Membranes for low-slope roofs

By - , Build 159

Building consent authorities are finding roofing systems specified for low-slope roofs that are unsuitable for the expected traffic use. What membranes can be used on roofs with regular maintenance foot traffic?

Typical example of low-slope multi-layer membrane roof.
Typical example of low-slope multi-layer membrane roof.

LOW-SLOPE ROOFS are commonly used in both commercial and residential construction.

Low-slope roofs are defined as roofs where the pitch is less than 10°. Low-slope roof is a more appropriate term than flat roof as all roofs must have some slope to enable water to drain off them. The low pitch generally means that a roof below 3° must have a waterproofing membrane.

Design for function – any traffic?

Not all membrane roof systems are trafficable – able to carry maintenance traffic.

The first step in designing a low-slope roof is to determine the roof’s function. If it has occasional or frequent pedestrian access for equipment and roof maintenance, the selected membrane must be able to withstand this level of wear.

Building Code requirements

New Zealand Building Code clauses that apply to membrane roofs include clauses B2 Durability and E2 External moisture.

Clause B2 Durability deals with durability, and Acceptable Solution B2/AS1 to clause B2 requires roof claddings to be durable and remain waterproof for at least 15 years.

Clause E2 External moisture deals with weathertightness or the roof’s ability to cope with external moisture and direct it to a drainage system.

The Acceptable Solution E2/AS1 to clause E2 requires that membrane roofs must have a pitch no less than 2°. The coverage of acceptable membrane roofs in E2/AS1 is limited to EPDM and butyl membrane roof systems applied over plywood. Other membrane and substrate options must be evaluated at the consent stage as an alternative method.

Three broad categories of membranes

A range of membrane roof systems is available that can be classified in three broad categories:

  • Single-ply membranes.
  • Multi-layer or built-up membrane systems.
  • Liquid-applied membrane systems.

Single-ply membranes

Single-ply membrane roof systems comprise one layer of sheet membrane laid over a substrate. There is a wide range of membrane types and options, including thickness, sheet size, and fixing and jointing methods.

Butyl rubber

This is a flexible synthetic rubber with low permeability and good resistance to UV, acids and micro-organisms but with poor resistance to mineral oils and solvents. Therefore, it should not be used:

  • with substrates containing bitumen
  • with light organic solvent preservative (LOSP) treated plywood, where it will be in contact with solvent
  • with petroleum-based products
  • over uncured or wet cement products.

Where there is light pedestrian traffic only or for roofs, a 1.0 mm thick membrane may be used.

In areas with regular maintenance access, a 1.5 mm thick membrane should be used. For higher pedestrian traffic, additional protection should be considered such as installing pavers, panels or slatted timber decking on proprietary support pedestals or timber packers over the membrane.

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EPDM

Ethylene propylene diene terpolymer (EPDM) is a durable, synthetic elastomeric rubber that has very good resistance to UV, temperature extremes and corrosive chemicals. As with butyl rubber, it should not be used over bituminous surfaces or be in contact with organic solvents.

For light maintenance traffic, a 1.0–1.2 mm thick EPDM sheet membrane may be used. Where heavier traffic is expected, a 1.5 mm thick membrane should be used and additional protection considered to limit the potential for damage.

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PVC membranes

Polyvinyl chloride (PVC) membranes include VET (vinyl ethylene terpolymer) and KEE (ketone ethylene ester) membranes.

They have UV absorbers and stabilisers to provide flexibility, durability and puncture resistance for roof use. Easily cleaned, they are suitable where there is regular maintenance traffic.

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TPO membranes

Thermoplastic polyolefin (TPO) membranes are single-ply polyester-reinforced membranes installed as fully bonded or mechanically fixed systems. Accessories are welded directly to the membrane so there is no reliance on sealants.

They have good UV resistance and puncture resistance, can be used for roofs and are generally coloured grey or white for good solar reflectivity. TPO membranes are not suitable for regular maintenance traffic.

Multi-layer or built-up membranes

Multi-layer or built-up membrane roof systems typically consist of two or more layers of a bitumen-based membrane applied over the substrate. Options include:

  • polymer-modified bitumen sheet (PMBS) membrane, which consists of a modified bitumen reinforced with polyester or glass fibre
  • mastic asphalt, which consists of limestone and aggregates mixed with bitumen to create a jointless roof covering. This is seldom used now.

Benefits of multi-layer membranes include:

  • a minimum two layers of waterproof material results in a thicker membrane than single-layer or liquid-applied membrane systems
  • with sheet membranes, the joints in the first layer are protected by the offset joints in the second layer
  • the extra thickness means the membrane has better puncture resistance and joint security.

Their disadvantages include:

  • they require protection from UV exposure (for example, a coat of ceramic/mineral chips or an acrylic finish)
  • the finishes are susceptible to damage from foot traffic so require walkways over the roof for protection
  • they are affected by temperature changes and will soften and become more pliable in hot conditions
  • they should not be used with hydrocarbon products such as diesel, oils and fats.

Liquid-applied membranes

Liquid-applied membranes, as the names suggests, are applied as paint-on systems in several coats to create a jointless, flexible membrane that is fully bonded to the substrate.

They are generally reinforced with fibreglass matting to improve wearability and the ability to withstand building stresses. Liquid-applied membranes may be waterborne or solventborne applications.

The three main categories of liquid-applied membranes are:

  • multi-coat polymer gel membranes
  • acrylic paste membranes
  • aliphatic polyurethane membranes.

They generally have good resistance, such as to common pollutants, dilute mineral and organic acids, mineral and vegetable oils, neutral detergents, alcohols and petroleum spirits. They also accommodate occasional maintenance foot traffic.

Disadvantages include:

  • the waterproofing ability relies on a thin layer of membrane that is susceptible to being punctured by sharp objects
  • recoating may be required every 5–7 years
  • the surface tends to hold dirt and lichens, potentially speeding up the rate of deterioration
  • the membrane can be affected by structural movement.

Jointing

Options for jointing membranes are:

  • lap tape – for butyl and EPDM
  • heat welded with a ‘leister’ vinyl welder – for PVC, TPO and KEE
  • torched joint – for PMBS.

Pointers to select the right system

General requirements when selecting a membrane roof system are to:

  • determine what the roof will be used for and the expected traffic on the roof, rainfall intensity, snow loads and other potential environmental impacts
  • obtain product-specific design and installation instructions from the manufacturer before making the final selection
  • source all components of the membrane system from the same manufacturer or supplier
  • provide full support by a substrate to all membrane systems
  • meet the supplier’s requirements for roof space ventilation
  • not use membranes where they will be permanently stretched or stressed
  • specify installation by an experienced (licensed) applicator.

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Typical example of low-slope multi-layer membrane roof.
Typical example of low-slope multi-layer membrane roof.

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