The pitch of a house roof impacts the aesthetic appeal of a streetscape. It also affects water drainage, thermal comfort, material selection, storage space and cost. This bulletin discusses the pros and cons of different pitches.
Categories of house roof by pitch
Roof planes may be any pitch, and a roof may contain planes at widely different pitches. However, most roof planes are one of a few standard pitches, and most roofs have multiple planes at the same pitch. Described below are the standard categories of roof pitches in Australia.
Flat roofs are pitched under 5o. These roofs are usually hidden by parapets.
Low roofs are pitched between 5 o and 18 o. These are typically a ‘skillion’ roof with one or two planes.
Medium roofs are pitched between 18 o and 30 o. Hipped and gabled roofs in Australia usually have a 22.5 o pitch. Some have a 25 o pitch.
High roofs are pitched at 30 o or more. They are uncommon in Australia.
Issues relevant everywhere in Australia
Aesthetics
The roof pitch influences how a building is seen from public viewpoints such as the adjacent street. Flat and skillion roofs tend to be seen as ‘modern’. Medium pitches, if symmetrical, tend to be seen as traditional. High pitches make the roof more prominent, but are more expensive than lower pitches.
A flat roof hidden by parapets.
The example below shows the same façade with a 22.5 and a 25 degree roof pitch. The façade is 14 m wide. The 25 degree roof pitch gives the façade better proportions, and adds height to the gable and pillar feature.
The same house with different roof pitches - 22.5 degrees on the left, 25 degrees on the right.
The example below of a 25o and a 30o roof shows how a higher pitch can make the roof become the dominant element. In this case the 25o roof pitch gives the façade better proportions.
The same house with different roof pitches - 25o on the left, 30o on the right.
Thermal comfort
The volume of the roof cavity assists insulation and ventilation. As hot air rises, hot air from a medium to high pitched roof will escape more easily than from a flat or low-pitched roof. The larger the roof cavity, the more easily hot air can rise away from the ceiling.
Cladding
In Australia, the roof cladding material is usually tiles or metal sheeting. Tiles can be used on roofs pitched between 12o and 70o. Metal sheeting can be used on any pitch greater than 2o.
Shedding rain
The primary function of a roof is to protect the interior of a building from water. The steeper the roof, the faster debris and water falls off it, and the longer the roof’s likely lifespan.
PV panel placement
The pitch of the roof influences the efficiency of solar panels attached to it. The most effective angle for a solar panel is similar to the location’s latitude. Darwin has a latitude of 12o, Hobart 43o. As the sun is lower in winter, and the days shorter, to maximise the energy collected throughout the year, a pitch of between 3o and 6o less than the latitude is optimal. See here for the best pitch.
Solar panels on an angled mounting frame for optimal sun.
Costs
The amount of roofing material used increases as the pitch increases. This increases the cost, but only marginally. The cost of the builder’s labour varies little for roofs with less than a 30o pitch. However, a steeper roof requires a different tradesperson, with different tools and equipment. This increases the cost substantially. Changing a roof pitch from 22.5o to 25o adds approximately 3 % to the cost of the roof. Changing it to 30o adds about 6 % more.
Storage
The roof cavity can be used for storage or as extra living spaces. The higher the roof pitch, the more headroom that is available. Using this space is uncommon in Australia. A 25o roof may provide enough extra headroom to make habitable rooms in the ceiling space viable.
An extra room in the roof cavity of a house with a 25o roof.
Issues relevant in particular regions
Bushfire prone areas
If a house is within an area of bushfire risk, the house roof should have a simple form that doesn’t collect leaf litter or embers. A roof pitch over 15o reduces the amount of litter caught on the roof. Parapets, valleys and multiple ridges are likely to be problems, but may be found by a building bushfire assessment to be acceptable.
The image below shows where embers may get caught, and how to change the design to a safer roof form. This is also relevant for houses near deciduous trees, as leaf litter may build up.
Roof design improvements to minimise fire risk in bushfire prone areas.
Cyclone prone areas
Cyclones bring strong winds. These winds can rip the roof off a house if it is not well connected. A roof pitch of less than 20o is subject to strong uplift force, a roof pitch over 35o a strong wind load. The best roof has a pitch of between 20o and 30o and a curved shape. A 30o four sided hipped roof is recommended to minimise uplift while being simple to build. Also to minimise uplift, the recommended maximum eave overhang is 500 mm. See more here.
The wet tropics
In the wet tropics, heavy rain can fall for hours. The steeper the pitch, the less likely the roof is to leak or be damaged by the heavy rain. Another issue in tropical climates is heat and thermal comfort. The flatter the roof, the more heat is trapped close to the inside of the house, increasing the internal temperature. To counteract this, use a steep pitch and roof vents. The recommended roof pitch for rainfall run off and thermal comfort in tropical areas is at least 30o.
Alpine areas
In alpine areas, ice and snow can build up on the roof. The steeper the pitch, the less likely the roof will be damaged by ice and snow. If the estate is in an area that receives frequent snow, adding a minimum pitch requirement to the design code can assist lot owners and builders with choosing a safe option. The recommended pitch in an alpine area is 20o to 40o.
A house with a high pitched roof reduces snow buildup.
Design standards
The most common roof pitch is 22.5o. This is cost effective for the builder and lot owner and gets water and debris off the roof quickly. However, a 25o roof pitch on single storey houses usually looks more attractive, at very little extra cost. A street full of houses with 25o roofs looks much nicer than one of 22.5o roofs. Double story houses already have more built form, more street presence, and a less visible roof. As such, a 25o pitch is less important.
“If the house is single storey, the roof pitch must be at least 25o. If the house is two storeys, the roof pitch must be at least 22o.”
Some design codes limit the minimum and maximum pitch of a hipped roof to control the appearance of the streetscape. The Design Review Group recommends at least 20o. We see no aesthetic benefit above 30o, except in alpine and tropical areas.
“The pitch of gabled or hipped roofs must be between 20o and 30o.”
