Water harvesting

Why a skillion is good for this

All the water from the entire roof surface runs to one end. You do not need a complicated gutter layout or multiple downpipes -- one good gutter at the low end catches everything. On a hip or gabled roof you have to gutter all four sides and join it all up. On a skillion you have one job.

The gutter

A standard quad profile aluminium gutter screwed to the fascia board at the low end of the roof. Size it to match your roof area: a wider gutter (115 mm quad or larger) will handle a heavy downpour without overflowing. One downpipe is usually enough for a shanty-sized roof unless yours is very long.

Slope the gutter slightly toward the downpipe end -- 1:500 is enough -- so water runs to the outlet rather than sitting and breeding mosquitoes in a flat gutter.

First flush

The first water off the roof after a dry spell carries dust, bird droppings, and anything else that has settled on the corrugated steel. A first-flush diverter sends that initial volume (a few litres per 100 square metres of roof) to a separate, slow-draining chamber before letting clean water into your tank. For drinking water or aquaponics, a first flush is worth fitting. For general garden watering, you can leave it out.

IBC tanks

A 1000-litre IBC tote sits directly under the downpipe. For my shanty I have two tanks plumbed in series: the first fills before overflow runs into the second. Two IBCs give 2000 litres, which is a useful reserve in a dry spell and is not hard to move if you need to change your layout.

IBCs come up regularly on Marketplace from food and chemical manufacturers. The food-grade ones (which usually held syrup, oil, or food-safe chemicals) are fine for garden water. Avoid anything that held solvents, petroleum products, or harsh chemicals -- they are not worth the risk even after washing.

Plumb the outlet of the first tank into the inlet of the second tank at the base level, so both tanks fill evenly, or run a simple overflow pipe from the top of tank one into the top of tank two. Either way works.

How much water can you collect

The rough formula: roof area in square metres times annual rainfall in millimetres, times 0.9 (for losses to splash and evaporation), gives litres per year. A 50 m2 shanty roof in a 1500 mm annual rainfall area will collect around 67,000 litres a year -- or about 185 litres per day on average. That is more than enough to run an aquaponics system and keep a garden watered.

In practice you collect in bursts during rain events and draw down in dry spells, so the tank volume matters as much as the annual total. Two IBCs is a sensible start; more tanks give you more buffer.