Description
A sedum blanket or wildflower green roof only performs as well as what it's rooted into. Ordinary topsoil is the wrong growing medium for a roof: it's heavy when saturated, it drains poorly, and it's too rich in nutrients — sedum and other extensive-roof planting are adapted to lean, free-draining, low-nutrient conditions, and a rich, water-retentive topsoil will encourage soft, waterlogged growth that thins out or dies back within a season or two. This is a purpose-engineered lightweight mineral growing substrate for extensive green roofs: a free-draining aggregate blend designed to sit above a drainage layer and root barrier, hold just enough moisture and nutrient for sedum, wildflower and similar low-maintenance planting, and stay light enough for a domestic flat roof structure to carry when saturated.
It's the layer that's been missing from a proper build-up: root barrier underneath to protect the waterproofing, a drainage board to move surplus water to the outlets, this substrate to grow into, and a sedum blanket or seed mix on top.
Specification
| Type | Lightweight mineral aggregate growing substrate for extensive green roofs (not general garden topsoil or compost) |
| Composition | Predominantly crushed/expanded lightweight mineral aggregate with a smaller proportion of low-nutrient organic matter, blended for free drainage and low fertility |
| Dry bulk density | Approximately 0.95–1.1 tonnes/m³ depending on batch moisture content |
| Saturated weight | Higher than dry weight once wet — always confirm actual saturated load with your structural engineer or building control before specifying a green roof build-up; we don't publish a load figure here because it varies by depth, moisture and drainage design |
| pH | Typically around pH 8 (mildly alkaline), within the normal range for extensive substrates |
| Peat | Peat-free formulation |
| Recommended depth | Commonly 60–100mm for sedum-only extensive roofs, deeper (100–200mm) for biodiverse wildflower roofs — always check your sedum blanket or seed supplier's minimum depth |
| Formats | Small Bag (25 litres) and Bulk Bag (1m³) |
How much do I need?
Coverage depends on the depth you lay, not just the roof area — always calculate by volume (area × depth), not by area alone:
| Format | Volume | Coverage at 25mm | Coverage at 80mm (typical extensive depth) |
|---|---|---|---|
| Small Bag | 25 litres (0.025m³) | 1m² | ~0.31m² |
| Bulk Bag | 1m³ (1,000 litres) | 40m² | 12.5m² |
Worked example: a 12m² garden room roof at a typical 80mm extensive depth needs 12 × 0.08 = 0.96m³ — just under one Bulk Bag. A small patch repair of 1m² at 25mm needs one Small Bag. Manufacturer or specifier guidance can call for a different depth depending on the planting scheme, roof loading and local climate, so treat 80mm as a general planning figure rather than a fixed rule.
Pair it with the rest of the build-up
This substrate is designed to complete the same green roof system as our other layers — order them together for a full specification:
- Green Roof Root Barrier Membrane — protects the waterproofing beneath from root penetration
- Green Roof Drainage & Water Reservoir Board — moves surplus water to your outlets and holds a reservoir for dry spells
- Sedum Green Roof System — Complete Blanket Kit — the pre-grown planting layer that roots directly into this substrate
FAQs
How much substrate do I need for my roof?
Work out the volume, not just the area: multiply your roof area (m²) by your planned depth (m). For example, a 20m² roof at 80mm depth needs 20 × 0.08 = 1.6m³, so two Bulk Bags. Always allow a small margin for settling and an uneven deck.
Can I use ordinary topsoil or multi-purpose compost instead?
No. Standard topsoil and compost are too heavy once saturated for most roof structures, retain far more water than a roof build-up is designed to hold, and are too rich in nutrients — sedum and wildflower roofs need a lean, free-draining substrate or they become waterlogged, encourage weed growth, and put unplanned extra load on the structure.
What depth do I need?
Extensive sedum-only roofs are commonly specified at 60–100mm; biodiverse wildflower roofs typically need more, often 100–200mm. Depth affects both plant establishment and structural load, so check the minimum depth stated by your sedum blanket or seed mix supplier, and confirm loading with a structural engineer.
Do I need a root barrier and drainage board underneath?
Yes, for a proper build-up. This substrate isn't a waterproofing or drainage product — it needs a root barrier below it to protect the roof membrane and a drainage layer to move surplus water to the outlets. See the linked products above.
Is this substrate suitable for a garage, shed or garden room roof?
Yes, extensive green roof substrate is specifically formulated for lightweight, low-maintenance domestic roof applications like these. As with any green roof, confirm the structure can carry the saturated weight of the full build-up (membrane, drainage, substrate and planting) before installing.
Ordering & installation guidance
Order the Small Bag for repairs, top-ups, samples or very small roofs, and the Bulk Bag for full roof installations — calculate your volume first using the table above. Bulk Bags are heavy and palletised; check delivery access and have a plan to move or barrow the bag to roof level before it arrives. Lay the substrate evenly over your drainage board using a rake or float, avoid compacting it more than necessary, and lightly firm and level before laying your sedum blanket or sowing a wildflower mix on top.
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