Coco coir works in a green roof substrate as the organic fraction, not the bulk of the mix. The German FLL guideline that most of the industry specifies against caps organic matter at under 65 g per litre for extensive roofs, which is roughly 5 to 8 percent by mass in a mineral mix, and research blends keep organic components under about 20 percent by volume. Inside those limits, coir pith raises water storage and lowers weight compared with the same volume of aggregate, and its lignin makes it slow to break down. In living walls, coir appears as fibre mats and as the fill in felt pockets, where its job is to hold water between short, frequent irrigation pulses.
Key facts
- FLL reference values for extensive multi-course substrates: organic matter under 65 g/L, maximum water-holding capacity 35 to 65 percent by volume, air content above 10 percent at maximum water capacity, pH 6.0 to 8.5, water-soluble salts under 3.5 g/L, fines below 0.063 mm under 15 percent by mass.
- Organic matter is limited because too much of it shrinks the substrate as it decomposes. For intensive media with a bulk density above 0.8 g/cm³ the limit works out to about 6 percent by mass.
- Coir pith has a dry bulk density of roughly 0.04 to 0.08 g/cm³ against 0.7 to 1.15 g/cm³ for compacted mineral substrates, so every litre of aggregate you swap for coir takes weight off the roof.
- In a two-year rooftop trial at 20 percent by volume, coir held more water than mushroom compost, composted yard waste or rice hulls but grew the smallest sedum, because it supplies almost no nutrients on its own.
What a green roof substrate has to do
A green roof substrate has to carry plants through drought on a few centimetres of depth, drain fast enough that a storm doesn't pond, keep its volume for decades without being topped up, and weigh as little as possible when saturated. That last point is the one the structural engineer cares about.
So these substrates are mostly mineral. Expanded clay, expanded slate or shale, lava, pumice, crushed brick and sand make up 80 to 95 percent of the volume. They don't rot or shrink, and their particle size sets the drainage. The organic fraction sits in the gaps, holds water the aggregate can't, adds cation exchange capacity so nutrients stay put, and gives roots something to grip while the plants establish.
| Roof type | Substrate depth | Weight range (built-up system) | Plants | What the substrate is tuned for |
|---|---|---|---|---|
| Extensive | 60–200 mm | 60–150 kg/m² | Sedum, moss, drought-tolerant herbs | Low weight, fast drainage, low fertility |
| Semi-intensive | 120–250 mm | 120–200 kg/m² | Perennials, grasses, small shrubs | More water storage and nutrients |
| Intensive | 150–400 mm or more | 180–500 kg/m² | Lawns, shrubs, trees | Water and nutrient storage, structural stability |
Research papers usually draw the line between extensive and intensive at 15 cm of substrate.
The FLL limits that shape the mix
The German FLL guideline is the reference most European and North American specifiers use. The table gives its values for an extensive multi-course build-up, with notes on what coir does to each.
| Property (FLL method) | Extensive multi-course reference value | How coir affects it |
|---|---|---|
| Organic matter (loss on ignition at 550 °C) | < 65 g/L | Coir is almost entirely organic, but it's so light that 10% pith by volume adds only about 4 to 8 g/L. Compost is much denser and hits the cap far sooner |
| Maximum water-holding capacity | 35–65 vol% | Raises it. Pure pith holds 60–80% of its volume in water |
| Air content at maximum water capacity | > 10 vol% | Fine pith lowers it. Fibre and chips raise it |
| Water permeability (saturated) | 0.001–0.12 cm/s (0.6–70 mm/min) | Fine pith slows drainage. Too many fines fail the grading curve |
| Particles below 0.063 mm | < 15 mass% | Pith dust counts. Screened or coarser grades keep you under |
| pH (CaCl₂) | 6.0–8.5 | Coir sits at roughly 5.5 to 6.5, so it pulls alkaline brick and lava mixes toward neutral |
| Water-soluble salts | < 3.5 g/L | Unwashed coir can push this up. Washed coir is low |
| Nutrients: P₂O₅ < 200, K₂O < 700, Mg < 200, N < 80 mg/L | Upper limits | Coir adds potassium. Compost adds more of everything |
Single-course extensive substrates, where one layer does drainage and growing, are tighter: manufacturer sheets quote organic matter under 40 g/L and a water capacity of only 20 to 25 percent by volume. A pumice-based lightweight substrate for roofs with almost no load reserve runs at 3 percent organic matter by mass, 390 to 410 kg/m³ dry and 830 to 950 kg/m³ saturated.
The reason for the organic cap is settlement. Organic matter loses volume as it mineralises, and a roof substrate that drops in depth exposes the drainage layer and dries the plants out. Coir is slow to decompose because of its lignin content, which is why it's an attractive organic fraction, but it's still organic matter under the ignition test and the limit still applies.
The organic figure is a loss-on-ignition result on dry material, expressed per litre of substrate. It's not the same as the volume of compost or coir you tip in. Dry coir pith weighs only 40 to 80 g per litre, so a mix with 20 percent pith by volume carries roughly 8 to 16 g/L of organic matter from the coir, well under the 65 g/L cap. With coir, the tests you're more likely to fail are fines, air content and permeability, not organic content. Compost is much heavier per litre and runs into the cap much earlier. Either way, you can't convert from one material to the other without a lab test on the finished blend.
Where coir fits, and the ratios people use
Published and commercial mixes put coir anywhere from 5 to 20 percent by volume.
| Mix | Coir share | Rest of the mix | Context |
|---|---|---|---|
| Tropical extensive roof, 2.5 cm deep | 5% coir | 10% compost, 5% rice husk, 40% sand, 40% crushed recycled brick | Lab screening of 16 blends. The ones with coir and crushed brick had the most suitable properties |
| US rooftop trial | 20% coir (v/v) | 80% expanded shale and clay fines | Compared against mushroom compost, composted yard waste and rice hulls at the same rate |
| Regional council guidance | 5–20% organic (v/v) | Mineral aggregate | Higher rates help plants but add weight |
| Lightweight research substrates | Under 20% organic (v/v) | Scoria and other aggregates | Coarse and fine coir both tested as the organic component |
A sensible starting point for an extensive sedum roof is 10 percent coir by volume in a graded aggregate, with the lab result on fines and air content deciding whether you move up or down. Semi-intensive and intensive roofs can carry more.
Coarse and fine coir are not interchangeable here. Fine pith stores the most water but drains slowly and adds fines. Fibre and husk chips store less water but keep air in the profile and don't fail the grading curve, so a pith-and-fibre or pith-and-chip blend is more forgiving than pith alone. The numbers for each grade are in the guide to coir water holding and air porosity.
What coir does to saturated weight
Here is the engineer's calculation with round figures. Take a 100 mm extensive substrate. A typical compacted mineral mix weighs about 1,150 kg/m³ dry and 1,350 kg/m³ at maximum water capacity, so 100 mm of it is 115 kg/m² dry and 135 kg/m² saturated.
Replace 10 percent of the volume with coir pith, which is 10 litres per square metre. Dry pith at 0.04 to 0.08 g/cm³ weighs 0.4 to 0.8 kg. At field capacity it holds 60 to 80 percent of its volume in water, so those 10 litres carry 6 to 8 kg of water and the saturated coir fraction weighs 7 to 9 kg/m². The 10 litres of aggregate it replaced weighed about 13.5 kg/m² saturated.
The swap cuts the saturated load by around 5 kg/m² per 10 percent coir, a 3 to 4 percent saving on a 100 mm roof, while holding more plant-available water than the aggregate did. Small, but it runs the right way. Two caveats: coir rewets more slowly than compost after a dry spell, so the first rain after drought soaks in less than you'd expect, and manufacturers quote 8 to 10 percent tolerance on their weights, so a lab test on the actual blend beats arithmetic.
Coir versus peat and compost as the organic fraction
Peat was the default organic fraction in Northern Europe for decades: it holds water, it's acidic enough to balance alkaline aggregates, and it's light. With peat extraction being restricted, compost is the usual replacement and is what most German commercial substrates use.
Compost brings nutrients, which sedum roofs don't want much of and perennial roofs do. It also brings fines and keeps decomposing. In a survey of 22 German extensive substrates, the grading curves that failed the fines limit failed because of the organic part of the mix, and the organic-rich substrates carried the most nitrogen.
Coir brings water storage without the nutrients. In the two-year rooftop trial at 20 percent by volume, coir modules had the highest volumetric water content during and between storms, but the sedum in them grew less than in the compost treatments and showed more leaf scorch. Nutrient availability, not water, limited establishment. For a sedum roof, a slow-release feed at planting covers that. For a perennial roof, blend coir with compost rather than choosing one.
| Organic fraction | Water storage | Nutrients supplied | Fines added | Decomposition rate | Weight |
|---|---|---|---|---|---|
| Peat | High | Low | Moderate | Slow | Low |
| Green waste compost | Moderate | High | High | Fast | Moderate |
| Coir pith | High | Very low (some potassium) | Moderate | Slow | Low |
| Coir fibre or chips | Low to moderate | Very low | Low | Slow | Low |
Salt and EC
Raw coir can carry potassium, sodium and chloride from retting. FLL puts water-soluble salts under 3.5 g/L for the finished substrate, and lightweight commercial substrates test as low as 0.1 g/L. Even at 10 percent by volume, a coir with a high raw EC can move the whole mix toward the limit, and sedum on a thin roof has no reservoir to dilute it.
Specify washed coir and ask for the EC of the batch. CVM Fiber and Industries washes its coco peat with water to bring EC down and uses no calcium nitrate or other chemicals, so it isn't buffered. On a roof that's usually fine, because brick and lime-bearing aggregates supply the calcium, but for a perennial roof it's worth reading how coir EC and pH behave before you settle on a rate.
Coir in living walls
On a roof, coir is a minor fraction of a loose mix. On a wall it's often the whole growing medium, held in a pocket or pressed into a mat.
Felt pocket systems. The pockets are usually needle-punched synthetic felt, often recycled PET. The fill is a light potting mix or straight coir, because anything heavy pulls the pocket out of shape. One widely specified pocket panel is rated at 50 kg/m² fully saturated and is irrigated from the top row with 1.9 L/h pressure-compensating drippers, run for 1 to 10 minutes about every two hours outdoors. Water moves down through the felt by capillarity and wets the pockets below. Coir in the pocket catches that pulse and releases it to the roots over the two hours.
Coir fibre mats. Mattress-grade coir fibre needled into a mat, sometimes with a latex or synthetic binder, is used as a root mat in modular panels and as the growing layer in some hydroponic walls. The mat drains vertically, so the top dries first and the bottom stays wet, and species placement follows that gradient. The fibre is the grade described in the coconut fibre grades guide, and the mat construction is close to the needle-punched blankets used in coir erosion control.
Coir in rigid modules. Trays filled with a coir-based mix behave like a shallow container stood on its edge. The mix needs more fibre or chips than a pot would, because gravity drains a vertical 100 mm profile fast.
In a four-month indoor trial that compared coconut fibre, geotextile felt, a synthetic epiphyte substrate and tree-fern fibre, all four grew plants adequately and the wall cooled the room by an average of 4 °C. The geotextile cooled best but used the most water. The coconut fibre's weakness was degradation. In a separate green wall study, a coir and rice husk blend distributed irrigation water more evenly across the wall than sphagnum moss, with no difference in plant biomass.
Water and nutrients on a vertical surface
A living wall is irrigated little and often because it has no reservoir. Every pulse runs top to bottom and the medium's job is to catch a share of it on the way past. Coir does that well, but it holds nutrients loosely, so the feed has to be in the irrigation water rather than in the medium. Run-off from the bottom carries salts out, which is why coir walls rarely build up EC the way a pot does. Outdoor walls in wet climates can leach out instead, so monitor the drain EC rather than guessing.
Breakdown and replacement
Coir in a wet, aerated, warm wall breaks down faster than the same fibre on a slope or in a roof mix. In the indoor trial above, degradation of the coconut fibre was already a problem within four months. As the structure slumps, water channels past the roots and the panel holds less. Plan on rebuilding fibre mats with the plants lifted and reset, and on topping up pocket fills as the coir settles. Inspect the lower panels first, where the medium stays wettest. On a roof the same material lasts far longer because it's protected in a mineral matrix that dries out between rains.
Specifying coir for a roof or wall project
For a roof substrate blender: state the grade (pith alone, pith with fibre, or husk chips) with a maximum fines content, ask for washed material and the EC of the batch, and get the finished blend tested to FLL methods rather than the coir on its own. Start at 10 percent by volume for extensive and up to 20 percent for semi-intensive and intensive, and let the test move you.
For a living wall: use pith with a share of fibre or chips in pocket fills so they drain between pulses, specify fibre grade and thickness for mats and ask how they're bound, and plan nutrition through the irrigation.
CVM Fiber and Industries, in Kanyakumari, Tamil Nadu, India, supplies coco peat blocks and briquettes, husk chips and coir bristle fibre, and can prepare pith, fibre and chip blends to a stated ratio as a custom blend. If you're putting together a roof or wall substrate and want to talk through grades and ratios, get in touch.
FAQ
Can a green roof substrate be 100 percent coco coir?
No. Coir on its own is almost entirely organic matter, and extensive roof guidelines cap organic content at under 65 grams per litre, with intensive roofs at about 6 percent by mass. A pure coir roof would also shrink as it decomposed, blow away when dry and fail the fines and permeability tests. Coir belongs in the mix at 5 to 20 percent by volume alongside mineral aggregate.
How much coir can I add and still pass FLL?
Usually more than the organic cap suggests, because coir is so light. Dry pith at 40 to 80 grams per litre means 20 percent by volume adds only 8 to 16 grams per litre of organic matter against a 65 gram limit. With fine pith you'll hit the fines, air content or permeability limits first. The only reliable answer is a lab test on the finished blend.
Does coir make a green roof lighter or heavier?
Lighter, slightly. Dry coir pith weighs around 0.04 to 0.08 grams per cubic centimetre against roughly 1.15 for a compacted mineral mix, and even saturated it weighs less than the aggregate it replaces. Swapping 10 percent of a 100 millimetre substrate for coir saves roughly 5 kilograms per square metre at saturation while raising water storage.
Is coir better than compost for a sedum roof?
For water storage and stability, yes. For plant establishment, compost wins because it supplies nutrients and coir does not. In a two-year rooftop comparison, coir modules held the most water but grew the smallest sedum. The practical answer is coir for the water and structure, plus a slow-release feed or a share of compost for the nutrition.
Why is organic matter limited in green roof substrates at all?
Because organic matter decomposes and the substrate loses depth as it does. A roof that settles exposes the drainage layer, dries out faster and can't be easily topped up. The limit is set by measuring weight loss when a dry sample is burned at 550 degrees Celsius, so it covers coir, peat and compost alike.
What does coir do in a living wall?
It catches the short irrigation pulses that run down the wall and releases the water to roots between pulses. It's used as fill in felt pockets, as needled fibre mats, and in coir-based mixes inside rigid modules. Nutrition has to come through the irrigation water, because coir holds nutrients loosely and the wall is flushed every cycle.
How often does coir in a living wall need replacing?
Faster than on a roof, because the wall is wet, warm and aerated, which speeds decomposition. In one indoor trial the coconut fibre was already degrading within four months. Fibre mats slump and channel and are rebuilt with the plants reset, while pocket fills are topped up as the coir settles. Inspect the lower panels first, since they stay wettest.
Do I need washed or buffered coir for a green roof?
Washed, yes. The finished substrate has to stay under 3.5 grams per litre of water-soluble salts and sedum on a thin roof can't dilute a salty medium. Buffering is usually unnecessary because the mineral aggregates, especially brick and lime-bearing material, supply plenty of calcium to the mix. Ask for the EC of the batch either way.