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Green Roof Vegetated Tray Rainwater Retention Calculator

Calculate the rainwater volume a green roof's vegetated trays retain before runoff, based on their retention depth.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Retained rainwater volume

330.22 gal

Check your inputs

Retention depth is the manufacturer's rated figure for the specific tray/media system at saturation — actual retention varies with antecedent moisture and rainfall intensity/duration.

At the values currently entered, the retained rainwater volume works out to 1250. Note that total vegetated tray area sits at the low end of the range this calculator was checked against, so treat the output as indicative rather than settled. Confidence is moderate: the method is sound, but real materials and site conditions vary. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Retained volume = tray area × the tray system's rated water retention depth, the standard method for estimating a modular green roof system's stormwater retention capacity

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Frequently asked questions

Where does the rated retention depth come from?
It's the manufacturer's rated figure for the specific vegetated tray and growing-media system at saturation, typically published in the product's technical data sheet.
Will the trays always retain this exact volume?
No — this is a rated capacity at saturation. Actual retention on any given storm varies with antecedent soil moisture (how wet the media already is) and the rainfall's intensity and duration, so treat this as a design-capacity estimate, not a guaranteed outcome.
Does this size downstream drainage or overflow?
No — this only estimates the volume retained by the tray system itself. Overflow and scupper/drain sizing for water beyond the retention capacity should be calculated separately.