Roofing Physics

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:
  1. Night-time condensation – During clear cool nights, the temperature of the metal roofing sheet itself can be lower than that of the surrounding air. Condensation forms on the top side and underside of the roofing sheet (as natural dew). If the roofing system is not catered for this condensation water, it will have a damaging effect on the roof system from within.
  2. Water vapour diffusion - Air itself contains water vapour. Higher temperature allows the air to contain more water vapour. The moisture content of air can be expressed as partial pressure of water vapour. If the moisture content reaches saturation (dew) point, or in other words when the saturation pressure is reached, condensation will occur. Partial pressure is dependent on relative humidity and temperature. Varying partial pressure is the drive behind the diffusion of water vapour through the roof. For example, due to differences in both temperature and relative humidity, the indoor and outdoor partial pressures will be different. To reach equilibrium, water vapour from high partial pressure will be driven to low partial pressure space through the roof. In between the moisture diffusion path, it is possible that dew point is reached causing condensation.

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.

 

Noise is an annoying sound which must be controlled to provide a comfortable indoor environment. Sound Pressure Level (dB) is used to indicate the loudness of sound. Typical Sound Pressure Levels of some general areas are:

  • Steel workshop near grinder:110dB
  • Quiet office:50dB
  • Inside a bedroom at night:30dB

Human audible sound frequency ranges from 20 to 20,000 Hz. However, the human ear is not equally sensitive in all these frequencies. At high frequency or low frequency, higher sound pressure level (dB) is required to produce the same loudness as the mid-range frequencies.

To control noise, we can:

  1. increase the distance from the noise source
  2. create a sound absorption enclosure or barricade
  3. reduce the noise at source
  4. use sound insulation in building components

In considering the roofing acoustics, there are three parameters to be considered:

  1. Ln for rain impact noise
  2. STC or Rw of the roof build-up
  3. NRC for interior sound absorption
Rain Impact Noise
Metal roof has generally been termed as a type of noisy roof as majority of its corrugated profile has hollow sections and many "ribs”. Rain drumming on these hollow metal surfaces is the main culprit. This is a far cry from the Double Standing Seam Profile. By having a wide, flat surface and full backing support, there is less vibration of the sheet metal that causes the drumming sound. Furthermore this noise is deadened/ minimised by a full substrate underlay or noise-absorbing insulation.

Air-Borne Sound Insulation

Sound level is reduced when transmitted through a roof structure/ layer (referred as Sound Transmission Loss). In general, the heavier the roof structure and the lesser the linkage between each layer, lesser noise will be passed through the roof. The depth of air space and the use of sound absorbing materials within the air space in the system also play a role in sound transmission loss. This insulation property is referred as Sound Reduction Index of the material/layer, and it varies from low to high frequency.

"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

A roof/ cladding system may have an array of materials co-exists side by side. Extreme care has to be taken when installing different materials either in direct or indirect contact to prevent electro galvanic reaction which may lead to accelerated corrosion.

This reaction is caused by difference in electrical potential between the metals. Higher potential metals will decompose a lower potential metal.

So compatible materials must be used whenever there is direct contact. For example the metal grounds for lightning conductors, the fixture onto the roof for solar panels, etc. shall compatible with the roofing material.


(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
Titanium 0.008mm/moC
Copper 0.017mm/moC
Stainless Steel (304) 0.017mm/moC
Titanium Zinc 0.022mm/moC
Aluminium 0.023mm/moC
Steel 0.012mm/moC




 

Going Green

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