Stand under almost any iron roof in Kenya during a short rains downpour and you will see the same thing: sheets of clean water pouring off the edge, digging a trench along the wall and running away down the path. A week later the same household is buying water by the jerrycan. The roof is already the best water source on the property. What is missing is usually a few metres of gutter, a downpipe, a simple diverter and a tank that is set up properly.
We have already covered the arithmetic of roof catchment and tank sizing in our main guide to rainwater harvesting in Kenya. This article takes a different, hands-on angle. It is about the build: choosing and hanging gutters so they actually catch the water, making a first-flush diverter from ordinary pipe fittings, setting the tank inlet, overflow and screens, and keeping the whole system clean through the rains. It applies to a house, a cowshed, a poultry house or a store, because every roof on the farm is a catchment.
Start With the Roof You Have
Every millimetre of rain falling on one square metre of roof is one litre of water. Not all of it reaches the tank: some splashes, some wets the roof, and some overshoots the gutter in heavy storms. Rainwater handbooks commonly plan on capturing around 80 percent from a well guttered iron roof, so treat any calculation as an estimate. The figure below shows how quickly it adds up.
Measure the roof footprint, not the sloping length of the sheets: length times width of the area the roof covers on the ground. A steel measuring tape or a laser distance meter makes this a five minute job.
Check the roof itself before you invest in gutters. Galvanised iron and painted mabati sheets give clean water. Avoid collecting drinking water from roofs painted with old lead based paints, from asbestos sheets that are breaking up, or from thatch, which sheds organic matter and stains the water. Cut back branches that overhang the roof; they drop leaves and give birds, rats and monkeys a route on to it.
Hanging Gutters That Actually Catch the Water
Most failed systems fail at the gutter. The water runs over it in a storm, behind it into the wall, or sits in it and breeds mosquitoes. Four details make the difference.
Position. The gutter must sit under the edge of the roof sheets so that water leaving the sheet falls into the middle of the gutter, even in heavy rain. If the sheets end short of the gutter, water runs back under them along the fascia; if the gutter sits too far out, the storm jets over it. The roof sheets should project a little way over the gutter, and the outer lip of the gutter should be lower than the roof line so that overflow spills outwards, away from the wall.
Fall. A gutter must slope steadily towards its outlet so it empties completely after rain. A level gutter holds water in puddles, which silt up and breed mosquitoes. Too steep a slope on a long run means the low end sits far below the roof edge and misses the water. Run a builder's line from end to end and set each bracket to it, so the fall is even with no dips.
Support. Fix brackets to solid fascia board or rafter ends, at the spacing the gutter maker gives. A gutter full of water is heavy, and one that sags between brackets forms a low spot that never drains. On mud and pole houses without a fascia, fix a straight timber along the rafter ends first.
Size and outlets. Undersized gutters overflow in exactly the storms that deliver most of the water. If you are unsure, go one size larger, and on long roofs fit an outlet at each end or in the middle so no single run carries too much. Fabricated sheet metal gutters can be made to size by a fundi; our guide to sheet metal troughs and gutters covers that route.
Building a Simple First-Flush Diverter
Between rains the roof collects dust, bird droppings, dead insects and leaves. The first water off the roof washes most of it into the gutter. A first-flush diverter catches that dirty first water in a closed pipe and lets only the cleaner water that follows go on to the tank.
The simplest version needs only a tee, a length of pipe the same size as the downpipe, and an end cap. The downpipe comes down from the gutter into the top of the tee. The bottom of the tee feeds a vertical standpipe, closed at the base with a cap. The side branch of the tee runs on to the tank inlet. When the rain starts, water fills the standpipe first. Once it is full, water backs up to the tee and flows sideways into the tank.
Drill a small hole in the end cap, or fit a small tap, so the standpipe drains slowly over a few hours after the rain and is empty and ready for the next storm. Many builders also fit a screw-off cap so the sludge that collects at the bottom can be washed out.
How much water to divert
There is no single correct figure, and researchers still debate how well the simple area-based rules work. Suppliers' rules of thumb range from about half a litre per square metre of roof on clean, open sites to around two litres per square metre where there are overhanging trees, bird droppings or heavy dust. Our main rainwater guide uses a working range of 0.2 to 1 litre per square metre. Pick a figure, then adjust it after watching what the water looks like.
Work out the standpipe length from the pipe size. A metre of 110 mm pipe holds roughly 8 litres, depending on its wall thickness. So a 60 square metre roof diverting half a litre per square metre needs about 30 litres, which is close to four metres of 110 mm pipe. If that is too tall to fit below the downpipe, use a wider pipe, or split the roof between two downpipes, each with its own diverter.
Tank Inlet, Overflow and Screens
The tank end of the system matters as much as the roof end. Set it up so that only screened water gets in, overflow goes somewhere safe, and nothing can breed or drown inside.
- Leaf screen at the gutter outlet. A coarse mesh at the top of the downpipe stops leaves and seeds before they reach the diverter.
- Fine mesh on the inlet and overflow. Mosquitoes breed in tanks, gutters and open drums. Cover every opening, including the overflow pipe, with fine insect mesh and keep the lid tight.
- Overflow sized like the inlet. The overflow must carry away everything the downpipe brings in during a storm, or water will back up the gutters. Lead it well away from the tank base and the house foundations, ideally to a soak pit, a garden bed or a farm water pan.
- Outlet tap above the bottom. Fit the draw-off tap a little above the tank floor so sediment stays behind, and a separate drain valve at the lowest point for cleaning.
- A firm, level base. A full tank weighs about a tonne for every thousand litres. Set it on a level concrete slab or a properly built stand; our guide to tank stands and gravity flow explains how height gives pressure.
Choosing the tank itself, by material, size and placement, is covered in our guide to water storage tanks. If you link a roof tank to a cattle trough, a float valve keeps the trough full without overflowing.
Using the Water: Household, Livestock and Garden
Clean, well diverted roof water is a good start, but it is not automatically safe to drink. Birds and animals can still contaminate a roof, and tanks can develop slime. For drinking, treat the water by boiling, chlorination or a suitable filter; our guide to farm water treatment compares the options. For livestock, washing and the kitchen garden, screened and diverted rainwater is usually ready to use.
Moving water from the tank to where it is needed is often the next step. A short braided hose from the tank tap, or a small solar water pump lifting water to a raised tank, turns stored rain into piped water around the compound. For a vegetable plot, a raised tank feeding a gravity drip kit stretches a few thousand litres a long way into the dry season.
Keeping the System Clean Through the Rains
A rainwater system needs a few minutes of attention through the season. Most problems come from skipping these small jobs.
| When | Job |
|---|---|
| Before the rains | Clear gutters, check falls with a bucket test, cut back branches, clean the diverter, check screens |
| After each rain | Check the diverter has drained; remove leaves from the gutter outlet screen |
| Every few weeks in the season | Look for standing water in gutters, sagging brackets and torn mesh; wash out diverter sludge |
| Every year or two | Drain and clean the tank; our tank cleaning guide covers the safe method |
Never send anyone inside a tank alone or without good ventilation, and never use strong chemicals without rinsing thoroughly before the tank is filled again.
Your Gutter and Diverter Checklist
| Part | Check |
|---|---|
| Roof | Suitable sheets; overhanging branches cut back; footprint measured |
| Gutters | Under the sheet edge; even fall to outlet; firm brackets; no puddles after a bucket test |
| Downpipe | Leaf screen at the top; sized to the gutter outlet |
| Diverter | Standpipe sized for your roof; slow drain open; cleanout cap |
| Tank | Firm level base; tight lid; fine mesh on inlet and overflow; tap above the floor |
| Overflow | As large as the inlet; led away from walls and foundations |
Equipment for Your Rainwater System
Fuga stocks the hoses, pumps and measuring tools that complete a roof catchment system. For current prices and stock, see the price list, browse the full water and irrigation range, or WhatsApp 0716 961 018 with your roof size and what you want to use the water for, and we will help you plan the setup.
Key Takeaways
- One millimetre of rain on one square metre of roof is one litre; plan on capturing roughly 80 percent from a well guttered iron roof.
- Hang gutters under the sheet edge with an even fall to the outlet, on firm brackets, and test them with a bucket of water.
- A first-flush diverter is a tee, a capped standpipe and a slow drain; suppliers suggest roughly 0.5 to 2 litres per square metre of roof.
- A metre of 110 mm pipe holds roughly 8 litres, so size the standpipe to your roof and split large roofs between downpipes.
- Screen every tank opening with fine mesh, size the overflow like the inlet and lead it away from foundations.
- Treat roof water before drinking, drain the diverter after each rain and clean the tank every year or two.
Frequently Asked Questions
How much water should a first-flush diverter hold?
There is no single correct figure. Suppliers' rules of thumb range from about half a litre per square metre of roof on clean, open sites to around two litres per square metre where there are trees, droppings or dust. Start in that range and adjust after watching the water quality.
How do I make a simple first-flush diverter?
Fit a tee in the downpipe. The bottom of the tee feeds a vertical standpipe closed with a cap that has a small drain hole or tap; the side branch goes to the tank. The first water fills the standpipe, then cleaner water flows on to the tank.
Why does my gutter overflow in heavy rain?
Usually because it is too small, has too little fall, sags between brackets, or has a blocked outlet. Fit a larger gutter or extra outlets, set an even fall with a builder's line, add brackets and keep the leaf screen clear.
Is rainwater from a roof safe to drink?
Not automatically. A first-flush diverter and screens improve quality a great deal, but birds and animals can still contaminate the roof and tanks can grow slime. Boil, chlorinate or filter rainwater before drinking it.
