SettingsSettings for this calculationUS
The total green roof area to receive the drainage layer.
Measure the full area of the green roof assembly that will sit above the drainage/water-retention layer, before the growing medium is placed.
The area covered by a single roll of drainage/water-retention mat.
Check the manufacturer's data sheet for the roll's net coverage area — a common roll size covers roughly 100 sq ft (about 9.3 m²).
Drainage layer rolls needed
10 rolls
They open the calculator with your figures already in it
Vegetative (Green) Roof Drainage Layer Calculator: 10 rolls — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Roll count = ceil(roof area ÷ roll coverage area), the standard method for estimating rolled drainage composite/mat material installed beneath a green roof's growing medium
Inputs used
- Green Roof Area
- 970 sq ft
- Roll coverage area
- 100 sq ft
What this calculation does not cover
- Counts rolls as bare area divided by the coverage figure you enter. Manufacturer-required side and end laps, cuts around drains, vents, upstands and edge trim, and offcut waste on a narrow or irregular roof all add material this number does not include — unless the coverage you typed was already the lapped net figure rather than the roll's own dimensions.
- Takes the area exactly as typed and makes no adjustment for slope. On a pitched green roof the surface the mat has to cover is larger than the plan area scaled off the drawing.
- This is a material take-off, not a roof drainage design. It does not check that the drainage layer can move the design storm across the roof to the outlets, and it does not size drains, scuppers or overflows — those stay with the drainage design and the code requirements that apply to it.
- Does not choose or verify the product. Required core thickness, reservoir depth and in-plane flow capacity follow from roof slope, the drainage path length to the outlets and the specified water retention. Enter a roll whose performance has already been confirmed against the specification; this only counts it.
- Covers one layer of the build-up. Root barrier, protection mat, filter fleece above the drainage layer, gravel and edge margins and the growing medium are separate take-offs, and the water this layer is designed to hold adds saturated dead load that a structural engineer has to verify against the roof's capacity.
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.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Roll count = ceil(roof area ÷ roll coverage area), the standard method for estimating rolled drainage composite/mat material installed beneath a green roof's growing medium
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