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Wellington Information and Details

This page has information about the wind effects on roofs in the wellington region. For details about Wellington roofing contractors, please click this link, or visit our homepage above.

 Living in possibly the windiest city of the world throws up some interesting roofing problems. Wellington sits next to the cook straight which is a small channel of water between the North and South islands of New Zealand. The wind here is famous, blowing 37.6% of the time. The strongest gust on record was recorded at Hawkins Hill and measured 248km/h. For this reason the roofs have to be made double secure to negate the force of the wind. It may be useful to roofers in other windswept areas to look at the methods used to make the roofs secure.

 Roofing tiles are not usually "blown off" the roof in the same manner that a tree may be blown over. The roof tile is sucked upwards as the wind moves over the roof. As you still have a gaping hole in your roof either way, you may consider this of little importance. However, if we are to minimise the risk of damage it is important to understand the forces applied by the wind.

In windy conditions the roof surface is subjected to uplift. This results when the air pressure in the loft is higher than the air pressure above the roofing. As wind travels over the structure the pressure on the surface decreases while the pressure inside the roof space increases, causing wind uplift. As a consequence an upward suction results on the windward and leeward sides of the roofing. Put simply, the tiles are sucked upwards. The fact that low pitch roofing suffers more wind damage than high pitch roofing is evidence of wind uplift.

It can be understood from this that the traditional way of fixing roof tiles may not be the most effective. The old way of securing roofing tiles was a single nail or peg through the top or head of the tile. This only leaves the dead weight of the roof tile to hold down the tail. In fact it has been proven that a tile clipped at the bottom can withstand 5 times the force of an adjacent roofing tile head nailed. While roofing in Wellington I have noticed the effectiveness of many forms of roof tail fixings. The most efficient method I have found is to wire the tail at the bottom. The wire feeds through an eyelet between the supporting tiles and around the roofing baton. I am of the opinion that this method out performs modern tile clips. Modern tile clips can work loose with the conditions in Wellington. The wire holds the roof tile down hard and stops tile chatter. Tile chatter is the noise created when loose roofing tiles are constantly lifted and dropped by the wind suction. As an extra precaution against tile chatter a small bead of silicone may be applied between tile courses. Of course not all roofing tiles lend themselves to wire fixing and tile clips will be used. In these instances it is important to make sure the clips are bedded down tight and at right angles to the tiles.

A lot of older properties in Wellington are constructed with no roofing felt or building paper under the tiles. If you are planning to construct new roofing in Wellington it is important to use good quality roofing paper. The waterproofing qualities of the paper are less important than its use as a wind barrier. The primary purpose of roofing felt is to stop wind uplift. The paper helps prevent the change in air pressure under the roof covering. It should be laid horizontally across the roof with a minimum of 150mm laps. Make sure there is a roof baton fixed over the laps to give a wind tight seal.

Metal roofing in Wellington requires the use of screw fixings. The old style roofing nails often work free causing the metal sheets to work loose. Again, the maximum fixing recommendations of the manufacturer should be followed.

The initial increase in cost of extra fixtures is far outweighed by the cost of replacement tiles and roofing sheets. More importantly they provide the homeowner with a sense of security during heavy storms. If you are roofing in areas of high wind, err on the side of caution and use extra fixings.  By simon cowham Google