Waterproofing green roofs

A watertight roof is critical to successful green roof construction. While some roofs are intrinsically waterproof when built, most will require some form of treatment to prevent water entry into the building. Waterproofing treatment must provide a strong but flexible layer that allows expansion under physical or thermal movements of the building structure, without compromising watertightness. Vegetation generally should not be installed over areas such as expansion joints, where regular inspection of the waterproofing will be necessary.

 

Advice should be sought from a specialist waterproofing manufacturer to find the most suitable type of waterproofing treatment for the roof structure and the proposed green roof design. The manufacturing and installation of waterproofing membranes should comply with Australian Standards (see here for more information).

 

In Australia, waterproofing is likely to be installed by a third party waterproofing contractor. The involvement of an independent contractor means that a clear agreement between all parties must be established for responsibility of the waterproofing membrane once it has been installed and certified as watertight. An independent leak detection specialist should test the waterproofing after its installation, and again after the green roof build-up is installed, prior to handover (see also here).

 

The following tables contrast the two major types of waterproofing – liquid applied treatments and preformed sheets.

 

  • Liquid applied treatments can be composed of bitumen emulsions, modified bitumen, polymer cement systems, polyurethane, polyurethane modified acrylic, acrylic or two-part polyurethane hybrid elastomers that require mixing prior to application.
  • Preformed sheets are asphalt-based or comprised of thermosetting polymers or thermoplastic polymers.

Preformed waterproofing may suit green roofs with gentle slopes and large uninterrupted areas. A green roof with many fixings onto the roof deck or penetrations, such as for lighting, power or ventilation, may be more suited to liquid applied waterproofing that is sprayed on or rolled on to form a cohesive single layer.

 

Waterproofing membranes must be protected from physical and chemical damage. This includes cuts and tears, the action of invasive roots and rhizomes, and exposure to the elements. All membranes will become brittle over time, and this is accelerated by exposure to cold, heat and UV rays from sunlight. A green roof will shield the membrane from damage and can significantly lengthen its life. Some preformed membranes have a surface coating that provides additional protection.

 

Ensure that the waterproofing material is certified root resistant, suitable for the substrate, and installed by experienced, trained and certified professionals.

 

Root resistance may be built into waterproofing membranes either by the addition of root-inhibiting chemical treatments, or because the composition of the membrane provides an impenetrable barrier to root growth. Root resistant waterproofing is quicker to install than separate waterproofing and root barrier layers, but can be more costly. Examples include certain types of ethylene propylene diene monomer thermosetting, thermoplastic PVC and thermoplastic polyolefin membranes; however, the root-resistance of a product must be confirmed with the manufacturer, with certification provided.

Common waterproofing treatments

Liquid applied waterproofing treatments

 

Suitability

 

Advantages

 

Disadvantages

Complex designs with many upstands1, corners or curves

Roofs where access for sheet installation is difficult

 

·    Seamless

·    Often trafficable

·    Flexible and capable of elongation

·    Easy to apply

·    Tolerant of some degree of surface imperfection

·    Easily repaired by re-application over breaches

·    Bonded to the roof

·    Pin-holes may develop on poorly prepared roof surfaces2

·    Solvent-based treatments become brittle over time and with exposure to sunlight and high temperatures

·    Root resistance

·    Not easy to ensure uniformity of thickness

·    Not recommended for use in permanently wet conditions, as they absorb moisture and soften over time

 

Preformed waterproofing sheets

Membrane composition

Application methods

 

Advantages

 

Disadvantages

Asphalt-based

Loose-laid

Fully bonded ‘torch-down’

Partly bonded

·    Loose-laid membranes quick to install compared to bonded

·    Fully bonded more resistant to wind uplift than partly bonded

·    Fully bonded recommended by some green roof installers and lower risk of uplift makes insurance easier to obtain

·    Easier to locate leaks with precision with fully bonded membranes

·    More resistant to wind uplift than loose-laid

·    Loose-laid not recommended if high risk of wind uplift

·    Soft structure means membranes are usually not trafficable, UV resistant or root-resistant

·    Prone to failure and root penetration along seams due to softness of composition

·    Installation requires a high degree of skill to ensure high quality detailing of seams and around upstands1

·    Become brittle over time through exposure to sunlight, heat and cold

Thermoplastic

Fully bonded

·    UV stable and weatherproof

·    Seams can be welded with heat or solvent3; or heat only4

·    Bonding of membrane to the roof with heat, solvent, water-based adhesive, or tape reduces risk of wind uplift

·    PVC membranes are breathable, and well suited for bonding to the roof deck with glue or tape adhesives

·    PVC, thermoplastic polyolefin, ethylene vinyl acetate and ethylene butyl acrylate membranes are likely to be classified as root resistant: check manufacturer’s specifications

·    Mechanical fastening to the roof deck may be suitable for some applications

·    With classes of membrane other than PVC, condensation of moisture may soften glue or adhesive tapes used to attach membrane to the roof deck, increasing risk of wind uplift

Thermosetting

Fully bonded

·    Ethylene propylene diene monomer (EPDM) and Butanoyl® membranes are likely to be classified as root resistant: check manufacturer’s specifications

·    UV stable and weatherproof: long lifespan

·    Bonding with glue or adhesive tape reduces risk of wind uplift

·    Condensation of moisture may soften glue or adhesive tapes used to bond waterproofing to the roof deck

·    Carbon content of EPDM/Butynol® membranes makes them intrinsically electrically conductive and therefore unsuitable for electronic leak detection

1        Upstands are structural penetrations from the roof such as vent pipes that will need to have the waterproofing layer brought up (‘dressed up’) around them to terminate above the level of the substrate.

2     Note that the waterproofing consultant must certify that the roof is fit to receive the membrane prior to installation.

3     Thermoplastic polymers composed of chlorinated polyethylene, polyvinyl chloride (PVC), thermoplastic polyolefin, ethylene vinyl acetate and ethylene butyl acrylate.

4     Thermoplastic polymers composed of ketone ethylene ester.