If you are thinking of designing a building with a green roof or walls, you may be about to cause yourself some problems. Basically, buildings and plants don’t mix.

The idea behind such additions is noble. Greenery helps combat the negative effects on the natural environment of ever expanding urbanism, which reduces natural vegetation and creates heat-sink effects. As well, we all feel better living in greener cities.

Even though the whole commercial concept is fairly new, there are many manufacturers and providers of green roof and wall products, and many building owners and designers are thinking of building green roofs and walls.

Much has been publicized about green roofs and walls, and all of it seems positive, which actually is a bit lopsided because there are a lot of serious concerns with this form of construction. It seems people just haven’t fully thought it through.

Here are some general problems that can happen:

• Root damage to man-made elements
• Soil moisture differentials where larger plants take moisture from the soil, creating a difference in moisture content from where there is no planting, leading to ground settlement differences which can structurally damage buildings
• Damage from falling or moving branches
• Leaves and other debris clog up drainage systems
• Dropped fruit, leaves, and seeds creating pedestrian slip-resistance hazards
• Moisture retention where sun and wind can’t dry building elements, leading to building fabric deterioration
• The blocking of winter sun into buildings
• Access into the building of vermin and pests
• Waterproofing and building envelope damage

Going and putting on to a building a green roof or wall may be just creating trouble. There are five areas of more specific concern with green roofs and walls:

1. supporting structure
2. waterproofing
3. maintenance
4. safety and liability
5. reduced rainwater and solar harvesting capability.

Supporting structure

Basically, green roofs are heavy. Significant extra structural implications and costs are involved in supporting green roofs, and these impositions are multiplied to adapt existing non-green roofs to be green. How sustainable is it to consume the extra materials and energy to build the structure to support the green roof?

Waterproofing

Green roofs need to be waterproofed under the soil to stop moisture entering the building below. No matter how good the waterproofing system is, one day it will fail and need replacement. To rectify this means digging up the soil and plants, replacing the waterproofing, then reinstating the soil and plants – a massive undertaking. This needs to be repeated again in the future each time the waterproofing needs to be replaced.

Maintenance

Here are some maintenance considerations:

• Green roofs need to be constantly maintained. Due to the critical location of the landscaping (ie on a roof), a thorough and professional maintenance program (preferably produced and overseen by a horticulturalist) needs to be adhered to.
• The establishment of unsuitable plants (especially trees), either done negligently or accidentally (such as via germination of windblown seeds) needs to be constantly guarded against.
• A constant water supply is required, so permanent piping and/or limited rainwater harvesting needs to be installed. Collecting rainwater can be problematic as it can’t be harvested from a green roof.
• Drainage needs to be resolved. Drainage from landscaped areas is likely to be chemically affected by the soil, which can have implications down-stream environmentally and to piping.
• The landscaping needs to be constantly fertilized, composted and mulched.
• Maintenance tools need to be stored or carried to the roof. Constant access of maintenance personnel, landscaping supplies and equipment is also required. Is this access via the building public lift?
• How are building insurance premiums affected by installing building systems which may reduce building life and/or performance?

Safety and liability

Here are some safety and liability considerations:

• Roofs are high places. Will the roof edge need to be fenced? Is there danger of landscaping supplies and equipment falling to the street below?
• Planting also burns very well. Is a fire danger being created? Are there compliant fire escape exits?
• How is access to the roof achieved? Do ladders, stairs and walkways need to be installed?
• How will safety aspects impact on Public Liability Insurance premiums?

Reduced rainwater and solar collection capability

A roof covered with landscaping prevents rainwater collection and/or solar panel installation.

Alternatives to green roofs

Green roofs may not be the fairy tale we were promised. There are however, alternatives. The roof can be standard construction and used to collect rainwater and solar energy. Also, plants grow best in the ground, so why not use the opportunities for greening at ground level both on public (such as roads and footpaths) and private land? Plants in the ground are far easier to install and maintain, and more can be planted.

A lot of hard pavement area can be turned into landscaping. Being creative will provide opportunities for this, even in small places. Also, some pedestrian and low-use vehicular areas can be paved using turf laid on hidden reinforced matting. Sure, all this needs more maintenance than concrete paving, but it is infinitely more easier than maintaining landscaping 50 metres above ground level.

Plant selections are always best made by a horticulturalist. Many problems are caused by inappropriate plant selection.

New buildings can be designed with an attempt to reduce the ground level footprint, leaving more space for landscaping. The ground floor could be designed to incorporate a just a lobby and lifts/stairs, with upper floors overhanging, providing a shaded landscaped area.

Large decks to lower floors only (to help landscape maintenance) can also be greened. These decks should not have habitable enclosed areas under them so as to minimize disruption from failed waterproofing.

Other alternatives include potted planting and freestanding green walls.

Green walls

Green walls can either be fixed to a building or other structure, or be freestanding (self-supporting, away from a building or other structure).

Green walls attached to a building or other structure can be problematic due to:

• Moisture retention where sun and wind can’t dry the building or other structure, leading to building fabric deterioration
• Access into the building of vermin and pests
• Waterproofing and building envelope damage
• Inaccessibility for maintenance to the building, including re-painting of walls
• Hindrance or prevention of building maintenance inspections
• Fixings for the green wall structure penetrating wall cladding causing leaking
• How are building insurance premiums affected by installing building systems which may reduce building life and/or performance?

Freestanding green walls however can be very successful. These are self-supporting without any need to have them close to a building or other structure.

They need to have a corrosion resistant structure, a watering system (with fertilizer added to the water), and maintenance access. Advantages of freestanding green walls include:

• They occupy a small area of land in comparison to the amount of vegetation they hold.
• They can be placed to provide visual privacy and to hide unsightly views.
• They can be placed to improve acoustic resonance (echo) in hard surfaced environments.
• They can be placed to reduce noise transfer (such as between a busy road and pedestrian areas).
• They have all the advantages of planting including visual amenity, cleaning of the air, support of some habitat.

The industry talk is so positive about green roofs and walls. The basic concept may be commendable, but there are serious concerns with green roofs and walls that no one seems to want to talk about.

Balanced thinking and presenting all the facts to building owners is essential. Owners who build vertical forests may quickly feel trapped between the huge cost of maintaining their forest and the unthinkable aesthetic and practical problems of not maintaining it. Maybe green roofs and walls are not always such a good idea after all.