The Underlay Dilemma

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.E3 (internal moisture).

This article is the first of three which discusses the designer’s role and the roofer’s responsibility in this process which includes condensation, insulation and ventilation of roof spaces.

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 3604: 2011 removed section 11 the building envelope (which included underlay) from its scope, while the new E2/ AS1 published on August 1 now cites NZS 2295:2006 as the compliance document for underlays. 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. So hello- what’s going on? 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 requiring some insulation present exactly the same issues.

This situation has not just occurred, it has been going on for years and perhaps the best place to start is with a bit of history.

Way back we used heavy absorptive bitumen impregnated roofing felt as an underlay under metal roof cladding supported on sarking which was made of native timber boards about 11/2 inch (40 mm) apart. 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. NZS 3602: 1975 said:

24.2.3.1.Roof cavities including cavities beneath flat roofs should be ventilated by such means as:
a) grilles in eaves;
b) louvre frames in gables;
c) a continuous gap in the roof soffit:
d) ventilating ridging: or
e) other suitable means

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 1980 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.

The fire retardant additives were based on antimony trioxide/chlorine chemistry and dispersed in highly toxic petroleum based solvents. About 1989, for health and safety reasons, the solvent base was replaced with either chlorinated paraffin waxes or a water-based fire retardant system incorporating antimony trioxide/chlorinated emulsions. Both of these systems had serious inherent shrinkage issues.

There were many examples of the failure of kraft fire retardant underlay due to shrinkage over the next few decades. Everybody blamed everybody else. The manufacturer stated that the underlay should not get wet, the roofer said he only put the roof on and didn’t design the structure underneath while the designer had no regulatory requirement to provide any ventilation under the underlay.

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 1965 when New Zealand adopted the British Standard 1521 and called it NZS 873: 1969. Later the British Standard 4016 1992 was adopted as NZS 2295 1988.

Meanwhile NZS 3604: 1981 cited a previous version of NZS 2295 1969 entitled Building papers breathing type which had the absorbent test included (> 100g/m2).

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.

However in 1999 NZS3604 adopted AS/NZS4200 called ‘pliable building membranes’ before (fortunately) changing it back again in 2000 to NZS 2295.

About the time that Leaky Homes were being built 1993- 1999 synthetic building wraps were introduced to the market and touted as being a replacement for kraft underlay.

Then when the leaky homes scandal broke (2002), BRANZ conducted a number of seminars around the country entitled ‘Wrap it up’ but the confusion of the name ‘wrap’ and the purpose of the underlay created a number of failures by using the wrong material. This is evidenced by the number of synthetic underlays
and wraps now withdrawn from the market. Some started life as envelopes or protective apparel while others started life as babies’ nappies.

It was not until about 2007 that synthetics which were specifically designed to meet and exceed the properties of ‘good ol’ black building paper’ were locally manufactured and there was general acceptance that this material was an excellent, even if a more expensive, replacement.

One of the problem issues was that the role of underlay had never been clearly defined or agreed as underlay manufacturers saw their role as a sales one and it was left to the roofing industry to put the’ system’ together. The NZMRM Code of practice published in 2003 said:

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 purposes of an underlay are:

• To provide a temporary means for the accumulation of condensation by absorption.
• To provide a permeable membrane to allow the passage of water vapour
• To prevent the ingress of external moisture into roof or wall cavities.

The good thing about the second 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 4.3.

While the latest NZS 2295:2005 did include synthetic materials for wall cladding they were not permitted for use with roof cladding.NZS 2295:2006 does not include foils and although they are sometimes referred to as underlays, they are not – they are VCL’s -vapour control layers If they are sealed they are termed vapour barriers.

NZS 2295 is a standard that covers underlays for all buildings in all situations. An anomalous situation has arisen because the NZBC Acceptable Solution E2/AS1 only covers the scope of NZS 3604 The requirements of E2/AS1 by referencing NZS 2295 and quoting R1 & R2 as the only underlay types of roofing underlay (kraft) has meant that designers and BCA’s alike are caught between a rock and a hard place. The regulations are out of step with the market place as nearly 50% of roofing underlay used is now synthetic.

E2/AS1 was also amended in August 2011 changing the rules yet again for support, for pitch and for span.

E2/AS1 2005

8.1.5.1 Underlay support Roof underlays shall be installed in a manner that prevents ponding of water by:
Run horizontally or vertically when the roof pitch is not less than 8°
a) Allowing roof underlays classed as extra heavy or heavy in AS/NZS 4200 to span no more than 1200 mm in one direction, or
b) If supported by a corrosionresistant material:
i) roof underlays classed as medium light or extra light in AS/NZS 4200 shall span no more than 300 mm in one direction, and shall be used only at pitches of 8° or above, or
ii) roof underlays classed as extra heavy or heavy in AS/NZS 4200 shall be used at pitches less than 8°.
Appropriate corrosion-resistant materials are polypropylene tape or a minimum 0.9 mm diameter steel wire mesh galvanized in compliance with AS/NZS 4534.
(It should be noted that the industry has never regarded either packaging strapping or 0.9 mm diameter galvanized steel wire mesh netting as being either appropriate or corrosion resistant in severe environments, simply because they do not last 50 years.)

E2/AS1 2011

Run horizontally for roof pitches below 10°
• Run horizontally or vertically for roof pitches above 10°
8.1.5.1 Underlay support Prevent sagging of roof underlay by either:
• For Rl underlays, fully support with a corrosion resistant material
• For R2 self supporting underlays, laid to maximum 1.2 metre span between adjacent supports

It should be noted that the pitch has been upped by 2° -or has it? as a roof of 10° appears to be in limbo!! The requirements of both of these E2/AS 1 versions appear to take it for granted that self-supporting underlay is exactly that, and does not need any support if it spans no more than 1200 mm.

The MRM Code of practice has always required support even for self-supporting kraft underlays below 8°.simply because the instruction was not to pull underlay taut because it would tear on shrinking.

4.3.8. 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.

Table C2 in NZS 2295 appears to contradict E2/AS1 by allowing R2 underlays to be used at greater than 1200 mm if it is supported however it also omits to qualify the statement by any roof pitch.

Table C2 also doesn’t make sense because it is the purlin spacing not the rafter spacing that determines the span.

It also appears to contradict again: A3.4. If the roof underlay runs over and is fixed to the top of the purlin then it should be run vertically.

The 1200mm dimension is also incorrect. Self-support and synthetic underlay is usually 1250mm wide and requires a 150mm lap.

COP 4.3.8. requires
Horizontally laid underlay must be fixed to the purlins on both edges by the roof fastenings i.e. The purlin spacing must be 150mm less than the width of the underlay. If it exceeds this spacing it must be laid vertically.

Because horizontally run underlay is fixed at each purlin the minimum dimension becomes 1100mm. 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 but only on the condition that the underlay does not remain wet. This is very likely in severe and very severe environments particularly at the gutter line where metal profiles are open to salt aerosol. (see COP 2.4.5.) If the time of wetness is such that the metal remains wet then a different separation material is recommended. This material is termed underlayment and is described in the Code of practice 4.3.11.

Such material is termed ‘underlayment’ which can provide a ‘slip joint’ between cladding and the structure and can also provide an air gap. 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.