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Warm roofs — is this the future of roofing?

Warm roofs — is this the future of roofing?

A warm metal roof is a generic term for insulated panels, aka SIP (Structural Insulated Panels), Sandwich panels, Coolroom panels, Composite panels and Stressed Skin panels. Section 12.0 of the MRM Code of Practice describes all of these.

Although they can be made up of different materials, the majority of metal insulated panels are made from foam insulation with metal cladding on both sides. There are also many types of different foams with different fire and thermal performances, but for the roofer they are all about the same.

Some warm roof systems are hybrids — ie, they use a modified bitumen polymer membrane as the exposed roof surface.

There are a lot of the dreaded acronyms used in this part of the industry, including:

  • ISP: Insulated sandwich panel

  • EPS-FR: Fire retardant expanded polystyrene

  • PIR: Polyisocyanurate

  • MF: Mineral fibre

  • SPS: EPS Phenolic Hybrid

Warm panel roofs have an advantage called a “synergy” — ie, the product strength is much greater than the individual strengths of each of the component parts.

Very large spans can be obtained by increasing the depth of the insulation which can vary from 40mm to 150mm. Increasing the thickness not only increases the insulation value but also the panel’s strength.

The panels act like an ‘I’ beam, with the top and bottom metal cladding acting as the flanges and the insulation acting as the web.

Easy to erect in a short time

The major difference when using panel construction is that there is no need for an underlay, insulation or safety mesh, and being in long lengths it is very easy to erect a lot of finished roof in a short time.

Too good to be true? Yes, there are some issues with panel construction and that is what this article is all about. The first would be cost.

Although costs have come down recently, this type of construction, on the face of it, is more expensive. However, it depends on how you do your sums.

Although they are a safe way to erect roofs quickly, safety edge protection and nets must be in place and a harness must still be worn. Scheduling a panel job is more crucial if the advantage of the speed of erection is not to be lost.

Some manufacturing companies have their own fixing and erecting teams, but the majority of standard building insulated roofs are going out for tender.

This type of construction has been around for a long time, but is becoming more popular with architects, particularly those designing school buildings, supermarkets and pools.

Perhaps this story is showing Thomo’s age, but in 1984 he won first prize in an open building design competition for “Stressed Skin Design”. The prize was a trip to Europe, and entry into a number of factories producing insulated panels around the world.

The uniqueness of the winning design was the detail at the ridge where specially curved panels structurally held the pitched panels apart. The structural steel frame ran the opposite way — instead of a portal frame, the frames ran the length of the building and became the purlins.

Unfortunately the building was never built because the company went into receivership — but I did get my trip!

When Thomo first joined NZ Steel as development engineer in 1982, one early job consisted of testing metal clad insulated panels under simulated wind load.

Over about 100mm thickness and using 6m spans, it became difficult to fail the panels as the tests were on simple spans and the fixings were not able to restrain the panels.

The potential for this type of construction was publicised and a number of manufacturers started up but, unfortunately, without enough jobs to support the large capital outlay for their manufacturing plants, a number of companies fell by the wayside.

Warm roofs are not much different to cold roofs (the ones you are building all the time).The only difference is that they have to be closer to perfect!

If the warm roof did not have any holes (penetrations) in it, life would be easy but, as you know, “it don’t work that way”.

Vapour barrier

What we are doing is creating a vapour barrier on the underside of the panel which has to be perfectly sealed at the laps to avoid any moisture ingress from the inside — if there are any gaps there is trouble ahead.

Swimming pools are the great example. There are very few that have not had problems at some stage, and some pool roofs have had to be rebuilt because of the penetration of water vapour.

While supermarkets are usually air-conditioned, schools are not, so attention to detail is important. Ventilation at the moisture source is fine in theory but, again, we are looking for perfection.

Unlike waterproofing from above on cold roofs, air seals, strip butyl or gun sealants are relied upon for water vapour sealing in warm roof panel construction. Some joints have to be insulated on-site with expandable foam, but equal to the fire rating of the panel.

Tanks can’t be nearly watertight — just as warm roofs can’t be almost sealed.

Panel roofs are not a panacea for condensation. Although many buildings using panel roofs are air-conditioned and control humidity and have a large volume, small buildings such as mountain huts still require ventilation if condensation is to be avoided.

Thomo investigated a winery that was said to be leaking, and no one could find the leaks. The owner was not pleased when on entering the offending building he saw a smile on my face.

“This is not a laughing matter,” he said. It was then pointed out that the blower heaters run off a bank of gas cylinders, so this was a problem of his own making. The answer to the “leaking” problem was not to heat the winery by unflued gas, and was received in disbelief.

The penetration limitation is recognised by most architects who usually design their penetrations in such a way that they can be easily sealed, but the plumber with his vents, the air-con man and the television aerial man appear oblivious to any problem as they usually are with cold roofs.

The other detail which causes heartache with designers and the BCAs is when joining panels end to end. Waterfall or stepped joins are not easily fixed and sealed, but then nor are lapped joints.

If you are pricing warm roofs it pays to double your time allowance over cold roofs for flashing this detail. One manufacturer does do factory cut-backs though.

Even with joins and limitations with transport, some long lengths can be achieved, so the same expansion provision should be made for fixings as for cold roofs with lengths over 12m.

Screw fixings get very long with the thicker panels which, in itself, can be a good thing because there is some “fastener roll” that can occur — although it is only minimal because of the stressed skin effect of panels.

Some panels have a separate snap-on cover strip which can protect the fastener from the weather and contaminants. But the majority have top fixings which are subject to the same problems of fastener corrosion that we are currently addressing.

Edge details vary from different manufacturers but are generically classified into rib fixing (as per cold roofs) and flush fixing which are dependent on rain and vapour sealing.

Rooflights must be insulated with warm roofs. Most manufacturers have their own details for these, and joining these up becomes a tricky bit.

Gutters also have to be insulated, as any uninsulated path seriously diminishes the fire and insulation performance — one simple way to do this is to use panel as gutter support.

There is an Australasian Code of Practice published by the Insulated Panel Council of Australasia (IPCA) which has details for cool-room panels, and they have a panel certification scheme in place. Their web site is worth a visit: www.insulatedpanelcouncil.org.

The hybrid systems are assembled on site and are said to equate in fire ratings and performance but require a lot more input from the roofer.

In a similar manner to single skin roof cladding, simply supported roof (one span) panels do not have equal spanning performance to multi-spans.

Similarly, dark colours can produce expansion problems so light colours are always recommended.

Safety issues are minimised but edge protection, safety nets and/or a harness must be used.

Some roofers may be wary of what they might regard as new innovations, but warm roofs are likely to play a much greater part on the roofing scene in the future.

There is nothing really to be concerned about except upping the standard of workmanship and understanding the problems, and knowing how to deal with the tricky bits before you start.
Maybe it is time to have a closer look at panel roofing?


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