Metal roof underlayment options trading
Metal roof and wall claddings contribute a lot more to buildings than just keeping the water out. They are part of the moisture management of the building and a necessary part of the regulatory system as required by the NZBC. This first part discusses underlay and its changing role in the changing circumstances of the last few decades. It is understandable that designers are confused about underlay. The new NZS This document does not permit synthetic materials for use as a roof underlay, however in a recent document published by the Ministry of Education, only synthetics are permitted to be used for school buildings.
This issue is not only important but topical as the increase in the number of complaints about excessive condensation and the role of underlay and ventilation is right up there. While this is predominately a residential building problem, the use of foils and synthetic underlay in Commercial and Industrial buildings metal roof underlayment options trading some insulation present exactly the same issues. This situation has not just occurred, metal roof underlayment options trading has been going on for years and perhaps the best place to start is with a bit of history.
The attic space was pretty drafty because permanent gable air vents were fitted and when any condensation occurred it was absorbed by the felt and would soon dry out due to the sometimes excessive ventilation.
Roof cavities including cavities beneath flat roofs should be ventilated by such means as: This principle continued for many years with the substitution of roofing felt with a heavy bitumen impregnated kraft paper but the venting of the attic space was overtaken by the energy crisis and we forgot about it. About solvent based fire retardant kraft paper underlay came into general use, the pink, green and red stuff and although black bituminous underlay was perceived to be a fire risk it still was the most popular underlay.
Both of these systems had serious inherent shrinkage issues. There were many examples of the failure of kraft fire retardant metal roof underlayment options trading due to shrinkage over the next few decades. Everybody blamed everybody else. The Roofing Industry asked the manufacturer by this time there was only one to withdraw kraft fire retardant underlay products from the market which, after a few frustrating years, was eventually done. The underlay standard has been changed many times over the years starting back in when New Zealand adopted the British Standard and called it NZS Prior to this everyone knew that underlays needed to be breathable and absorbent but by how much was not quantified till this time.
This standard also required underlay to be run horizontally and lapped 75mm. About the time that Leaky Homes were being built synthetic building wraps were introduced to the market and touted as being a replacement for kraft underlay. This is evidenced by the number of synthetic underlays and wraps now withdrawn from the market. The performance and properties of an underlay must be related to the purpose for which it is required and because an underlay performs more than one function, a definition is necessary.
Although the principles are the same for other claddings and their use is similar, an underlay as referenced in this Code of Practice is used in conjunction with metal roof and wall cladding. The good thing about the metal roof underlayment options trading generation synthetic underlays is that they are fire retardant and do not shrink like kraft papers do when they are wet; the bad thing about synthetic underlays is that not all of them comply with the requirements that have been proven over the years to be required for an underlay.
These can be found tabulated in the Code of Practice 4. While the latest NZS NZS is a standard that covers underlays for all buildings in all situations. Appropriate corrosion-resistant materials are polypropylene tape or a minimum 0. It should be noted that the industry has never regarded either packaging strapping or 0. To avoid splitting due to shrinkage or damage from structural movement, roofing underlay should be fixed securely and should not be laid taut but tensioned sufficiently to give fall on low pitches.
It also appears to contradict again: If the roof underlay runs over and is fixed to the top of the purlin then it should be run vertically.
The mm dimension is also incorrect. Self-support and synthetic underlay is usually mm wide and requires a mm lap. The purlin spacing must be mm less than the width of the underlay. If it exceeds this spacing it must be laid vertically. Because horizontally run underlay is metal roof underlayment options trading at each purlin the minimum dimension becomes mm. While it could be said that the DBH Acceptable Solutions were directed towards residential roofs, because, as it has been pointed out many times before, these documents become the default ones as they are applied to commercial and industrial roofs alike.
This is not such a support problem simply because safety mesh is normally used as support underneath underlay on commercial and industrial roofs for safety reasons. The issue now is to categorise synthetic underlays as to if, or when, the requirement for support is necessary. Wire netting has been shown to be the cause of corrosion of metal cladding in severe environments and therefore should not be used as underlay support in these environments.
Underlay has assumed another role, that as a separation between treated timber, dissimilar metals, or materials and works quite well metal roof underlayment options trading only on the condition that the underlay does not remain metal roof underlayment options trading. This is very likely in severe and very severe environments particularly at the gutter line where metal roof underlayment options trading profiles are open to salt aerosol.
If the time of wetness is such that the metal remains wet then a different separation material is recommended. This material metal roof underlayment options trading termed underlayment and is described in the Code of practice 4. Some such materials are a three-dimensional mesh made from polypropylene synthetic monofilaments 6mm high which is tangled and welded where they cross and provides a ventilating and drainage gap between the metal cladding and the batten.
This separation also will reduce noise transmission. If this material is used it is not necessary to provide an underlay as a separator between copper preservative treated timber and metal cladding, however an underlay is still normally required. Skip to main content. NZ Metal Roofing Manufacturers. The purposes of an underlay are: