Snow
Each zoned area has a different snow load, with the altitude being the determining factor. The Canterbury Alps and Plains is regarded as the most severe snow zone in N.Z.
Roof pitches of 30° and less are considered to be unable to shed snow, and roof pitches above 30° have a reduction factor applied to them, which effectively reduces the snow load proportionately, reaching zero at 70°.
In snow areas, designers using roof pitches between 30° and 70° should recognise that shedding snow from a roof can be a safety hazard during the thaw, and snow guards should be installed over doorways or pathways. For areas subject to 1.0 kPa snow loads, roof slopes in excess of 10° should have snow guards at a minimum of 50 mm high. Snow guards should be able to resist the dynamic weight of sliding snow (which depends on the roof pitch) multiplied by the tangent of the roof pitch. Static load = 1 kN/m x tan. (see Roof Pitch Tangent.)
Secret-fix roofs have a lower friction coefficient, because of their lack of protruding fastenings, and purlin spacings should be reduced if there is a likelihood of de-indexing at the lap.
Areas on roofs where natural shedding is prevented — such as at valleys, parapets, sawtooth and butterfly roofs, internal gutters, and high roofs above low roofs — are subject to accumulated drift and should be designed for twice the normal snow load.
Designers of structures required to resist snow loads should ensure that their deflections under load will be within the same prescribed limits as for non-snow load buildings. The roof cladding span should be reduced to withstand the point load plus the snow load to comply with the minimum pitch as prescribed in Minimum Recommended Pitch.
Side lap stitching should be considered to ensure there is no gap between sheets at the side lap, which would allow the ingress of snow. Powder snow can infiltrate tiny gaps in flashings, so neat fitting and sealing are required to a higher waterproofing standard than that for wind alone.
Load-spreading washers and sealed laps should be used on all roofs where they are to be subject to snow loads in excess of 1 kPa.
Where a penetration such as a chimney pierces the roof cladding in a severe climate, it presents two problems.
- It offers an obstruction to snow drifts.
- Leakage of heat from a flue will melt snow near the chimney, which can turn to ice and can cause leakage at the lap when it thaws.
For this reason, chimney flues should be placed on a wall or a ridge in snow areas.
Continual low temperatures inhibit the normal diurnal temperature fluctuations of roof cladding, and if there is a heat source within an uninsulated building a considerable amount of condensation will flow down the sheeting.
If there is no heating, or if the building is well insulated, sustained temperatures below freezing will allow a build-up of ice on the underside of the sheeting, which will be removed safely only if the provisions of section underlays , have been complied with.
This condensation is often mistaken for leakage from the roof, because of the quantity and the continuous dripping that can be expected from a snow submerged roof.
The back and side curbs of penetration flashings in snow-prone areas should have full height, be sealed, and are required to be a diverter or cricket design.
Internal gutters should be avoided in snow-prone areas. If they are fitted, the use of snowboards or gutter snowboards is required to avoid consequential flooding from gutter and outlets blockage. (See Snow/Hail Board)
In snow areas, the gutter should have additional depth, and be open-ended to ensure full capacity overflows.
External gutters should have snow brackets as well as standard brackets to be able to withstand the added weight of the total volume of the gutter filled with snow.
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| 1 m³ of fresh snow weighs approx. | 100 kg or load of 1 kPa |
| 1 m³ of old snow weighs approx. | 400 kg or a load of 4 kpa |
| 1 m³ of water weighs | 1000 kg or a load of 10 kPa |
| 1 m³ of ise weighs approx. | 900 kg or a load of 9 kPa |
As snow is 90% air, it is impossible to generate any head or water pressure until the thaw. Water, by its definition, is above 0°C and heavier than snow, so rain falling on snow will fill the air gaps, melt it, and run underneath the snow to find its own level.
Prolonged sub-zero temperatures are unlikely in New Zealand below an altitude of 500 m. (See Snow Loadings Map). Often the mitigating circumstances of wind, ventilation and heat escaping from a building mean that the roof cladding temperature will be above freezing, at which time snow will run as water.
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