ROOF SAFETY MESH - AS/NZS 4389 UPDATED
AS/NZS 4389:1996 Safety Mesh has been updated, including a new date and name, to AS/NZS 4389:2015 Roof Safety Mesh. The new Standard is not significantly different from the old Standard, but it does set out things more clearly and explains things better.
The main purpose of roof safety mesh is to prevent somebody falling through it. The Standard has stringent requirements on the mesh design and installation, and also requires it to be tested with a 165kg weight dropped from 1.4m. It is made of a minimum 2mm wire mesh, with longitudinal wires (spanning between purlins) at 150mm centres welded to transverse wires (parallel to purlins) at 300mm centres. Mesh is normally galvanised but PVC coated is available for visual purposes or for sea spray zones and chemical environments. Stainless Steel is sometimes available for extreme environments, but you should check availability in the area.
Roof safety mesh has traditionally been used in two situations;
- Under some profiled plastic roofing to prevent fall through
- As a safety platform for the roofers to work on, particularly over high bay situations like factories and warehouses.
Although its use under plastic roofing is likely to continue, recent (and proposed) changes in health and safety requirements may mean a change in the general use of mesh. The Contractor is now having to adopt a wider range of safety devices during the construction process, which may mean the use of mesh as a safe working platform will not be necessary. The Contractor is responsible for the safety systems used and as such it is possible they may or may not elect to use safety mesh. Excluding plastic roofing, this removes the mesh from being a specification item for general construction safety, however it still could be considered for long term maintenance safety (replacing roofing). If the contractor elects to build-in mesh then you will want to control the quality and installation in the specification.
Some plastic roofing does meet the requirements for safety from falling without mesh when new. However, plastics may lose strength with age and become brittle, reducing their resistance to fall through. Most roofing plastics would not be expected to retain their full strength for more than 5-10 years, the best performing reinforced GRP roofing is expected to retain strength for up to 20 years. The proposed health and safety reforms on the horizon are expected to put more liability on the designer, so consider the use or non-use of safety mesh under plastic roofing very carefully.
By its very nature safety mesh can cause abrasion and dissimilar metal problems with some roofing materials. It is not recommended under Aluminium roofing and other soft materials that have a high expansion rate. Plastic roofing must be separated from the mesh by protection strips, usually self adhesive foam 3mm to 6mm thick and the width of the purlins. PVC coated mesh can help a little in these situations, although some precautions may still be necessary, consult with the roofing manufacturer.
The use of safety mesh and wire netting is sometimes confused. Wire netting has no tested safety from falling capacity and is simply used to support roofing underlays where required. particularly for;
- non-supporting underlays (NZBC E2/AS1, 188.8.131.52)
- self supporting underlays for pitches under 8° (NZMRMI CoP)
- underlays under some dark colour roofs (NZMRMI CoP)
- some wide purlin spacings
For some extra long spans safety mesh is sometimes used to support underlay instead of netting.
All related Masterspec work sections have been amended to include the new 2015 Standard AS/NZS 4389 Roof Safety Mesh.