Principles of Roofing Physics
This is a brief introduction to nature’s elements and driving forces that affect a building envelop. By developing ways to control its effects, a comfortable and energy-efficient building can be achieved. It will save costs in the long run through reduced maintenance and energy consumption.
Designers, Architects, Clients and Building Owners have to understand its principles when deciding on the most optimum system. The study should start from design and conceptual stage where its’ determining factors can be weighted out properly.
Heat, often the highest feared element in our tropical climate, demands the highest attention. Through intelligent adoption of thermal resistant materials, heat ingress into the building can be minimised thus reducing the heat stress to the occupants of a non air-conditioned building or minimise the energy consumption for air-conditioned building.
Singapore, for example, has a mandatory thermal insulation standard for the purpose of energy conservation in buildings. It regulates the maximum roof thermal transmittance values (U-value) of both the air-conditioned and non air-conditioned buildings.
U-value is the heat transfer through 1m2 of roof area if the temperature difference is 10C (W/m2 K). It is the reciprocal of total thermal resistance, R, of the roof build-up (U-value = 1/R). Calculation of the resistance of surface air films, roofing materials, air-gaps and thermal bridging will give an estimation of heat loss through the roof.
Some other design aspects to consider in minimising heat ingress include the building’s occupancy rate, orientation of the building, shape and slope of its roof, Solar Reflectance Index (SRI) colour scheme, ventilated and non-ventilated roof construction.
The use of heat insulation materials and radiant barrier are effective ways to reduce heat tranfer through the roof.
Moisture. Condensation occurring in the roof system is detrimental to the building. Aside from possible water dripping into the building, the condensation water will damage the roofing material. There are two situations where condensation occurs:
Incorporation of ventilation space and/or vapour barrier are ways of preventing condensation from occurring in the roof system. Vapour barrier can reduce the amount of water diffuses into the roof structure, thus eliminating condensation.
Air-Borne Sound Insulation
"When designing a new building, or converting an existing building, likely sources of noise should be considered and an assessment made of the possible effects on neighbours of noise generated within the building. Where there is a risk of disturbance from noise it will usually be possible to control the noise, as perceived by listener, by careful attention to various factors of the design.” (Clause 4.1 BS 8233: 1987)
The effectiveness of the construction to attenuate sound is measured by Sound Transmission Class (STC) or Weighted Sound Reduction Index (Rw). A roofing acoustic performance of Rw = 45dB does not mean it will reduce the noise by 45dB at all frequencies. Sound level varies with different frequency.
The most effective way of determining the sound reduction index or STC value of a construction is by laboratory testing. Measurement on a completed building usually results in lower value because there are other factors affecting the measurement like doors, windows, features of the building, and flanking.
Other ways of estimating the STC value like calculation based on mass law gives an idea of the sound insulation but it is not accurate.
Wind. Roof covering can be lifted up by strong wind due to poor detailing and workmanship, ignorance of the wind load at critical areas, and use of inferior fixing materials.
Engineers calculate the wind load in accordance to national standards using the correct reference wind speed, surrounding topography, building height and geometry. Particular attention must be paid to the effect of wind along edges and corners of roof as these areas experience the greatest wind suction force.
From the wind uplift force, the fixing distance is determined in regards to fixing method, panel width, and thickness of material.
Basic Material Property
(1) Materials Compatibility
(2) Thermal Expansion & Contraction
Metals expand or contract when experiencing changes in temperature. Calculated measures such as the incorporation of sliding clips in the system so that it can accommodate these thermal expansion and contractions are vital. It will prevent detrimental thermal stress and wearing over time of the roofing material.
Listed below are the linear thermal expansion co-efficients of different metals. For every degree Celsius change, they expand/ contract the lengths. For roof and cladding surfaces, a variance of 60 degree Celsius is very common.
|Architectural Material||Thermal Expansion Co-efficient|
|Stainless Steel (304)||0.017mm/moC|
With the advent of global warming concerns, the sustainability of buildings has become the utmost importance criteria for majority of the building owners. Green buildings with LEED certifications will command a better recognition for now and beyond.
However, one of setback of achieving a green building is the higher initial cost which may not be understood or accepted well by the owners and designers. Overcoming the initial costs, and the later benefits of these greener buildings are many such as higher efficiency for energy consumption, less carbon footprint, higher productivity of occupants and better building value.
Do consult us to find out and understand better the many greener options that we provide. Alternatively, to find out more on achieving green building index certification in Malaysia, please log on to www.greenbuildingindex.org