Roofing: When The Roof Load Becomes An Issue - Engr. Osaz’ Enobakhare

‘Wear the cap on whom it fits’ so the saying goes. There is a lot of sense in this one and a real good line of thought too.  Although caps cannot stop you from getting wet under the rain, your roof can. Caps are better worn on whom they fit and so should roofs be. The passion for sky-scrapping roofs suddenly envelope nearly all classes of players and participants in the built environment back here and the aesthetic impression it gives to a building quickly blindfolded many to its structural implications. Now some bungalows have roofs that are five times as high as the building frame itself.  Likewise, the crave for curvy, straight-stepped and composite parapets built mostly of reinforced concrete made a mockery of traditional wooden fascia that normally flushes with external asbestos ceiling that was the norm up to the mid 90’s.

Apart from preventing direct exposure to external weather conditions, roofs are important structural elements of buildings. They constitute a significant fraction of the entire load of the structure and must be well designed so that they do not portend danger to the overall strength and stability of the building. For instance, roofs are designed to be structurally able to withstand wind loads (weight imposed on the roof from wind pressure forces) and ice load too (in country where this is applicable). Hence they have to be strong enough to perform these functions but that should not mean that a roof should become too heavy.

Some people do not take cognizance of the roof load implication on the foundation to which it is transmitted and with time, the roof load begin to take its toll on the foundation causing it to experience excessive settlement or failure. The roof carcass and parapet are very integral parts of the roof load that have heavy members. The combined load of a timber roof carcass and parapet for a residential building can reach up to 1kN/sqM. That is almost equivalent to 2 bags of 50kg cement on every square meter of the roof. Now that’s a lot!

An overweight roof is often a disaster! It is therefore generally advisable to reduce the load of the roof so as not to impede on the structural efficiency of the building. To achieve this, the use of light-weight yet rugged parapet fascia (e.g. wired polystyrene, aluminum, etc.) as well as using light-weight roof carcass (like treated timber, light steel, etc.) and fitting them in such a way as to deliberately reduce the weight on the structure. Where reinforced concrete is to be used, light-weight (aggregate-less) concrete with damp-proof membrane could be used.