Can your roof manage its own weight?
Roof collapse incidences are not uncommon in Australia. The reasons behind their occurrence vary from the quality of materials used for roofing, to the design, type of workmanship and the weather.
What makes a good roofing structure?
A roof should be able to fulfill two basic functions:
- Act as a climate barrier, and
- Carry its own weight
Any building contractor will tell you that the basis of a good roofing structure starts with an understanding of how individual components combine to keep the weather elements out and keep the warmth in. The first test of a good roof is its ability to keep out water.
Strength and stability
The roof is made up of various components which give it stability in keeping the weather elements out. Apart from the roofing material itself, there are trusses, rafters, fascia boards, sheathing, battens and underlay that add to the roof’s strength. Obviously, these also contribute to the roof’s total weight.
A roof should be able to carry its own weight (known as dead load) as well as any other load imposed on it, transferring as little weight as possible to the supporting walls below it. To achieve this the design that is as light as modern technology can allow.
Dead loads, live loads, and environmental loads
The roof’s own weight is referred to as dead load, while any other external load can be classified as live or environmental. Live loads are brought on by personnel on the roof, or other materials and equipment.
Environmental loads come about mainly due to weather factors such as:
- Snow and ice
- Rain which causes ponding
- Frost heaving
Types of roofs
Roof designs are described in terms of their shape. Each roof type can be designed in different ways using different materials. Most roofs are categorised as pitched or flat.
Pitched roofs have a slope which carries away rainwater.
Pitched roofs have four main designs: gabled, hipped, shed and mansard. The simplest is the shed type (with one slope) used mostly for lean-to structures, while complex roofs have several hips (joints between adjacent slopes of a roof) that give it the hipped shape. Gabled roofs have more structural support provided by the gable – a triangular extension of the end wall.
The more complex the roof design, the more materials are likely to be used in its construction, hence more of a dead weight.
Different types of roofing material also vary the amount of dead weight the roof has to bear.
Types of materials:
- Plain tiles made from concrete or clay, are fairly small in size, and have to be laid double lapped which makes them very heavy
- Interlocking tiles made from concrete, unlike plain tiles, are large in size and have a single lap making them 40-50% lighter than plain tiles
Other roofing materials
- Metal roofing is light compared to tiles. Examples include color bond roofing, corrugated iron sheets, tin roofs,
- Shingles (wood, slate, asphalt or asbestos). These are lighter than tiles
- Polycarbonate sheets are used for sunroofs which are used where extra lighting is needed. They are the lightest in weight.
How different designs are affected by environmental load
Snow is usually the biggest culprit in cases of roof collapse. The weight of one foot of snow can vary between 3 to 21 psf (pounds per square foot), depending on how fresh it is. When this weight is combined with other factors such as additional weight from AC systems or satellite dishes, then roof collapse becomes inevitable. Residents in areas such as Snowy Mountains, Thredbo and Perisher Valley in New South Wales can attest to this.
The layout of the individual tiles and shingles also determines your roof’s ability to keep out rain water, and therefore avoid structural damage. Certain materials such as shingles are more porous making them susceptible to rain or snow damage. Water resistant materials such as color bond, on the other hand, are able to do quite well during heavy storms.
Flat roofs and roof decks
Flat roofs are used mainly for domestic extensions, although they have also been used in the past to cover entire roofs. They are made from bitumen felt applied in separate layers or asphalt. Single ply membrane, a modern material, is also used to construct flat roofs and is made from a thick polymer sheet.
Roof decks are also flat roofs usually made from timber in a kind of balcony design.
Circumstances that cause flat roofs to collapse
Flat roofs collapse mainly due to ponding, which is the accumulation of rainwater on the roof. Ponding is mainly caused by the failure of the rainwater drainage system, or high intensity of rainfall which occurs during storms.