Minimum Roof Slope

Roof pitch, in layman’s language, simply refers to the steepness of the roof. The pitch of a roof is arrived at by dividing the vertical rise and its horizontal span, or what people in the roofing industry would refer to as the slope. There are different kinds of roofs and all these have particular specifications for their installation. The minimum roof pitch is also subject to the standards set forth by the various building codes used in the respective regions.

Importance of a roof pitch

The chief reason for having a roof pitch is to redirect water and snow away from the roof and avoid any percolation that might result from stagnation of water on the roof. As standard practice, places with heavy rainfall or snow storms will have higher roof pitches compared to places that experience less rain and snow.

Other than directing away water and snow, there are still a number of reasons as to why the pitch is such an important component and should never be ignored in the design and installation of any roof type. It will dictate a number of other roof features such as the roofing material to be used, the architectural properties of the roof, the walkability on the roof and the roof sections to be covered. The pitch also affects the economic aspects of the installation. For instance, steep pitches lead to more roofing material being used. In turn, this will result in a higher roof installation cost.

Factors affecting minimum roof pitch

Due to the fact that there are many roofing styles and different kinds of roofing materials, there is no universal minimum roof pitch. In most cases, manufacturers will recommend the minimum roof pitch for their roofing materials but ultimately, it lies upon the installer to come up with a considerate pitch since every roof is unique and requires different considerations. Below are some of the recommended minimum pitches for different kinds of roofs:

MINIMUM PITCH FOR METAL ROOF

The minimum roof slope for metal roofs is dependent upon the roof profile. With regards to metal roofing, the profile refers to the shape adopted by the metal sheets when they are bent to form the panels. To get the minimum pitch for a metal roof, the vertical rise per foot is divided by the horizontal slope. Most metal roofs will have minimum pitches of three in twelve, but this might sometimes go down depending on whether there are special installation procedures issued by the manufacturer regarding that particular roofing material. To be on the safe side, however, it is recommended that you first have a look at the manufacturer’s recommendations about the minimum pitch for metal roofs before you decide on any form of customization.

FLAT ROOFS

Flat roofs form the bulk of the roofs for commercial and industrial buildings. Though their construction appears to be simple, there are a lot of technical considerations that come into play during the construction and installation phases.

A well designed and constructed flat roof should facilitate the quick and effective flow of water from the roof surface to the gutters, without allowing for the formation of ponds on the surface. Formation of ponds on flat roofs is detrimental to the longevity and effectiveness of the roof. The ponds will do nothing but lead to siltation which will create stress on the roof membrane, causing the distraction of the roof cover. It is imperative that the right slope for the flat roof be adopted to avoid all these potential problems.

For water to flow on a flat roof, the minimum slope must only be greater than 10. However, this is only possible in theory; it is very difficult to achieve in practice. Due to this, there are other factors that are taken into account when working out the ideal slope for a flat roof. Some of the factors include:

  • Deflections on the structural members – Potential deflections on the structural members can alter the overall slope of the flat roof and inhibit the free flow of water or snow. These members mainly include the timber, metal decks or the decking under the roofs. If possible, the deflections should be rectified before deciding on the final slope of the roof.
  • Inaccuracies in construction – There are certain occasions when inaccuracies in construction might lead to a different slope being adopted for the flat roof.
  • The type of waterproofing material used – There are different types of waterproofing materials used in flat roofs. Since they vary in quality as well as method of installation, the choice preferred for a flat roof will ultimately have an effect on the final slope of the flat roof.

In general, the minimum slope for a flat roof is 10 but as stated earlier, this is only possible in theory. It takes the manufacturer’s recommendations, the unique conditions of the roof as well as the skills and the expertise of the installer to arrive at the right slope.

SLATE ROOFS

Slate roofs are very attractive when completed, but the process of installing the slates is never an easy one. It is one of the most complex roof types to install in the industry. Many people think that due to the simplicity of the slates, the installation process is also simple. However, there are a number of factors that must be considered to ensure a successful installation. These factors include the pitch of the roof, the type of slate being used, the size of the lap, the size of the slate as well as the site’s exposure rating.

With regards to the pitch, the building standard code BS 5534 recommends a minimum pitch of 20for slate roofs. But since there is still a wide variety of slate roofing systems, there are occasions when the minimum pitch can go down to as low as 15depending on the uniqueness of the roof.

MINIMUM ROOF PITCH FOR SHINGLES

It is important for shingle roofs to function properly, and one method of ensuring that this is achieved is to observe the pitch requirements for the shingles. The recommended minimum roof pitch for shingles is 2/12, or simply two units of vertical rise to every twelve units of horizontal span. However, it is regrettable that many installers usually ignore this requirement due to the desire for a quick installation process. However great a shingles roof installation might look, if the minimum slope requirement is not observed, there are no guarantees that it will last long, let alone serve its purpose correctly.

MINIMUM PITCH FOR TILED ROOF

Just like with the other roofing materials, manufactures will always give a recommendation for the minimum pitch for tiled roofs. There are however some instances when the slope adopted can go below the recommended minimum. This normally happens when an underlayment or special fastening technique is used for the installation of the low slope tiled roof. The same can also be the case when the tiled roof has a reduced exposure.

MINIMUM SLOPE FOR ASPHALT SHINGLES

The requirements for an asphalt shingles roof are contained in the International Building Code. According to this code, the minimum slope for asphalt shingles is 17 degrees or two vertical units to twelve units in the horizontal. In case a slope greater than this is necessary, then a double underlayment application must be used as per the requirements of the International Building Code.

MINIMUM PITCH FOR CORRUGATED METAL ROOF

Just like other metal roofs, the minimum pitch for a corrugated metal roof largely depends on the profile of the roof. In most occasions, a pitch of three in twelve is preferred, but this should also take into consideration the manufacturer’s recommendations as well the unique condition of the corrugated metal roof. These slope considerations also apply to standing seam metal roofs.

Drainage

The main aim of having a drain or drainage is to redirect runoff away from the roof to other designated places. Water will flow as long as the drainage angle is greater than zero; however, not just any pitch above zero is suitable for all kinds of drainage. It can be argued that the minimum roof slope for drainage is 10 since this is sufficient to allow water to flow, but there are other design considerations that must be taken into account when determining the final minimum slope.

These considerations include the length of the roof, the roofing material, the size of the roof as well as the rainfall characteristics of the region in question. A wide roof with a steep slope will lead to faster runoff, hence necessitating a slightly steeper slope for effective discharge of the rain water. A small roof with a shorter length on the other hand will not collect a lot of water within a short period of time and will work just fine with the minimum roof slope for drainage.

Evidently, there are set standards for the different kinds of roof slopes. It is important to recognize that the standards are set for overall guidance and they are rarely used according to the book during practical application. When it comes to the actual installation process, the roofing experts will be compelled to make informed choices in situations when the set minimum roof pitch poses a challenge in the design and installation. This is why there can never be one single minimum pitch which will work for all roof types.