A guide for homes and extensions

Rainwater planters in Ireland: sizing and planning

A rainwater planter takes runoff from a roof downpipe into a planted container. It provides temporary storage before water leaves through the designed drainage arrangement. Rainplanter helps you estimate the planter size for your roof and see whether extra storage is needed.

By Rainplanter · Updated 30 October 2026

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How does a rainwater planter work?

Roof water enters the planter, where space above the soil and voids within the growing medium and gravel provide storage. A suitable outlet allows water to drain after the storm. An overflow and a safe route for excess water are part of the site design.

A rainwater planter is one option within sustainable drainage systems, often called SuDS. It is different from a water butt, which primarily stores water for later use. A planter’s overall outside volume is not its usable water storage: soil, gravel and construction materials occupy some of that space.

How big does my rainwater planter need to be?

There is no single planter size that suits every roof. The calculation depends on the connected roof area, local rainfall, the number of planters, the available storage depth and whether a suitable drainage outlet is available.

Enter your location and roof area. Use the roof area draining to the proposed planter system, rather than the area of the whole property.
Check the downpipes. Rainplanter assumes the roof area is shared equally between the selected downpipes, with one planter per downpipe. Check that this matches the actual roof layout.
Select the drainage arrangement. Only choose a drain or soakaway connection if a suitable connection is available. This selection changes the storage calculation.
Adjust the dimensions. In the planter calculator, compare required storage with provided storage. If a practical planter leaves a shortfall, include the additional attenuation identified in the results.

The initial recommendation is a practical starting point. Confirm the space, levels, construction and drainage connections before using it on site.

What design assumptions does Rainplanter use?

The calculator uses location-based Irish rainfall data for a 1-in-100-year rainfall event, a 20% climate-change allowance and a roof runoff coefficient of 0.95. With an outlet, it compares storm durations and subtracts the assumed restricted outflow from the runoff volume.

Without a normal base outlet, the calculator uses a 30-minute design storm and assumes no normal base discharge. This does not demonstrate how the planter will empty between storms. Infiltration, emptying, overflow and safe exceedance arrangements need to be resolved for the actual site.

These are the tool’s design assumptions, rather than a statement that every Irish local authority requires the same method. Review the design basis shown with your result against your project requirements.

Can I use the pack for a planning application?

The drawing and calculation pack is intended to help explain a concept-stage drainage proposal for a home extension or one-off house. Your architect or project team should check it against the site and the relevant local authority’s requirements.

The pack does not guarantee planning permission or drainage approval. It does not replace a drainage survey, structural design or other site-specific professional input. Check inlet levels, foundations and supports, the proposed connection, overflow and where water can safely go if capacity is exceeded.

What is free, and what does the €10 pack include?

You can size the planter and adjust its dimensions for free. The optional drawing pack costs €10 as a one-off purchase and is provided as a PDF after confirmed payment.

Plan, elevation and section drawings for the selected design.
Construction and storage information.
Preliminary materials quantities / bill of quantities.
Design basis, calculation information and limitations.

Review your project inputs and design before purchasing. Read the drawing pack terms for its scope and supply conditions.

Common questions

Is a rainwater planter the same as a soakaway?

A planter provides planted storage at the surface. A soakaway is designed to infiltrate water into the ground. Selecting a soakaway as a planter outlet does not size or prove that soakaway: it needs its own assessment. For a separate sizing tool, visit soakaway.ie.

What if one planter is not enough?

Use the additional storage figure in the results to inform the wider drainage design. Adjusting planter dimensions or adding another suitable SuDS feature may help. Changing the number of planters must also reflect the actual roof and downpipe arrangement.

Does the planter need maintenance?

Check that the inlet and overflow remain clear, look after the planting, and inspect the outlet for blockage. Follow the maintenance requirements for the final construction and check the system after heavy rain.