The Affect Aircraft Wake Vortices have on a Tiled or Slated Roof.

The Affect Aircraft Wake Vortices have on a Tiled or Slated Roof.

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On a clear day it is possible to see the path of an aircraft high in the sky, travelling to some far off destination. The path is clearly marked by a vapour trail that has a defined width, and remains in the sky for a long period after the aircraft has passed. The vapour trail is technically known as an aircraft wake vortices. At many hundreds of miles per hour, the strength of the vortices generated by the wings and fuselage cutting through the air, and added too by the thrust from the jet engines, are enormous.
The disturbance generates spiralling cones of air (the vortices), much like a tornado. The force of the vortices takes a long time to dissipate as there is little in the upper atmosphere to slow it down.

Closer to the ground large aircraft travel much slower, and the atmosphere is much thicker, so the vortices that are generated are less powerful and dissipates quicker. The most critical period is when a large aircraft is coming into land, when it is common for the aircraft wake vortex to reach the ground. The force of the vortex can suck tiles or slates off the roofs close to the flight path.
The main risk factors
The risks of being affected by an aircraft wake vortices can be assessed by looking at various know factors
Is the location of the building under the flight path into a major airport or military air base? The flight path can be up to 10 degrees either side of the centre line and up to 6Km from the touchdown point.

The direction of landing. More damage is created by aircraft coming in to land than taking off, due to the shallow rate of decent, relative to the steep rate of climb when taking off. Coming in to land the aircraft are lower beyond the end of the runway
The size and weight of the aircraft, and the speed at which they land can make a big difference. It is the wide-bodied aircraft that record the most strikes, as there are more surfaces on the aircraft to generate the vortices.
The weather conditions have a major effect. Windy and wet weather breaks up the structure of a vortex quicker than still clear conditions.
Vortex strikes
Unlike hurricane force winds, which affect the perimeters of a roof (especially the ridge and verge), aircraft wake vortices damage occurs in the centre of a roof slope, as the edges of the roof break up the vortices rather than help it. The average vortex is approx. 500mm wide, will travel at about 5 Knots, and last for approx. 3 minutes in clear air. Once it comes into contact with a roof, the vortices can exert their total force of up to -1200N/m² on just one or two slates or tiles for a fraction of a second, before they break up and loses their energy. If the tiles or slates are not fixed securely, the sucking and twisting action of the vortices can lift them out of place.

Fixing specification
To resist the force of the vortices it is essential that the tiles and slates can not lift at the tail of the tile or slate, and can not rotate. This can be achieved with tiles by being head nailed to the batten and tail clipped with a rigid clip. The smaller the tiles the more fixing can be installed per square metre of roof. Plain tiles should be fixed with either ring shank nails with a thick strong nail head or screwed. For double lap slate, centre nailing with ring shank nails should be adequate. Roofs clad with metal sheeting or built up systems do not appear to be vulnerable to aircraft wake vortices damage as the small footprint of a vortex relative to the large surface area of the panel will allow the load to spread to a greater number of fixings.
Program of repairs
The existing roofs that are under the flight path into major UK airports, starting with London Heathrow, are likely to be subject to a planned or programmed roof replacement scheme. However for all new buildings under a flight path it is the responsibility of the designer/ specifier to ensure the correct fixing specification is used to ensure no roof damage is caused by the effect aircraft wake vortices. In most instances the airport authority will be able to advise if the size of aircraft using the airport is an issue, and the exact line of the flight paths. With this information, the assistance of the roof tile or slate manufacturer should next be sought to determine the correct fixing specification for the roof.
Further reference
To assist designers and engineers, the Building Research Establishment has produced a Digest number 467 entitled "Slate & Tiled Roofs: Avoiding Damage from Aircraft Wake Vortices", giving all the information needed, including sample calculations.
Conclusion
The number of pitched roofs in the UK that may be affected by aircraft wake vortices is very small, but the affect an aircraft wake vortex can have on a pitched roof can be very disruptive. The development of some brown field sites under the flight path into airports, and the extension of existing airports to take more air traffic will increase the overall number of roofs that could be affected. It is therefore important for building designers and specifiers, who intend to build near an airport to understand and be aware of the potential problems that aircraft wake vortices can have and where to get the information needed to ensure it does not become a problem for their project.

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