Zinc, a resistant material quite at home in cold weather
In mountainous regions, the higher you climb the more common zinc roofs become. It is often chosen as the roofing material for ski resort hotels, mountain top restaurants, cable car stations etc. The zinc clad Helbronner cable car station, at 3500mts above sea level in the Alps, is a good example. In other regions of world where very cold winters are the norm, zinc on the whole is much more commonly used than in most countries with a temperate climate, The Nordic countries are a very good example.
1. Behaviour of zinc in cold climates
- Being impervious, zinc is unaffected by freeze thaw processes that can break up other materials
- Once installed, it is not affected by low temperatures.
- Enjoys a very long life.
Snow lying against the zinc for long periods of time does not affect the surface finish, be it naturally patinated or pre-weathered, but on occasion after especially snowy winters some white discoloration can be visible in the spring as the snow melts and reveals where any meltwater was retained by ice damming. This discoloration then gradually returns to the normal colour of zinc. This does not occur with an elZinc Rainbow® or elZinc Advance® finish.
One other effect of snow lying on a new or relatively new natural zinc roof for a few days is that it the patina more uniform, reducing the initial irregularity associated with naturally patinating roofs, if the snow cover is relatively even and defrosts relatively quickly.
2. Design considerations in cold climates
There are only two important issues that must be addressed – ice dams and snow retention.
Common to roofs of all types (tiled, slated, shingled, metal etc.), ice dams are a localised build up of ice on the roof due to melt water refreezing.
This can be caused by:
- Snow melting from the underside due to heat loss thawing and freezing again at the overhanging, colder, eaves.
- Snow melt due to solar thawing draining onto shaded areas and refreezing.
This can cause the standing seams to be flooded, leading to water ingress. If there is a structural underlay with a waterproof membrane under the zinc, this will drain the water down to the cold eave, where it will freeze under the zinc.
2.1. Underlying phenomena - behaviour of snow thaw on a zinc roof
Snow thaw on a metal roof can occur due to three phenomena:
- As it breaks the bond between the zinc and the snow layer, it can lead to a sudden shedding of the entire snow blanket if no retention systems have been installed.
- Flowing over shaded areas it can re-freeze and build up ice dams.
Heat loss thaw
2.2. Recommendations to minimise the formation of ice dams
- Insulate the roofs adequately, paying special attention to cold bridging. This will limit heat loss thaw.
- Venting the roof keeps the zinc colder by introducing an air space between the insulation and the decking supporting the zinc. This helps to protect against heat loss thaw. There are two options here:
Built either using open gap softwood boarding or wooden sheathing as the substrate. Inlets and outlets should be continuous at the eaves and ridge. Follow local and national guidelines.
Used in Alpine Europe (Parts of France, Austria and Switzerland) they provide extra protection against water ingress through the standing seams. These roofs typically incorporate a bitumen based waterproofing layer over the sub-deck to drain any water ingress (that has filtered through the standing seams and dripped through the open gap boarding) or condensate away. This layer is not vapor permeable, and so must be vented underneath by the second air layer. Inlets and outlets must be continuous. Follow local and national guidelines.
- If possible, design the roof so that all slopes receive at least some sun during the day (east – west facing slopes are preferable to north – south facing slopes).
- Dark colours (elZinc Graphite®, elZinc Lava®, elZinc Oliva® and elZinc Rainbow® Brown and Black) will help absorb sunlight and induce sun thaw. Generally, this is preferable to heat loss thaw as it avoids the ‘cold eave’ problem that leads to eave ice dams. Dark coloured rainwater goods should also be used to avoid the meltwater freezing in gutters and downpipes.
- Seal standing seams at eaves using appropriate measures such as closed cell compressive foam strips. The sealing strips should extend well up the roof to at least 1,5mts from the eaves and in any case to beyond where there is a risk of the seams flooding. This should be carried out in all other areas where there is a risk of seam flooding.
- Consider using the detail below for the foot of the standing seams – the end tabs of the seam upstands are folded back within the seam, allowing for free slippage of snow and ice.
3. Design and installation considerations for climate with cold winters
- Snow and ice retention measures
The installation of a snow and ice guard system is commonplace in regions with frequent winter snow. Their function is to retain the snow on the roof until it melts away, stopping potentially dangerous slippages onto passers by below. It also protects the eaves of the roof from damage due to the weight of slipping snow.
- Standing seam roofing systems
These systems use aluminium pipes that are clamped to the standing seams, and therefore do not perforate the zinc or prevent the zinc bays from freely expanding and contracting due to temperature changes. One such system is shown below.
To reduce the amount of loading on individual runs, the roof is broken up into horizontal bands across the roof by the snow retention pipes. The higher the loading, the narrower the bands need to be.
Points to bear in mind.
- The clamps should be installed on every standing seam so that the forces generated by the snow loads are evenly distributed. Follow the manufacturer’s instructions regarding installation and dimensioning of the snow retention system. Important – do not overtighten and dimensioning of the snow retention system. Importante – do not overtighten the clamps – use a dynamometric wrench to tighten the nuts to the correct torque. This is to allow the clamps and pipes to break free from the roof under extreme loading, saving the roof from beign torn from the substrate.
- Enough clips should be fixed along the standing seams either side of where the clamps are to be placed to ensure that the loads are transmitted to the substrate without loosening the clips. The clamps should never by placed directly over a sliding clip, as the clamping action can press the zinc trays against the base part of the clip, preventing free thermal movement of the trays. To be safe, leave a small gap either side of the sliding clips for the placement of the clamps.
- Clip distribution. Even without retention systems, when snow settles on a roof, a strong bond is formed between the snow and the metal sheets. The extra loading due to the snow exerts a dragging effect on the zinc trays, tugging them down the slope of the roof, which can only be resisted by the band of fixed clips installed across the roof (as the sliding clips are free to move in this direction). As seen above, snow retaining systems clamp to the seams and so the dragging effect on the zinc is the same.
Therefore, with or without retention systems:
- The zinc trays should have the fixed band of clips located at the very top of the roof, to avoid bucking of the trays under the additional load of the snow pulling them down. This limits the maximum length of the trays to 8mts without using specially designed sliding clips.
- Consider whether the number of fixed clips in the band should be increased in snowy climates to ensure that the trays are effectively retained. Seek further advice from your local distributer, or direct from elZinc® if required.
4. Best practice for storage and transportation of elZinc® products
Storage of material
- The material should be stored in a well ventilated, dry space.
- Any changes in temperature should be gradual, to prevent condensation forming on the surface of the zinc (preferably in a temperature-controlled environment).
- For winter:
- Unpacking should be carried out immediately after unloading and placing within the warehouse, allowing any condensate which may form to evaporate.
- Cold air should not fall directly on the zinc.
- Material cold from transportation should not be placed in warm, humid spaces, as condensation forming on the zinc surfaces is likely. This applies even if the space is not heated. Packing should be unwrapped to let this condensation evaporate quickly, before covering again loosely with a breather membrane.
Transportation to the building site
The material should be properly protected from the weather, and securely packed to avoid damage during transport, even on short journeys. Protection during longer journeys should allow any condensation that forms during transportation to evaporate.
More information about Storage and transportation of elZinc® products is available here.
We recommend following regional and local regulations regarding design and installation, and seeking the advice of specialist installers with successful experience in the region.