harvesting system requires several interconnected components to collect, clean, store, and distribute water safely.

graph TD
    A[Rainfall] --> B[Roof Catchment]
    B --> C[Gutters & Downpipes]
    C --> D[First Flush Diverter]
    D --> E[Inlet Leaf Screen]
    E --> F[Storage Tank: Plastic or Concrete]
    F --> G[Suction Filter]
    G --> H[Booster Pump / Pressurized Tank]
    H --> I[Sediment & Carbon Filters]
    I --> J[Household Taps & Appliances]

1. The Catchment Area (The Roof)

The quality of harvested water depends on your roofing material.
* Suitable Roofs: Pre-painted iron sheets (e.g., Mabati), concrete tiles, and clay tiles.
* Unsuitable Roofs: Asbestos sheets (highly carcinogenic), rusted iron sheets, or roofs coated with lead-based paints.

2. Gutters and Downpipes

Gutters channel water from the roof eaves to the downpipes. In Kenya, homeowners can choose between:
* uPVC Gutters: Durable, rust-free, and easy to install. Popular brands include Appollo and Marley.
* Galvanized Iron (GI) Gutters: Stronger but prone to rust over time if not coated with food-grade paint.

3. The First Flush Diverter

The first 10 to 15 minutes of rainfall washes away accumulated dust, bird droppings, leaves, and organic debris from the roof. A First Flush Diverter is a vertical pipe chamber that intercepts and traps this initial dirty water. Once filled, a floating ball seals the chamber, directing subsequent clean water into the storage tank. A slow-drain valve at the bottom empties the dirty water over time, resetting the system for the next rain event.

4. Filtration Systems

To make rainwater safe for household use (washing machine, showers, toilet flushing), it must pass through:
* Leaf Screens: Wire mesh over gutters to block large leaves.
* Inlet Filters: Sand/gravel filters or mesh bag filters at the tank inlet.
* Carbon and Sediment Inline Filters: Installed after the pump to remove fine sediment, chlorine, and odors before the water reaches indoor taps.

5. The Storage Tank and Distribution Pump

Since gravity is rarely sufficient to push water from ground tanks back into house taps, a small booster pump (e.g., Pedrollo or Davis & Shirtliff dayliff pumps) combined with a pressure tank (pressurized water vessel) is installed to distribute water into the home's plumbing.

Storage Tank Options: Plastic vs. Concrete vs. Steel

The storage tank is the most significant cost driver in a rainwater harvesting system. Homeowners must choose between above-ground plastic tanks, underground concrete tanks, or pressed steel tanks.

Plastic Water Tank Prices in Kenya KES (2026 Estimates)

For above-ground storage, cylindrical plastic tanks are the most popular choice due to their simplicity and ease of installation. Prominent Kenyan manufacturers include Kentank, Roto, Kenpoly, Jumbo, and Technotank.

The table below outlines the average retail prices for standard double-layer, UV-stabilized plastic tanks in Kenya Shillings (KES), paid typically via M-Pesa or bank transfer:

Note: Plastic tanks must be installed on a perfectly level, smooth concrete base (plinth). Placing a heavy water tank (a 10,000-liter tank weighs 10 metric tons when full) on uneven soil or bricks will cause the plastic base to deform and split, voiding the manufacturer's warranty.

Overall Installation Costs for a Standard Residential System

Let's estimate the total cost of installing a complete rainwater harvesting system for a typical 3-bedroom bungalow (roof area approx. $150\text{ m}^2$) using a 10,000-liter Kentank as the primary storage unit.

1. Bill of Materials (BOM) & Labor Cost Breakdown:

1. Storage Tank: 10,000L UV-stabilized plastic tank – KES 85,000
2. Concrete Base (Plinth): Cement, sand, ballast, BRC mesh – KES 15,000
3. uPVC Gutters & Brackets: 30 meters of uPVC gutters, brackets, end caps, and connectors – KES 22,000
4. Downpipes & Fittings: 4 downpipes, elbows, clips – KES 8,000
5. First Flush Diverter Kit: Custom DIY assembly (PVC pipes, float ball, drain valve) – KES 6,000
6. Water Pump & Pressure Vessel: 1HP Pedrollo or Dayliff booster pump – KES 28,000
7. Filtration System: Leaf screen, 2-stage carbon & sediment inline cartridge filters – KES 14,000
8. Plumbing Accessories & Valves: PPR piping, non-return valves, bypass gate valves, union joints – KES 12,000

9. Plumbing & Masonry Labor: Professional installation fees – KES 30,000

10. Total Estimated Project Cost: KES 230,000

Checklist: Planning and Installing Your Rainwater System

Before you start purchasing components, run through this design checklist to ensure a safe, efficient installation:

• [ ] Calculate Your Roof Harvest Potential: Use the formula:
  $$\text{Harvest (Liters)} = \text{Roof Area } (m^2) \times \text{Annual Rainfall (mm)} \times 0.85 \text{ (efficiency factor)}$$
  (For example, a $150\text{ m}^2$ roof in Nairobi, which receives an average of 900mm of rain annually, can harvest up to 114,750 liters of water per year).
• [ ] Examine Roof Condition: Clean dirt, bird nests, and overhanging tree branches that drop leaves onto the roof.
• [ ] Design Gutters with Correct Slope: Ensure gutters slope towards the downpipes at a minimum fall of 1:200 (5mm drop per meter) to prevent standing water.
• [ ] Build a Solid Foundation: Make sure the concrete tank base is cured (at least 7 days) before placing the tank.
• [ ] Install a First Flush Diverter: Never skip this step if you intend to use the water for anything other than garden irrigation.
• [ ] Protect Tank Inlets: Ensure all tank entry points have tight-fitting mosquito mesh to prevent breeding insects and frogs from entering.
• [ ] Set Up a Dual-Source Bypass: Install a 3-way bypass valve system that allows you to switch your household plumbing between municipal/borehole water and rainwater.
• [ ] Prepare a Winterization/Maintenance Schedule: Plan to clean the roof and gutters at the start of the dry season and drain the first-flush chambers.

Maintain Your Water Infrastructure for Optimal Performance

Rainwater harvesting systems require periodic technical maintenance. Gutters sag and leak over time, leaf screens clog, first flush valves get blocked, and pump pressure vessels lose calibration, resulting in constant pump "cycling" that spikes your KPLC tokens. Regular disinfection of storage tanks with chlorine is also necessary to prevent bacteria buildup.

If you need a professional team to design and install a rainwater harvesting system, repair leaky gutters, clean out sludge from your water tanks, or service your booster pumps, we are here to help.

Submit a Maintenance Request today, and our expert plumbers will audit your water infrastructure, install high-efficiency filtration systems, and keep your water flowing clean and green.

Submit a Rainwater System Maintenance Request Now