DIY lawn irrigation design: zones, pressure, flow and head spacing

A well-designed system waters every part of the lawn evenly with the least water. A poorly designed one leaves dry patches and soggy corners however long it runs. Most problems come from three things: not enough water supply for the number of heads, mixing sprinkler types, and heads spaced too far apart. Good design avoids all three, and it happens on paper before you buy a single part.

This guide covers planning. For connecting to your water supply, check your water authority’s requirements first: work on the connection to mains water, including backflow prevention, may need a licensed plumber. The installing a pop-up system guide covers the hands-on work.

1. Draw the lawn

Measure the lawn and draw it to scale on graph paper, including:

• paths, driveways, garden beds, trees, sheds and fences;
• the tap or water supply point you’ll connect to;
• slopes, low spots and areas in heavy shade;
• where power is available for a controller.

A scale such as one square for every half metre makes head spacing easy to plan. Mark the lawn’s area too; the Lawn Area Calculator helps with odd shapes.

2. Measure your water supply

How many sprinklers can run at once depends on how much water your supply delivers.

• Flow: time how long it takes to fill a bucket of known volume from the tap you’ll use, with the tap fully open and no other taps running. Divide the volume by the time to get litres per minute. A 10-litre bucket that fills in 20 seconds means 30 litres per minute.
• Pressure: a gauge on the tap gives static pressure, the pressure with no water flowing. Pressure drops once water flows, and drops further through pipes, fittings and valves.

Do the flow test at the time of day you’ll water, because pressure can vary. Your irrigation supplier can use the flow and pressure to work out how many heads each zone can run and what pipe size you need.

If you’re using a rainwater tank and pump, design to the pump’s flow and pressure instead.

3. Split the lawn into zones

Each zone runs separately, so the supply only has to feed one zone at a time. Group areas that need the same watering:

• same sprinkler type (sprays and rotors on separate zones);
• similar sun and shade;
• similar soil and slope;
• lawn separate from garden beds, which usually suit drip irrigation.

A shady side lawn and a full-sun back lawn on the same zone will always have one over- or under-watered. Keep each zone’s total flow within what your supply delivers, with some margin. Add up the flow figure for each head and nozzle from the manufacturer’s chart.

4. Choose heads and nozzles

Read the choosing sprinklers guide for the full comparison. In short:

• Pop-up sprays for small and narrow areas;
• rotary nozzles where sprays would run off on clay or slopes;
• rotors for larger open lawns;
• strip and part-circle nozzles along edges and paths;
• sub-surface drip for narrow strips, odd shapes and greywater systems.

Use one range of heads and nozzles per zone, so that part-circle and full-circle heads apply water at matched rates.

5. Space the heads for overlap

Even coverage comes from overlapping patterns. A common approach is “head-to-head” spacing, where each head’s spray reaches the next head. Use the spacing recommended by the manufacturer for your chosen heads and the pressure you have.

1. Put heads at the corners first, using quarter-circle nozzles.
2. Add heads along the edges, using half-circle nozzles, at the recommended spacing.
3. Fill the middle with full-circle heads.
4. Check every point on the drawing is reached by more than one head.

Square layouts suit rectangular lawns; triangular layouts can cover open areas more evenly. Avoid stretching the spacing to save a head: a dry strip between heads is the most common complaint about new systems.

6. Plan the pipe

• Keep runs short and direct from the valve to the heads.
• Size the pipe to the flow. Small pipes over long distances lose pressure, so heads at the end of a run throw shorter. Your supplier can check pipe size against your zone flows.
• Use looped or branched layouts on larger zones so pressure stays even across the heads.
• Leave room for maintenance: valves in a valve box you can reach, and enough pipe flexibility to raise heads later.

7. Choose a controller

A controller with separate run times for each zone, a seasonal adjustment and a rain delay (or rain sensor) saves the most water. Weather-based “smart” controllers adjust automatically. Set it within your water authority’s allowed days and times; Icon Water’s permanent rules in Canberra, for example, set the hours when sprinklers can run in the warmer months and prohibit watering so heavily that water pools or runs off. See the smart controllers guide.

8. Parts list

A typical list for one or more zones:

• connection to the supply, with backflow prevention as your water authority requires;
• a filter;
• solenoid valves, one per zone, in a valve box;
• main and lateral pipe, fittings and clamps;
• sprinkler bodies and matched nozzles;
• swing joints or flexible risers, so heads can be raised and survive being knocked;
• controller and cable;
• rain sensor, if your controller supports one.

Products certified under the Smart Approved WaterMark program have been assessed as water efficient.

9. Install, then test

After installation, do a catch-cup test on each zone. Adjust nozzles, heads and run times until coverage is even. Re-test each summer.

10. Set the schedule

Use the catch-cup results and your watering target to set run times for each zone with the Sprinkler Run-Time Calculator. On clay or slopes, split runs into two or three cycles with a break between. Use the controller’s seasonal adjustment to scale run times down in autumn and up in spring.

A worked example

Here’s how the steps come together for a typical back yard. The numbers are for illustration; use your own measurements and your heads’ charts.

• The lawn: a 12 m × 8 m rectangle in full sun, plus a 2 m-wide side strip along the house that’s shaded most of the day.
• The supply: a flow test at the back tap fills a 10-litre bucket in 25 seconds, which is 24 litres a minute.
• The zones: the main lawn and the side strip get different sun, so they become separate zones. The garden beds go on drip, on a third zone.
• The heads: the main lawn is large enough for pop-up sprays with matched nozzles, or a few small rotors. The side strip suits side-strip nozzles or sub-surface drip, which keep water off the house wall.
• The check: adding up the flow figure for each head on the main lawn from the maker’s chart shows it needs more than 24 litres a minute. So the main lawn is split into two zones, east half and west half, each well within the supply.
• The schedule: after installing, a catch-cup test on each zone gives its application rate, and the calculator turns the watering target into run times.

The result is four zones instead of the one or two a first sketch might suggest, and every head gets the pressure it was designed for.

Planning for the future

• Leave spare capacity. A valve box with room for one more valve costs little now and saves digging later.
• Run cable for an extra zone while the trench is open.
• Think about the garden in five years. Trees grow and shade the lawn; a separate zone for the area under them lets you water it differently.
• Plan for a tank. If you might add a rainwater tank, ask your supplier how the system could switch between tank and mains.
• Keep the drawing. Mark where every pipe and valve actually went. It’s invaluable when you need to repair a leak or add a head.

Before you dig

Contact Before You Dig Australia before digging, so you don’t hit underground services such as water, gas, power or telecommunications. Check with your council and water authority about any approvals, especially for work near the street or on the nature strip.

Common mistakes

• Too many heads on one zone for the water supply, so none of them work properly.
• Mixing sprays and rotors on the same zone.
• Heads too far apart, leaving dry gaps between patterns.
• Watering paths and fences. Use part-circle heads along edges.
• Lawn and garden beds on the same zone, when they need very different watering.
• Forgetting to test after installation. A catch-cup test finds problems before the lawn does.

How long does it take?

Planning on paper takes an evening. A flow test takes ten minutes. Installing a small two- or three-zone system is usually a weekend’s work for a confident DIYer, plus the time for any plumbing connection. The catch-cup test and fine-tuning take another morning. Spreading the work out is fine; just keep trenches safe and covered in the meantime.

When to get help

A good irrigation supplier can check your plan and flow figures for free or a small fee when you buy parts. For larger lawns, complex slopes, bore or tank pumps, or anything involving the mains connection, a licensed plumber or irrigation installer is worth the cost.