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Flat Roof Construction – Common Problems


Many properties will have some form of flat roof which may form a larger or smaller percentage of the overall roof covering. A flat roof is defined as having less than a 10 degree pitch and these forms of roof have been common in the UK for over 90 years. Historically, pitched roofs were almost universal except for some grand Georgian and Regency properties where large expanses of lead flat roofing could be found. Flat roofs were also used over bays and small additions in older property which also utilised lead but these were the exception rather than the rule.

This blog will look at the more common types of flat roof and some of the problems that often occur.

Design Considerations:

A flat roof is essentially a simple structure that should allow rainfall to land on the surface and drain evenly away to a point where rainwater can be dealt with such as a gutter or hopper. This is why flat roofs are normally not truly flat but rather have a gradual slope in a chosen direction.

Where a roof is incorrectly designed it can lead to water ingress with a flat roof is generally more prone to failure than a pitched roof.

Poor detailing can lead to water ingress

There are many reasons why a flat roof may fail, poor workmanship is one of those reasons. For example if the joints are not correctly bonded on a flat roof with bituminous felt water ingress can occur. Similarly if materials are not carefully selected or wrong materials are put together such as in the instance of zinc and lead abutting (both of which are common roofing materials) then corrosion or other defects can occur.

The use of incorrect decking material below a flat roof covering also can cause problems. For example where chipboard is used this tends to absorb water to a higher degree then plywood and where plywood is used this should be a marine grade. Timber and high moisture levels are not good bedfellows with rot becoming a problem with sustained high moisture levels.

If a flat roof has been installed to early on in the construction process or if construction foot traffic on site is not managed then this can lead to damage of the covering. Felt for instance can be easily punctured if a ladder was footed directly from the covering.

If a flat roof is not given a fall or the fall is incorrect this can lead rainwater to pooling or indeed flowing into areas where it can cause problems. A fall is normally achieved using timber firring pieces, these are long slivers of timber which are thicker at one end and the other and are laid on top of structural joists below.

A flat roof should be properly detailed by a qualified and competent person, normally an architect and the detail should be checked during construction and on completion.


Modern flat roof construction should have a vapour check barrier, a membrane to stop warm moist air reaching the colder portions of a flat roof and condensing. Where there is no vapour control, high moisture levels can occur in the voids within a roof and if this is formed of timber, it can allow rot to occur.

Certain flat roofs should therefore normally be passively ventilated to allow the moisture to exit the structure. Identifying condensation can be difficult, the first thought where dampness occurs in a roof is to assume that there is a leak, but this may not be the case. Careful consideration is needed including understanding the use and occupancy of the structure, the nature of the flat roof and the location of the damp areas.

Ponding and Thermal Movement:

Where a flat roof has irregularities in either the structure of the roof covering, this can cause water to pond. In other words rather than freely draining, water will be retained on the surface of the roof and whilst this may not penetrate through the roof covering it will eventually cause problems.

This is because when exposed to direct sunlight the temperature of a flat roof will increase whereas the section containing the pool of water will have a lower temperature.

A flat roof be subject to extremes of weather like all external portions of the property. Materials when heated will expand and conversely when cooled will contract. In the short term this will not tend to be a problem however over extended periods of time this will cause stress fractures to occur in the roof covering and the material will weaken and eventually split allowing a passage for water.

Mineral Felt:

This is perhaps the most common form of roof covering is normally built up of two or three layers bonded together with a torch allowing for the bitumen to melt on and adhere to the material below. The topmost layer is normally formed with stone chippings to allow for weather protection and protection from ultraviolet light. Bituminous felt tends to have a lifespan of between 15 to 20 years although it can fail significantly before this. Older felt had a core of asbestos and so should be treated with some care, if in doubt should not be removed unless by a competent builder.

Blistering can occur in felt, this is normally caused by the expansion of small amounts of moisture and air trapped beneath the covering and blisters will tend to become larger and smaller depending on the temperature. The felt may expand permanently where the blister occurs and if damaged or left for prolonged periods of time can cause splitting and therefore water ingress.

Ultraviolet light can damage felt and therefore if the stone chippings or other form of weather protection has been eroded away the felt will quickly age and crack.

Initial construction can also cause problems if the felt is not properly adhered for example by incorrect or uneven heating or laps between felt layers being too small.

Mastic Asphalt:

This is a form of bitumen which has fine and coarse aggregates within it, asphalt is normally heated on site and applied with a trowel. If correctly applied the lifespan can be much longer then bituminous felt perhaps up to 50 years.

One of the most common problems with felt coverings its tendency to slump if not correctly laid which will inevitably lead to cracking and even tearing of the ash felt at certain joint locations. It is very important to apply the substrate correctly avoid this slumping.

Other problems include blistering which can occur for the same reasons as detailed above however with asphalt blisters are more prone to splitting.

Asphalt sagging at an upstand


Lead is a very durable roofing material and is still widely used however corrosion can occur, particularly on the underside where condensation due to poor detailing. Lead is also prone to expansion in high temperatures and therefore regular joints should be installed which allow for small amounts of movement. Where a large expanse of sheet have been installed without movement joints thermal expansion will cause splits and weathering over time.

Lead is also very prone to failure where it has been incorrectly installed and careful detailing is necessary. The Lead Sheet Association has a wealth of information with best practice guidelines:


This should not be considered a conclusive document on flat roofs and a single blog is not perhaps the best location for an exhaustive explanation of flat roof coverings and defects that can occur. Every property is different and as such there will be a set of circumstances particular to a property that will cause a defect and only careful assesment will correctly highlight a problem/s.

A competent surveyor will need to assess all the variables on site and this is precisely the sort of survey that The Hopps Partnership can provide, ultimately guiding you on whether to repair or replace your flat roof covering.

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