The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
The purpose of sealants used in the joints between metal roof and wall cladding and flashings is to prevent the entry of rain or snow and to exclude dirt, debris and moisture by:
- capillary action;
- end laps in flashings;
- side lapped joints between profiled sheets where the pitch of the roof is less than that permitted for the profile (curved roofs);
- end lapped joints between metal and plastic profiled sheets if the length of the roof cladding is less than the rafter length;
- side laps of plastic and metal roof cladding;
- any overlap that constitutes a joint designed to allow relative movement between flashings;
- penetration flashings where the back curb acts as a gutter;
- the passage of moisture vapour from within the building under the influence of vapour pressure differential; and
- joints or outlets in spouting and gutters;
This COP uses "sealant" to describe polymeric sealants used in roof and wall cladding, but which act differently to other sealants used in the building industry. The main use of sealants in the building industry is to accommodate thermal movement in a building without any through-fastening. Fasteners are always used in conjunction with sealants in the installation of roof and wall cladding because the sealant thickness sandwiched between two pieces of metal is insufficient to obtain the necessary shear strength for durability.
Polymeric sealants are formulated to provide a continuous bonding layer to both surfaces and to fill and seal a gap. They are cured either by polymerisation initiated by curing agents or by the evaporation of solvents. Polymeric sealants include natural rubber and generic types of synthetic elastomers of nitrile, neoprene, silicone, and butyl.
There are many sealants available and the designer and contractor should recognise that it is necessary to use the correct sealant, placed correctly and in the correct manner to avoid corrosion.
Only neutral curing silicone rubber sealants that are acetic acid-free are suitable for and compatible with all metals and organic finishes, including coated steel wall and roof cladding and flashing materials. Acetic cured silicone sealants can be used with stainless steel but are not suitable for zinc or zinc-aluminium coatings as the acid catalyst will attack metallic coatings.