As you drive through the suburbs, from the hillside homes of Cashmere and Mt Pleasant to down on the flat in Riccarton and Addington, you might notice a subtle but significant change in the skyline. The heavy, textured look of concrete tile roofs has slowly been outnumbered by the sleek, linear lines of long-run roofing iron.
This isn’t just an aesthetic trend. For Cantabrians, the choice between roofing iron vs tiles is increasingly becoming a decision based on structural integrity and seismic safety. If you are considering a roof replacement or building new in the Canterbury region, understanding the ins and outs behind this switch is a good idea. That is why we have created this quick guide into roofing iron vs tile roofing.
The Weight Factor And What It Means For Structural Load
To understand why Christchurch homeowners are making the switch, we first need to look at the numbers. The weight difference between traditional concrete or clay tiles and modern steel roofing is quite staggering.
- Concrete/Clay Tiles: A concrete or clay interlocking tile roof can weigh approximately 40kg to 50kg per square meter.
- Roofing Iron (Steel): Modern long-run iron typically weighs around 6kg to 7kg per square meter.
For an average-sized Kiwi home (approx. 150m² of roof area), a concrete tile roof puts roughly 8 to 12 tonnes of “dead load” on the house frame. In contrast, an iron roof weighs around 1 tonne.
That is a pretty big difference, especially when it comes to movement and structural load during an earthquake. The good news is both types of roofing systems are built and designed carry their respective loads safely and securely, but when we add in the effects earthquake ground movement there are some additional factors to consider including the seismic pendulum effect, lateral drift and falling hazards.
1. The Seismic Pendulum Effect
This is the core reason for the mass migration to iron roofing in Canterbury. When an earthquake hits, the ground moves laterally (side-to-side) and vertically and your house moves with it. If you imagine holding a broomstick upright in your palm and you tape a tennis ball to the top of the stick. When you shake your hand back and forth, it is relatively easy to keep the stick upright and stable.
But if you tape a bowling ball to the top of the stick. When you shake your hand, the weight at the top generates massive momentum. The stick might bend a lot and it takes much more strength to stop it from snapping. If you didn’t already get it – your house is the stick and a heavy tile roof is the bowling ball!
2. Lateral Drift
During a seismic event, a top-heavy house experiences significantly higher “lateral drift” (swaying). What this means is the inertia of 10 or so tonnes of concrete swinging back and forth puts immense strain on the wall bracing, the connections between the roof and the walls, and the foundation itself.
By switching to iron, you are effectively removing the bowling ball and replacing it with the tennis ball. You reduce the centre of gravity of the entire structure. This means that when the ground shakes, the house is likely to sway somewhat less violently, reducing the risk of structural failure, extensive plaster cracking, and frame distortion. For older villas and bungalows in Christchurch that may not have modern bracing systems, reducing the roof weight is one of the most effective structural upgrades available.
3. The Falling Hazard
Beyond the structural stress on the frame, there is the immediate danger of the roofing material itself. Traditional concrete and clay tiles are often held in place by their own weight and gravity, sometimes with wire ties or clips, but often just by interlocking patterns on battens. During the violent vertical accelerations Christchurch experienced during the quakes, many tiles were thrown off roofs.
A falling concrete tile weighs several kilograms. Falling from a height of 3 to 5 meters, it becomes a lethal projectile. We saw the aftermath of this all over Christchurch, with driveways, patios, and gardens littered with shattered heavy tiles. This unfortunately does present a hazard to anyone running outside for safety or caught off guard near the property.
What About Iron Roofs?
Modern roofing iron acts quite differently to concrete or clay tiling during a seismic event, as it consists of large, continuous sheets of high-tensile steel. These sheets are securely fastened into the timber purlins using long roofing screws with washer seals.
During a quake, an iron roof acts something like a diaphragm. It creates a rigid, unified skin that is tied securely to the structure. While the house still moves, the roof tends to move more as a unit. This means the sheets are less likely to dislodge, and they likely do not fall or slide around too much in the initial event or even during aftershocks.
Advantages Of Iron Roofing
Modern long-run roofing iron such as Colorsteel is designed for the extremes of the NZ climate and conditions here. Just a few of the benefits are:
- Thermal Movement: Iron sheets are installed to allow for thermal expansion and contraction without compromising the seal.
- Wind Resistance: Because they are screwed down, iron sheets are far superior to tiles in high-wind zones (like in the Port Hills).
- Pitch Flexibility: Iron can be installed on a much lower pitch (slope) than tiles. This is vital for modern architectural extensions or lean-tos where you want good water run-off without a steep angle.
- Aesthetics And Street Appeal: Ten years ago, there was a concern that swapping tiles for iron would “ruin the character” of a home. Now days, that perception has largely vanished. Modern profiling technology means iron is no longer just the “corrugated” look of a farm shed. There are other options including Trapezoidal and Tray Profiles.
- Lower Maintenance: No lichen or moss removal (a constant battle with rough concrete tiles). No mortar to crack and replace every 10 years
In the current Christchurch real estate market, a brand new long-run roof is often viewed as a premium feature. It signals to buyers that the “heavy lifting” (literally and financially) of maintenance has been done. It signals a dry, safe, and modern home. Looking for a reliable and qualified roofer in Christchurch to convert your tile roof to long run iron roofing? Give the team at Action Reroofing a call today!
Disclaimer: This article provides general information regarding roofing materials and structural load. For specific advice regarding your home’s structural integrity, please consult with a qualified structural engineer or a licensed building practitioner.
