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Flat Roof Coverings - Top 5 Defects

Friday, 2nd July 2010 | by: Peter Barry

1. Poor workmanship

Installing flat roof coverings, be they felt, asphalt or sheet metal is a specialised job. Many of the problems associated with felt coverings to flat roofs stem from poor workmanship. DIY felt jobs are the biggest culprit and are simply storing up problems for the future.

2. Ponding

‘Flat’ roofs are not technically flat but have a minimum gradient of 1:40. Where the appropriate fall has not been built in or has been altered by structural movement ‘ponding’ is likely to occur i.e. rainwater settling on the surface. Where water is allowed to sit on the surface of a felt roof for an extended period its lifespan will be drastically reduced.

Flat roofs that remain wet for long periods provide the ideal conditions for moss growth. Moss can attack the surface of the felt and the moss itself acts as a sponge soaking up water and leading to further problems as it freezes and thaws in winter.

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Peter Barry Surveyors undertake RICS Homebuyer Reports and Building Surveys throughout North London including the popular areas of Muswell Hill N10, Crouch End N8 and Islington N1. We can normally take instructions at short notice and reports are turned around within 2 working days.

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3. Thermal Movement 

A good quality solar reflective paint or a layer of light coloured chippings will reduce thermal movement and associated stresses to asphalt and felt roofs. Where the surface is not adequately protected creasing will occur and eventually lead to splits.

Sheet metal coverings such as lead and zinc have high coefficients of linear expansion and are therefore laid in small panels with overlaps known as ‘rolls’ or ‘seams’. It is essential that coverings of this type are given room to expand and contract or it is inevitable that the material will split. 

4. Blistering

This is a defect that affects built up felt roofs. It occurs when moisture becomes trapped between poorly bonded layers of felt. When the water evaporates in to a gas and expands it cannot escape and so forces the layer of felt above up to form a blister. The root cause may be something as simple as the roofer not adequately protecting the felt from rain during the installation process.

5. People

During a warm summer flat roofs become illegal terraces and their asphalt or felt coverings are subjected to point loads just when they are most vulnerable. The result is a series of pot marks which allow moisture to ingress and cause the covering to fail the following winter.

One of the most common causes of failure in sheet metal roofs is punctures caused as a result of people (including surveyors!) walking on the surface. It may be that the covering was laid on a surface that was not completely free of grit which subsequently gets forced through the covering when weight is applied from above. Alternatively it might just be a small piece of tile or slate that has fallen from a roof above which when walked on causes a puncture.

Categories: Surveying