Risk Assessment Thatched Constructions

Thatching is the craft of building a roof with dry vegetation such as: Long straw, Short straw, Water reed, Cape reed, Grass, Combined reed etc. The layering of the vegetation is to shed water away from the inner roof. It is a very old roofing method but is currently the choice of affluent people who desire a rustic look for their house and or recreational area. However, any thatch construction imposes a risk to property and caution should be taken in building any thatch construction. It is therefore essential that the thatcher or builder is a qualified and registered contractor.

During the time of the assessment/survey, the NID Risk Analyser will look at the following:

• Roof type
• Lightning conductor
• Lightning conductor distance from the main dwelling
• Lightning conductor coverage (45 degrees)
• Lightning density in the area
• Roof ridging
• Damage to roof (Weather related, age, or bird damage)
• Fireplace type
• Chimney
• Chimney high
• Spark arrestors
• Flashing membranes
• Electricity
• Separate electricity distribution box

Considering the fact that South Africa is rated the country with the highest lightning strike rate per square kilometer per year, and depending where the risk is located, it is of the out most importance to make sure that the thatch construction is properly protected against direct lightning strikes.

Your Thatch Building and Lightning

Lightning happens when a huge negative charge builds up in the ground, corresponding to a positive charge in the atmosphere, and the differential becomes sufficient to jump the gap. A lightning rod does not provide a spot for the lightning to strike. Rather, it dissipates the charge so that the strike never happens at all. This comes from early experiments with the Leyden jar; if a pointed metal rod were attached to the jar, it would not charge. The electrons are able to leap off a pointed tip, and into the atmosphere, dissipating charge as fast as it accumulates. The lightning rod does the same thing, on a larger scale; it dissipates the electrons into the air, so that the charge will not build up sufficiently, and the lightning never hits at all.

However, if the charge accumulates faster than the rod can dissipate it, there can still be a direct lightning strike, and the rod typically is the best and shortest route to ground. However, everything in the vicinity of the lightning rod will likely be damaged, most likely, your electronics.