Purple-Roof Green Roof Construction Specifications

Purple-Roof is a non-proprietary concept for a green roof that incorporates high retention and detention capabilities. This concept, or these specifications, may be seen as a "recipe".

This page explains the Purple-Roof concept in the following three formats:

3-Part CSI Construction Specification

For 3-Part CSI construction specifications in MS Word or PDF, select one of the options below:

Purple-Roof green roof construction specfication (retention & detention) with gravel and pavers

Purple-Roof concept (retention & detention) with gravel and pavers

Purple-Roof green roof construction specfication (retention & detention)

Purple-Roof concept (retention & detention)

Purple-Roof green roof construction specfication (retention & detention) with pavers

Purple-Roof concept (retention & detention) with pavers

Purple-Roof green roof construction specfication (retention & detention) with gravel

Purple-Roof concept (retention & detention) with gravel

Purple-Roof green roof construction specfication (retention & detention) - pavers only

Purple-Roof concept (retention & detention) - pavers only

Purple-Roof green roof construction specfication (retention & detention) - gravel only

Purple-Roof concept (retention & detention) - gravel only

Sponge Roof (mineral wool / retention) green roof construction specfication with gravel and pavers

Sponge Roof concept (retention) with gravel and pavers

Sponge Roof (mineral wool / retention) green roof construction specfication

Sponge Roof concept (retention)

Sponge Roof (mineral wool / retention) green roof construction specfication with pavers

Sponge Roof concept (retention) with pavers

Sponge Roof (mineral wool / retention) green roof construction specfication with gravel

Sponge Roof concept (retention) with gravel

Purple-Roof is a Non-Proprietary Specification

One of the first questions we usually get is, "is this a proprietary specification?" , which usually means "is there only one place I can buy it"?

Purple-Roof is a concept that may be implemented – and is intended to be implemented – by multiple manufacturers. This approach was chosen to allow competitive bidding , customization of assemblies for any market, and installation over various suitable roofing membranes.

Purple-Roof is a clearly defined and trademarked brand so that we can protect the integrity of solutions that claim to implement the Purple-Roof concept.

If you see the Purple-Roof name and logo, then you can rest assured that the manufacturer representing that product is following the specifications on this page, and is using data and research generated by Green Roof Diagnostics.

Each of the manufacturers below offers green roof assemblies that comply with Purple-Roof specifications. Please obtain manufacturer-specific product data from:

American Hydrotech

Garden Roof Plus

Knauf Insulation

Urbanscape Detention Roof

Next Level Stormwater Management

StormCap+ Detention

Sempergreen

Detention Roof

Sempergreen USA

Sempergreen Purple-Roof

Stormwater Capture Co.

StormCap+

Uniseal

Urbanscape Purple-Roof

High-Level Overview

Purple-Roof’s overall function can be described using two categories: Hydrological and Biological. Click the links below to learn more about each.

Purple-Roof diagram vegetation and soil

Biological

Purple-Roof diagram retention and drainage

Hydrological

Purple-Roof fits into a broader spectrum of vegetative roof covering concepts:

traditional sod roof green roof

Sod Roof

vernacular insulating concept

urban greening green roof

Traditional Green Roof

urban greening concept

high retention green roof

Sponge Roof

high-retention concept

stormwater retention and detention green roof

Purple-Roof

retention and detention concept

blue-green stormwater green roof

Blue-Green Roof

retention and detention concept

In many ways, the evolution of these concepts can be viewed from left-to-right.

  • Sod roofs are part of traditional Scandinavian architecture, and provide significant thermal insulation benefits.
  • The traditional/conventional green roof was developed in Germany in the 1960s and 1970s as a lightweight, economical way to green urban areas , and improve the fire resistance of buildings.
  • The Sponge Roof concept is a lighter-weight, higher stormwater retention version of the Traditional Green Roof concept. The incorporation of mineral wool generally hallmarks this concept as a means of doubling the stormwater retention of a Traditional Green Roof.
  • The Purple-Roof concept is an adaptation of the Sponge Roof concept, which maintains high retention but adds stormwater detention via a friction layer. Purple-Roof combines the detention abilities of Blue- and Blue-Green Roofs with the retention ability of a Sponge Roof for the ultimate combination and best value proposition.
  • The Blue-Green roof concept is a combination of a Blue Roof (ponding water on a roof with no vegetation) and either the Traditional Green Roof or Sponge Roof concepts, which essentially involves building a green roof over a blue roof . The Purple- and Blue-Green concepts are the only two concepts that provide significant stormwater detention.

Each of the concepts above has a place. And other concepts fit less neatly into the above continuum, such as the Brown Roof concept, which primarily focuses on biodiversity. (Yes, in this industry, we tend to name concepts after colors: Blue, Green, Blue-Green, Purple, Brown...)

The Purple-Roof concept was developed to solve a few specific challenges:

  • Incorporating detention into the Sponge Roof concept, to elevate green roofs to the stormwater management capabilities status of bioretention cells and detention tanks,
  • To convert gray roofs to green roofs , due to the above change, and
  • To provide a Blue-Green solution that works on a sloped roof .

Purple-Roof does these things by replacing the drainage layer with a detention layer. A detention layer slows down runoff, allows water to be temporarily stored within the green roof profile, and then water is slowly released after a storm.

Why retention-only roofs are not sufficient for stormwater management

Civil engineers consider retention-based solutions an excellent tool for annual volume reduction because the green roof is not saturated most of the time.

Most storm events happen in a sequence. This phenomenon is referred to as "2nd-day storms" . Traditional green roofs might retain about 50% of the annual rainfall volume, but the other 50% - usually during a 2nd day storm - passes through instantly to the drain.

For example, a traditional green roof might be mostly dry on Monday, and then rain starts, and the green roof holds water like a sponge. But then it rains again on Tuesday, and water pours out of the bottom almost instantly. During 2nd day storms, the "sponge" converts to a "pipeline".

The Purple-Roof concept detains 2nd-day storms and slowly releases water after the storm.

Component-by-Component Description

The remaining information on this page addresses the components within a Purple-Roof assembly. We list these in order from the lowest layer to the uppermost layer. Scroll down to view each layer.

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Detention Layer

green roof stormwater management detention layer

Why is a detention layer used?

Detention is drainage that occurs at a delayed rate versus rainfall (i.e. stormwater detention). All Purple-Roof assemblies incorporate a detention layer to facilitate this delay.

A detention layer is a drainage layer – just a slow drainage layer. It can also be placed under pavers and pedestal paver assemblies so that pedestrian areas can contribute to stormwater detention.

What is a detention layer?

A detention layer is the opposite of a typical drainage layer. A typical green roof drainage layer facilitates water flowing as rapidly as possible to the drain. A detention layer slows water down.

Green Roof Diagnostics tested a wide range of materials via hundreds of tests in 2017 to 2018 before finding the optimal solution.

The detention layer is a flexible layer comprised of vertically oriented polyester threads between two knit layers of tightly woven polyester fabric.

The optimal detention layer is 5mm thick, comprised of approximately 1,800 vertically stitched nylon fibers between woven nylon sheets, with stripes of alternating thread density about every 12mm to enhance turbulence of flow.

Currently, all Purple-Roof assemblies utilize the embodiment of the detention layer described above.

Typical detention layer

How does a detention layer work?

The detention layer replicates prairie conditions where water slowly meanders through dense vegetation. This is replicated via densely spaced vertical fibers that create friction , which can be thought of like an obstacle course for water.

As the detention layer restricts flow rates out of the profile, water is forced to remain within the profile. But, only for a short while, usually meaning several hours.

Detention is reliable when the drain down or "recharge" timeframe, the time it takes to empty, is neither too fast nor too slow . This is extremely important because the system needs to be ready to receive 2nd-day storms.

Pending roof size and geometry, the detained water will flow out within 6 to 18 hours.

Water flow through porous material can generally be categorized as laminar (think of water flowing in a pipe) or turbulent (think of rapids in a stream). Above a specific flow rate, the detention layer creates turbulence, such that water molecules bounce off each other and travel less efficiently: less efficient travel = slower travel. Slower travel facilitates stormwater detention.

The most significant benefit of turbulent flow is efficient peak flow reduction . During small storms, or at the leading edge of a large storm, a detention layer with turbulent flow provides minimal flow reduction – and that is a good thing because you don’t want to fill up detention storage too soon. As rainfall becomes more intense, the turbulence within the detention layer increases, which in turn provides proportionately more turbulence. A detention layer should work hardest when the storm is most intense , which is precisely what we want to achieve, and what is achieved via the Purple-Roof specification.

This is why this setup is preferred: turbulence is very effective at reducing peak flows of intense storms; these are the storms that otherwise cause flooding and CSOs (combined sewer overflows).

A detention layer should be installed in a continuous layer below the entire Purple-Roof assembly. Proper installation in the vicinity of the drain is critical for creating predictable flow rates. Installation in all other parts of the green roof is vital to ensure the entire green roof detains water uniformly .

Creation of friction below every square foot and every square meter of a Purple-Roof assembly ensures that all areas fill and drain at the appropriate pace and that the specified storage volume is achieved, and that the load is spread across the entire roof surface, even in a sloped scenario.

Continuity of friction allows the Purple-Roof concept to work on a slope! This is probably the most notable distinction between the Purple-Roof concept and Blue-Green concept, as Blue roofs and Blue-Green roofs tend to require zero percent slope roofs.

The detention layer should be un-cloggable . Immediately above the detention layer should be either needled mineral wool, or a honeycomb detention reservoir with needled mineral wool just above it. Needled mineral wool is an excellent filter (read more under "Mineral Wool" below), a much better filter than any filter cloth. Mineral wool is used in all Purple-Roof installations.

The second line of defense against clogging is redundancy. A detention layer will be installed in a continuous layer, and even if a portion of the material became clogged, the entire system would continue to function due to the redundancy of travel paths.

Note that when combined with the honeycomb detention reservoir, the detention layer’s resistance increases (transmissivity lowers).

How is a detention layer different from "normal"?

Traditional green roofs start to drain to a conventional drainage layer as soon as water percolates toward the bottom of the green roof media. Conventional drainage layers the expedite water to roof drains.

In contrast, a detention layer drains water very slowly, thus backfilling pore spaces within the green roof, and creating detention (delay).

A detention layer is also the primary difference between the Purple-Roof concept and the Blue-Green concept. Blue-Green roofs utilize some orifice restriction at the drain. A Purple-Roof utilizes friction throughout the entire green roof , versus only at the drain, where water would otherwise accumulate unevenly around the drain. Thus, the Purple-Roof concept can be used on a slope.

Blue-Green roofs usually employ some smart control to open and close flow restrictors, which allows for much greater flexibility in managing detention. The Purple-Roof concept is entirely passive and thus relies on friction within the detention layer.

Important detention layer material specifications

Look for this information in manufacturer-provided product data

Thickness5mm (ASTM D1777)
Thread count1800 threads per square inch (280 threads per square centimeter)
Material100% Polyester
Compression resistance(see table below)
Weight AppliedCompression Loss (% of manufactured thickness +/- 2%)
30 lbs/sf (145 kg/m2)3%
60 lbs/sf (300 kg/m2)4%
400 lbs/sf (2000 kg/m2)10%

Transmissivity: Complying with specifications below, per ASTM D4716, for the detention layer tested in isolation (transmissivity differs in combination with other components, which shall be tested separately).

Hydraulic GradientMinimum TransmissivityMaximum Transmissivity
0.0171.10E-031.40E-03
0.0509.00E-041.20E-03
0.1008.00E-049.95E-04
0.6675.50E-047.00E-04
1.0005.30E-046.50E-04

Important detention layer installation specifications

Look for this information in manufacturer-provided installation instructions

Lowest layerNo layer, other than the waterproofing membrane or extruded polystyrene insulation, should be below the detention layer.
OrientationAlign the detention layer so that stripes are perpendicular to the predominant direction of water flow.
ContinuityInstall the detention layer continuously below the green roof assembly with tight joints. Overlap joints 100mm (4 inches) at short ends of rolls.
FilteredCover the detention layer with a at least 25mm (1 inch) mineral wool that acts as a high-quality filter. (Below pavers or gravel, filter cloth may be used if needed.)
Restrict FlowRestrict horizontal flow within the green roof to only the detention layer. This involves multiple detailing considerations. All edging shall be non-perforated (solid-wall) edging. The detention layer shall tightly abut any edge containment. Avoid conditions that allow water to bypass the detention layer.
OutflowLocations, where the detention layer flows out to drains, shall be installed to the specified lengths.

Detention layer maintenance

A properly installed detention layer requires no maintenance.

Honeycomb Detention Reservoir

green roof stormwater management detention layer

Why is a honeycomb detention reservoir used?

A honeycomb detention reservoir ("honeycomb") is an often-used optional component in the Purple-Roof concept. The honeycomb detention reservoir is used to increase detention water storage volume.

What is a honeycomb detention reservoir?

Typical honeycomb detention reservoir

A honeycomb detention reservoir is a panel of small diameter, solid-wall tubes, vertically oriented, and fused as a panel. The honeycomb’s tubes detain water when placed directly above a detention layer. A honeycomb detention reservoir is quite simply almost entirely macropore space (>90% void space) with negligible horizontal flow.

How does a honeycomb detention reservoir work?

A honeycomb detention reservoir is always sandwiched between a mineral wool filter layer (above) and a detention layer (below). These two layers keep the honeycomb open and free of soil debris .

Since the honeycomb has negligible horizontal flow, flow can only occur above it or below it.

Flow below the honeycomb, including exiting the honeycomb, is regulated by the detention layer . Flow above the honeycomb is unlikely and only occurs after the honeycomb has filled.

Placement between the upper filter layer and lower detention layer allows the honeycomb tubes to fill with water, then empty very slowly, i.e. creating stormwater detention (delay).

The filter layer immediately above should be needled mineral wool within green roof assemblies, or an approved filter fabric when used below non-vegetated assemblies, such as gravel ballast.

A honeycomb may also be used below pavers , in which case the filter layer could be either needled mineral wool (preferred) or filter fabric, pending project requirements.

Honeycomb tube diameter should be small for optimal function and to avoid compression into the detention layer below to avoid altering flow rates.

At this time, 10mm is the recommended tube diameter; Green Roof Diagnostics is currently testing other tube diameters to determine the hydrologic impact of changing tube size. Small diameter tubes also prevent the needled mineral wool from sagging down into tubes, and small diameter tubes provide an optimal surface to receive paver pedestals.

A honeycomb detention reservoir also reduces flow rates of a Purple-Roof assembly. The combined transmissivity of a detention layer and honeycomb detention reservoir is lower, due to the increased friction. Hence, transmissivity continues to drop as the friction continues to rise with increased honeycomb thickness.

How is a honeycomb detention reservoir different from "the norm"?

There is no parallel to a honeycomb detention reservoir within the traditional green roof, but there is a parallel within Blue-Green roofs. Blue-Green roofs utilize some structure (stilts, open crates, etc.) to elevate the green roof above a blue roof. This creates detention water storage below the green roof. Such structures allow horizontal flow so that water may freely travel to the drain.

The Purple-Roof concept slows water down by friction , rather than a mechanical restriction at the drain, so the honeycomb holds water in place inside the array of tubes, thereby preventing horizontal flow. Instead, in the Purple-Roof concept only the detention layer allows horizontal flow to the drain.

Important honeycomb material specifications

Look for this information in manufacturer-provided product data

MaterialInert, stable material such as polypropylene or rigid HDPE
Compressive strength>= 140 psi (0.96 MPa) bare compression (ASTM C365)
ThicknessPer project stormwater calculations, minimum recommended thickness 12mm (1/2 inch), up to 150mm (6 inches)
Panel sizeMinimum typical panel size of 1m (39 inches) x 2m (78 inches) to minimize joints within the field of the roof.

Important honeycomb installation specifications

Look for this information in manufacturer-provided installation instructions

ContactInstall the honeycomb detention reservoir directly over the detention layer. Ensure the honeycomb makes good, uniform contact with the detention layer, such honeycomb tubes fill with water and drain as regulated by the detention layer, versus drain out rapidly through gaps. Cut the honeycomb at changes in the roof plane.
BallastEnsure at least 60 kg/m2 (12 lbs/sf) dry weight to maintain a tight fit between honeycomb and the detention layer.
FilterInstall needled mineral wool with overlapping joints, as a filter layer directly over the honeycomb.
Restrict flowInstall honeycomb in a running bond pattern (or herringbone pattern around drains) to minimize long-continuous joints between panels. As needed, as directed by manufacturer or inspector, stuff needled mineral wool into excessively long or wide joints to prevent unintended water flow, particularly in the vicinity of roof drains.

Honeycomb maintenance

A properly installed honeycomb detention reservoir requires no maintenance.

Needled Mineral Wool Retention Layer

green roof stormwater management detention layer

Why is mineral wool used?

Needled mineral wool is used to increase the stormwater retention of green roofs. Retention is the capturing of water in soil micropores and plant tissues and allowing the water to leave the roof only via evapotranspiration. That is, plants convert water to vapor, and release it into the atmosphere.

Mineral wool is a very lightweight material that retains up to 93% of its volume as water. When starting mostly dry, mineral wool has tremendous potential to lower runoff volume and increase the availability of water for plants.

Mineral wool also has excellent filtration properties, even for microscopic particles. Mineral wool is also very unlikely to become clogged or "blinded" in comparison with traditional filter fabrics. This beneficial property can be attributed to the three-dimensionality (rough surface) of the needled mineral wool surface.

The three-dimensional surface prevents the accumulation of fine particles along a single, flat plane (i.e. mineral wool prevents the type of clogging found in traditional filter fabrics).

Though mineral wool itself provides retention benefits and negligible detention benefits, mineral wool is an integral part of the detention function of a Purple-Roof.

This is because mineral wool has extremely low transmissivity (is highly resistant to water flow), which allows mineral wool to be used in several details of a detention roof that require restriction to flow.

Finally, mineral wool is an excellent rooting medium for plants. Though engineered soil media is the primary rooting medium, mineral wool extends the root zone for plants. Mineral wool retains higher volumes of water for a more extended period than engineered soil media and is thus highly hospitable to plant roots. Plants on green roofs with needled mineral wool have greater access to water during droughts and therefore outperform traditional green roofs in vegetative plant coverage.

What is needled mineral wool?

Mineral wool can be thought of as "cotton candy rock." The material is essentially made of thin strands of rock, needled together into blankets. Outside the green roof industry, mineral wool is a widespread material in building insulation, fireproofing, and horticulture.

Typical needled mineral wool water retention layer

How does mineral wool work?

Mineral wool is comprised of very little dry mass that contains millions of tiny pores that hold water tightly .

Most of these pores are micropores , i.e. pores that are small enough to hold water via capillary action. Capillary action is the physical process that allows sponges, fabrics, and materials like mineral wool to hold water that does not drain out. This is very closely related to retention .

All the tightly bound fibers of mineral wool are incredibly hospitable to roots and provide excellent filtration properties.

Because mineral wool is mostly pore space, it is exceptionally lightweight . Mineral wool provides an extremely high ratio of saturated weight to dry weight, making it the most efficient way to retain water in a green roof . Whereas retaining 1.4kg (3 lbs) of water might require 2.7kg (6 lbs) of green roof media, retaining 2.7kg (6 lbs) of water requires only about 0.45kg (1 lb) of mineral wool.

How is mineral wool different from what was done before?

Traditional green roofs utilize engineered soil media as a rooting medium, and sometimes a fibrous fleece for additional water storage. Mineral wool has up to double the water retention capacity of either material, with a much lighter dry weight.

The Purple-Roof concept uses mineral wool in much the same way as the Sponge Roof concept.

The primary difference between the Purple-Roof concept and some other concepts is that the incorporation of mineral wool is centered around a holistic, resilient approach , and thus any Purple-Roof-compliant assembly is free of nasty chemicals, such as Phenol-Formaldehyde.

Why would anyone incorporate chemicals such as Phenol-Formaldehyde, you might ask? This is because mineral wool requires some binding to hold the fibers together. We advocate for a non-chemical binding method , such as needling, which "punches" the fibers together with needles, versus using a carcinogenic glue such as Phenol-Formaldehyde.

Just think of the millions of square meters of green roofs that one day, 50 years from now, might need to be replaced. Needled mineral wool allows one to grind all the plants, soil, and mineral wool together, and blend it into the landscape, with no need for a landfill, which is needed for Phenol-based hardboard products.

Important mineral wool material specifications

Look for this information in manufacturer-provided product data

Retention capacity>90% maximum water retention per volume (ASTM E2397)
Non-toxicityFree of phenol-formaldehyde or any other carcinogens
DensityMinimum dry density of 128 kg/m3 (8 lbs/cf)

Important mineral wool installation specifications

Look for this information in manufacturer-provided installation instructions

Overlap jointsOverlap joints of the lowest layer of needled mineral wool to provide continuous filtration. (If using multiple layers, orient the upper layer at 90 degrees to the lower layer. Only the lowest layer needs to overlap.)
Cover immediatelyInstall needled mineral wool and cover with ballast (e.g. green roof soil) the same day to prevent wind uplift.
Walk lightlyArrange work sequence to avoid excessive foot traffic or equipment use over the mineral wool.

Mineral wool maintenance

Properly installed needled mineral wool requires no maintenance.

Engineered Soil Media

green roof stormwater management detention layer

What is engineered soil media?

Engineered soil media is a blend of aggregates, lightweight aggregates, organic matter, macro- and micro-nutrients, and sometimes some sand to produce a soil-like product.

We often refer to this as "soil," much to the chagrin of many within the green roof industry, over fears that someone will interpret that topsoil is an acceptable material. A high quality engineered soil media for a green roof is not topsoil, nor does it contain more than a tiny fraction of clay.

Our references to the material as "soil" stem from our focus on the biological properties of the material, which we want to make more like soil, i.e. "alive," versus the inert-sounding term "media."

High quality green roof engineered soil media is the most important factor in ensuring long-term plant health.

Why is engineered soil media used?

Engineered soil media is used as the primary rooting medium for plants. Plants structurally anchor themselves in the soil, use the soil as a source of nutrition , and as a source of water .

The soil offers live through a rich biota of microbes. These beneficial creatures are crucial to the short- and long-term success of the green roof.

Engineered soil media is by far the heaviest material within a green roof assembly, and it is located at the top of the assembly, which serves another fundamental purpose: ballasting all other materials to prevent wind uplift . Wind uplift requirements typically require at least 58 kg/m2 (12 lbs/sf) and up to 97 kg/m2 (20 lbs/sf) of dry media to weigh down other components.

How does green roof soil work?

Green roofs use an engineered media blend, rather than topsoil, as green roof soil must provide all the horticultural and hydrological functions of soil within a very shallow profile.

Green roof soil should contain little to no clay, as clay can clog filters and prevent proper drainage.

Just like high quality soil, high quality engineered soil media provides good proportions of organic matter to mineral, good air-to-water ratio, and an appropriate pH.

How is green roof soil handled differently in Purple-Roof?

Throughout the green roof industry, there is tremendous regional variability in specifications and product lines of engineered soil media due to the availability of local resources. Whereas manufactured layers (mineral wool, honeycomb, detention layer) predominantly focus on stormwater retention and/or detention, soil and vegetation are biological components, and thus should be customized to a specific climate . The quality and specification of the soil is the #1 factor long-term vegetative success (or failure) of a green roof.

We recommend that engineered soil media be explicitly designed for each climate zone . Within North America, we generally recommend a higher organic content (~10%) than industry standard (~5%), and a higher percentage of fine aggregates than the industry standard. Both of these modifications increase water retention and increase the potential for beneficial microbial soil life.

Important green roof soil material specifications

Look for this information in manufacturer-provided product data

Particle sizes≤ 5% of mass <0.002 mm, 100% mass <12.50 mm (1/2 inch)
Organic content8-12% loss on ignition

Important green roof soil installation specifications

Look for this information in manufacturer-provided installation instructions

Order by weightOrder media by dry weight, not by volume. Volume may expand or compress, and weight varies per moisture level. Dry weight is the most accurate measurement of mass.
Measure compacted thicknessRoll media with a ~150 kg (~300 lb) roller. Measure the thickness of compacted media.
Walk lightlyArrange work sequence to avoid excessive foot traffic or equipment use over placed media.

Maintenance of green roof engineered soil media

Sample media annually for nutrient levels, and apply the minimal amount of fertilizer necessary to sustain healthy plants while preventing nutrient runoff. Test according to protocols developed by Green Roof Diagnostics, which were designed specifically for high-flow-through porous green roof soils and have been proven on millions of square feet of green roofs. Good soil testing data minimizes the chance to apply expensive, unnecessary fertilizers that would just run off and pollute water bodies.

Vegetation

green roof stormwater management detention layer

Why is vegetation used on a green roof?

Without vegetation, a green roof is not green! Green roof plants are essential for evapotranspiration, erosion prevention, soil biology protection, and habitat creation.

Evapotranspiration is the primary driver of retention since the only way retained water leaves the roof is via evapotranspiration. Evapotranspiration is a two-fold process:

1) Plant transpiration is the process by which plants move water from the soil to the air.

2) Evaporation is the process by which water is removed from the soil as it is transformed from liquid to vapor without the help of plant transpiration, such as through sun and wind.

Evaporation is the primary mechanism by which green roofs cool buildings through "evaporative loss" , the process of intercepting radiant energy from the sun, and using that energy to convert water from liquid to vapor, rather than allowing that energy to heat the roof (and thus heat the building).

Plants are excellent at cooling buildings.

Habitat creation might include brown roofs, without many plants, as specialized habitats for spiders and certain nesting birds. But green roofs filled with nectar-producing plants significantly increase the habitat value of an urban area, particularly for bees and butterflies. Note: brown roofs work well in cool climates, but in warm climates, brown roofs become heat sinks similar to asphalt pavement.

What are green roof plants?

The staple of green roof plants is Sedums and their close relatives, such as Delosperma. “Sedums” is sometimes written in lowercase to refer to this type of succulent within multiple genera collectively.

Sedums are fool-proof. They can handle excessive heat, extreme cold, rapid environmental flucutations, excessive rain or drougth, and they need little nutrient to prosper. Sedums are not invasive, yet they regenerate effortlessly.

With enough depth of soil and enough irrigation, any plant can be grown on a green roof, but in practicality, vegetated roof plants are typically low-growing, herbaceous, drought-tolerant plants . Sedums and grasses are mainstays. Few others survive long-term.

Typical green roof mixed Sedum plant palette

How do green roof plants work?

Vegetated roofs are often harsh environments, exposed to intense sunlight, gusts of wind, periods of drought, occasional high moisture levels, and sometimes a bit of neglect. In these conditions, selecting the right plant is critical , and the use of the best installation technique is vital.

Green roof plants work by drawing moisture from the soil, and evaporating it into the atmosphere . In light of their harsh environment, reliably doing this requires tough plants .

The most common plants for extensive green roofs are Sedums and other succulents. Succulents conserve water when water is scarce. Sedums have an additional drought survival mechanism known as Crassulacean Acid Metabolism or CAM.

How is vegetation approached differently in the Purple-Roof concept?

We recommend that all Purple-Roof assemblies establish full, instant plant coverage (over 80% coverage). This is easily accomplished using pre-grown Sedum mats, which are already common in green roof installations worldwide.

We discourage allowing the soil to be exposed, which is the practice when planting with cuttings and plugs. Open soil creates dead zones where solar radiance and air flow through the porous media kills soil biota. This soil biota is needed for plant growth and these biota supply much-needed resources to the plants. Once these areas are bare, it is challenging for vegetation to fill in later.

If other plants are desired in addition to sedums, it is advised to plant plugs of non-Sedum species between, or within, the Sedum mats. If plants are desired other than sedums (for semi-intensive or intensive assemblies), we still recommend instant coverage, using a rapidly covering groundcover as appropriate.

Our recommended pre-grown Sedum mats are generally thinner (12-15mm, ½-inch) than industry norms (~25mm, 1-inch). The mats are field-grown versus greenhouse-grown, have matured at least nine months versus about 3-4 months, contain a mix of up to 15 species, are grown with a very lean low-organic media.

Many of the considerations above might be self-explanatory. The recommendation for a thin, low-organic Sedum mat is to encourage rapid root penetration from the mat into the engineered soil media.

Important green roof plant material specifications

Look for this information in manufacturer-provided product data

Coverage When Shipped85 percent
MaturityGrown for a minimum of 9-12 months in outdoor field conditions. No greenhouse-grown material shall be permitted.
SpeciesBroadleaf Sedums shall be matured into clumps prior to shipping, no more than ten broadleaf Sedums per square foot.
Thickness0.50 to 0.75 inches (1.3 centimeters – 1.9 centimeters).
MediaGrowing media on the mat shall be mineral-based, no peat-based media will be permitted.
CarrierBiodegradable vegetation blanket produced with substrate mix on a coir mat.

Important green roof plant installation specifications

Look for this information in manufacturer-provided installation instructions

Instant coverageIf planting with Sedums, use pre-grown mats to establish instant coverage.
WaterRegardless of planting technique, water plants thoroughly on the day of planting.

Green roof vegetation maintenance

The biggest task involved in maintaining green roofs is weeding: removing unwanted volunteer plants. The best strategy to prevent weeds is to maintain full, healthy coverage of desirable plants.

Occasionally trim low plants, such as Sedums, to prevent thatch and encourage low, continuous growth. At least once annually, remove excess biomass of taller plants such as grasses.

Purple-Roof is a concept with a specification, not a product. Products that implement the Purple-Roof concept are sold by third parties whose specific product information may vary from the specifications shown on this page. The User is advised to obtain specfic product data from specific manufacturers. Information on this page is provided to establish a standard against which individual product data may be evaluated.