Comparing Establishment Methods for Extensive Green Roofs 

Built-in-Place and Modular Vegetated Systems 

Traditionally, vegetation has been established on roofs using plugs, cuttings or seeds. These are called built-in-place vegetated systems. The initial vegetation coverage typically ranges 5-15% and it takes regular TLC (water, weed and feed) and patience (1-3 years) for the young plants to mature and establish.  

Increasingly, vegetation is first pre-grown in modules on the ground for quicker establishment on the roof. These are called modular vegetated systems. These pre-vegetated modules contain mature plants with 80+% vegetation coverage at delivery, and typically takes only one growing season to establish on the roof. 

If we could use an analogy comparing establishing a green roof to starting a front lawn, built-in-place systems would be akin to sowing seeds.  Modular systems would be equivalent to laying sod. Instant green, ease of installation, reduced establishment time and maintenance efforts are some attributes that make modular vegetated systems such as mats and trays popular in North America.  

Modular Pre-Vegetated Mat and Tray Systems

Modular pre-vegetated green roof systems are similar to laying sod.

Pre-vegetated mats contain vegetation planted in a shallow growing medium (2-3 cm) on a vegetation carrier. Natural carriers made of coir fibre matting decompose over time and integrate into the media. Synthetic carriers consisting of 3D entangled mesh provide enhanced root anchorage and mat integrity, which are suited for sloped roofs. They are durable and can be readily rolled back, even many years later, for membrane access. Pre-vegetated mat systems are loose laid, yet highly wind resistant. 

Pre-vegetated trays contain vegetation planted in containers resembling shallow planter boxes. They are made of plastics or metals and come in different sizes, typically containing 5–20 cm of growing medium. Some trays are designed to be fastened or interlocked together for stability and wind resistance. There are also trays that are made of natural materials that decompose and integrate into the media over time. 

Green roof market share in North America by planting types according to GRHC, 2016.

Why Vegetated Mat Systems Dominate

Green Roofs for Healthy Cities, a North American green roof industry association, conducted a detailed market survey in 2016. They showed that mats topped the North American market at 47% followed by trays at 39%. The difference is even higher when it comes to extensive systems 56% for mats vs. 29% for trays.

  • Lighter weight: 60-150 kg/m2 for mat systems vs. 200-300 kg/m2 for trays 
  • More economical: $150-200/m2 for mat systems vs. $250-350/m2 for trays 
  • No membrane puncture hazards: mat systems are flexible and do not contain any sharp edges or corners
  • Pre-vegetated mats are easy to cut and conform around drains, penetrations and curved and organic landscapes.

    Easy to cut and conform: mat systems are easier and less messy to cut around drains and penetrations, they fit curved and organic landscape design without looking “pixelated” and conform well to 3D contours such as dome or wavy shaped rooftops

  • Quicker installation: a typical mat is about 10X the size of a typical tray. Mats and baselayers are flexible and much easier to cut than the hard trays. These save as much as 25% labour on large roofs
  • Easy membrane access: mat systems, especially soilless, can be readily rolled back or cut for membrane access. It is more difficult with trays as they are either interlocked or “jammed” tight over time. 

Biodiversity in Trays and Mats

Trays, especially deeper ones (10–20 cm), can accommodate larger plants. Extensive mat systems can also incorporate larger plants with the BioBerm™ method.


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NLSM is Canada’s leading supplier of pre-vegetated roofs such as LiteN’Less™, Xeroflor® and StormCap™ systems. We customize to local requirements. Offer warranty& maintenance support, provide ​stormwater calculations specific to location ​and building/roof type, stormwater test data & related innovative hydrologic modeling. Email or call (416) 637-5772 Ext 1.