Tag: greenroofs
A Tale of Two Assemblies…CRA versus PMRA
Conventional (CRA) and protected membrane roof assemblies (PMRA) are the most common low-slope commercial roofing systems in Canada. The key difference is the placement of the roof membrane relative to the thermal insulation.
While installing a vegetated or green roof system on a conventional roof is relatively straightforward, component configuration can be trickier with PMRA.
Here, we untangle the two so you can design and construct a vegetated system on either roof assembly with confidence.
What is a Green Roof’s Conventional Roof Assembly?
In a Conventional Roof Assembly, the roof membrane is installed on top of the thermal insulation. Although many insulation products are suitable, choose ones with sufficient compressive strength to support the weight of the vegetated system. A cover board is recommended to spread the load across the insulation.
The vegetated system is installed above the roof membrane, starting with the root barrier and followed by the rest of the buildup. Some roofing manufacturers recommend laying a protection fleece or a protection board to protect the roof membrane from foot traffic during the vegetated system installation.
What is a Protected Membrane Roof Assembly (PMRA)?
In a Protected Membrane Roof Assembly or PMRA, the thermal insulation is placed on top of the roof membrane. It is also called Protected Membrane Roof (PMR), Inverted Roof Membrane Assembly (IRMA), or upside-down roof, referring to reversed positions of the membrane and insulation compared to the conventional assembly.
The insulation is now exposed to the elements and must resist water absorption, thermal expansion and contraction, freeze-thaw cycling…etc. The only insulation that is suitable for PMRA is extruded polystyrene (XPS), which is ballasted by aggregates, pavers or concrete topping to prevent flotation and wind uplift.
The amount of ballast required depends on the thickness of the XPS used, starting from 60 kg/m2 (12 psf) for 50 mm (2”) of XPS, and increases by 24 kg/m2 (5 psf) for every additional 25 mm (1”) of XPS. A filter fabric is usually placed between the XPS and the ballast, which not only prevents dirt from settling in the joints but allows the XPS to “raft” or float together, thus reducing the amount of ballast required.
Can a Vegetated System Act as Ballast in a Green Roof?
A vegetated system can replace some or all of the ballast required depending on its dry weight. This is the lowest weight when there is no moisture present in the system. This can be measured using ASTM E2397. Let’s look at an example where 60 kg/m2 (12 psf) of ballast is needed to hold down the XPS on a PMRA.
Let’s say “Vegetated System A” weighs 70 kg/m2 (14 psf) when dry and 100 kg/m2 (20 psf) fully saturated. It can replace all the ballast required as its dry weight is greater than 60 kg/m2 (12 psf). Now, we have a lighter “Vegetated System B” that weighs 40 kg/m2 (8 psf) when dry and 70 kg/m2 (14 psf) fully saturated. We will need at least 20 kg/m2 (4 psf) of ballast to make up the 60 kg/m2 (12 psf) required.
The ballast aggregates are installed on top of the insulation/filter fabric and double the duty as a drainage layer for the vegetated system. Note that if the vegetated system is used to ballast the XPS either partly or entirely, it cannot be removed without substituting alternative ballast in the future.
Where Does the Drainage Go in a Green Roof?
DuPont, a major XPS manufacturer, recommends that the drainage should always be above the XPS. This allows the water to drain off the top surface of the insulation and creates a “diffusion open” assembly. The drainage provides a ventilating air space for topside venting to create open diffusion.
Although installing a drainage layer under the XPS can divert water to drains quicker, unfortunately, it can create a “convective loop” below the XPS. Air can move through the drainage layer and potentially reduce the overall insulation value.
As a result, DuPont recommends installing the drainage above the XPS to ensure the insulation value is maximized. A good positive slope to drain is an effective and better way to divert water to drains in PMRA.
Root Barrier Installation for a Green Roof
DuPont recommends the root barrier be installed above the roof membrane and under the XPS. Root barriers are flexible sheets made of polyethylene or polypropylene that physically block root ingress to the roof membrane.
These plastic sheets are vapour impermeable. If placed on top of the XPS, water vapour in the system cannot escape and can potentially be driven back into the XPS, which can reduce its insulation value. Thus, a root barrier should be placed below the XPS to achieve “open diffusion”.
Green Roof Best Practices for Specifications
As the root barrier often falls under “Vegetated System” instead of “Roofing System” in construction specifications, this can create a workflow/coordination issue. Therefore, we recommend the root barrier be specified under “Roofing System” so it can be installed by the roofing contractor. It should also be referenced under the “Vegetated System” so the green roof contractor is aware it exists but focuses on the rest of the vegetated system buildup above the PMRA.
Further Reading and Reference Material
DuPont’s “Vegetative Roof PMR – A Design Primer” is a good reference that offers practical building science advice for designers who are interested in vegetating PMRA. All NLSM’s systems can be installed on conventional and PMRA.
Supplier of more than 2.5 M SQFT Coast-to-Coast
NLSM is Canada’s leading supplier of pre-vegetated roofs that are customized to local requirements. NLSM offers warranty & maintenance support, provides stormwater calculations specific to location and building/roof type, stormwater test data & related innovative hydrologic modelling. Contact NLSM for the best green roof option to meet your design goals.
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
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.
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
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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™.
Supplier of more than 2.5 M SQFT Coast-to-Coast
Canada’s leading supplier of pre-vegetated roofs, NLSM customizes to local requirements, offers warranty & maintenance support, provides stormwater calculations specific to location and building/roof type, stormwater test data & related innovative hydrologic modeling. Click here to get started on your next green roof.
More than 85 Years Experience
Next Level Stormwater Management (NLSM) is Canada’s leading supplier of pre-vegetated roofs such as LiteN’Less™ and StormCap™ systems. NLSM’s team has more than 85 years of combined green roof industry experience: helping with stormwater calculations and technical advice, design, planning and maintenance support, assisting with required documentation, issuing P.Eng-stamped wind reports and supplying and delivering the complete system package.
Enjoy this rare bird’s eye view of NLSM green roofs!
Supplier of more than 2.5 M SQFT Coast-to-Coast
Canada’s leading supplier of pre-vegetated roofs, NLSM customizes to local requirements, offers warranty & maintenance support, provides stormwater calculations specific to location and building/roof type, stormwater test data & related innovative hydrologic modeling. Click here to get started on your next green roof.