Thursday, April 23, 2015

Flat Roofs - Part 2 - Built up Felt - Problems to look for

A well constructed and properly maintained flat roof should remain in serviceable use for approximately 15 – 20 years. Life expectancy can vary depending on a number of different factors however if not regularly inspected and maintained there are numerous problems that can occur

In my last article I provided a brief summary of typical UK flat roof construction together with an explanation of different roof coverings.  As a result of the design/form of a flat roof they are often more vulnerable than pitched (angled) roofs to problems due to their reduced ability to discharge rain and other surface water adequately. A well constructed and properly maintained flat roof should remain in serviceable use for approximately 15 – 20 years. Life expectancy can vary depending on a number of different factors however if not regularly inspected and maintained there are numerous problems that can occur.  Below I will discuss a number of typical problems that can be found when inspecting a flat roof and how these can be identified.

You do not need to be a surveyor or have any detailed knowledge of property or construction to realise that there are clearly issues with the flat roof shown in the images below.  Some of these images show us the extent that people will sometimes go to carry out temporary ‘repairs’, when it is clear that there is a much bigger problem. 

Built up Felt Flat Roofs

Water Ponding – Water ponding can occur on a flat roof for a number of different reasons, for example the roof was not designed or constructed correctly in the first place and is not allowing water into guttering our outlets, there could be impact damage (possible people walking across) or heavy loadings such as plant or materials that have resulted in ‘depressions’ in the roof, outlets or guttering could be blocked etc.  In any event prolonged standing water on a flat roof can result in degradation of felt, stress and failure of joints, increased risk of condensation (as the internal roof surface temperature is reduced), and the build-up of moss and lichen.  Standing water will also freeze in sub-zero temperatures, further increasing the possibility of condensation as well as becoming a health & safety risk if there is a need to gain access to the roof at that time.

Source: Google Images
Although not the nicest of conditions, the best time to inspect a flat roof is in poor weather conditions particularly whilst it is raining.  This will highlight many of the problems with design, workmanship and general deterioration. Even in the warmer months when it may not have rained for a long time a surveyor will still look for visual clues that may indicate water ponding, such as a build-up of moss and debris, general staining of the roof covering or damaged coverings etc.  The image to the left provides an example of this.

Simple regular maintenance such as clearing gutters and outlets will help to reduce water ponding however this is something that is often overlooked by many property owners/occupiers.  This is likely to be because many flat roofs are out of sight (and therefore out of mind!) and unless the roof starts to exhibit problems many will choose to ignore it. Water ponding problems as a result of poor design or poor workmanship are often more problematic to deal with and consequently more expensive.  Again, many people will choose to live with these problems until of course the roof starts leaking and causing consequential damage to other parts of a building.

Blistering of Felt – A fairly common issue identified by surveyors when inspecting a built up felt flat roof is blistering of the felt roof surface.  This occurs where water vapour pressure becomes trapped underneath the roof covering between layers of felt that have been inadequately bonded.  Any trapped water or moisture will evaporate and expand.  As it is trapped and cannot escape it will force the felt to form a blister. The source of any moisture will need to be addressed before any repairs are undertaken otherwise the problem will re-occur.  Blistering sometimes occurs when the felt covering is laid onto a substrate that is not fully dry, possible when roofing works are being undertaken either after or whilst it is raining.

Cracks and Tears - Cracks and tears to felt roof coverings can be caused by thermal or moisture movement, saturation of insulation or sagging of the roof deck. Cracks and tears can also occur when there is thermal or moisture movement between the roof substrate and membrane. In most cases the impact of thermal movement is not visible to  the naked eye, however even though we cannot see it, building materials expand and contract by varying amounts when they heat up and cool down. This is due to their differing coefficients of thermal movement. Materials expand because an increase in temperature leads to greater thermal vibration of the atoms in a material, and hence to an increase in the average separation distance of adjacent atoms. Due to the fact that roofs in the UK can experience a significant temperature difference due to the varied climate, the roofing felt material will be reacting to these ever changing conditions by expanding and contracting continually.  This can sometimes result in crack or tears appearing in the felt surface which can often be repaired by cutting back the existing felt and allowing the area to dry before applying a new layer of felt with a suitable overlap between the new and existing material.

Other issues such as de-bonding at up-stands can also be encountered by a surveyor during an inspection of a built up felt flat roof as well as numerous other problems that occur as a result of poor workmanship.  In my next article I will discuss problems associated with mastic asphalt flat roofs, where some similar issues to those described above will be explored together with some other problems that may occur as a result of using a different material.

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