BUILDING BRIEFING: 
Flat Roofs
Whereas a traditional pitched roof can be expected to last in excess of 100 years without replacement, the lifetime of a flat roof will be considerably shorter - in the region of 10-20 years before the covering must be replaced. Phillip Shaw looks at some common problems.
It is unusual to encounter flat roofs being used for the whole roofing of houses and bungalows yet they are commonly used for additions such as porches, extensions and garages or for built in features such as bay windows. They have been used on many purpose built blocks of flats.
Flat roofs should provide protection against weather conditions whilst, at the same time, providing adequate thermal insulation. To provide protection from the weather they should be wind and watertight, strong and well secured. To be economically effective they should be reasonably long lasting.
The main justification for using a flat roof rather than a pitched roof is that the original construction will be cheaper - less and lighter materials are used which, in turn, can reduce the specification and, thus the cost, of the roof supporting framework. Sometimes design factors dictate the use of a flat roof for instance when roofing over a small gap between existing structures. There are prices to be paid for using the, initially, cheaper construction; flat roofs don't last as long as pitched and require regular, frequent maintenance - an important issue for those of us involved with the day-to-day management of property.
Insulation
Where a flat roof covers part of a living area, as opposed to, say, a garage, it will need some insulation within it to minimise heat loss. There are three standard ways this can be achieved in flat roofs that are built using timber.
There is a "cold" roof where the heat loss barrier of insulation is immediately above the ceiling, hence the area above it but below the top layer of the roof remains cold and has to be ventilated to avoid condensation.
On the other hand a "warm" roof introduces insulation immediately beneath the top decking of the roof but beneath the actual roof covering. The area between the ceiling and the insulation material is warmed from the living area beneath. In this case ventilation is not desirable since it may give rise to condensation.
The third type is a variation on the "warm" roof, it is an "inverted warm roof" where the insulation material is laid on top of the roof covering and held in place by some sort of heavy material, usually small slabs.
An essential element of each roof type is a vapour check barrier that must, always be positioned beneath the insulation material; the roof covering itself acts as this barrier in an "inverted warm roof". The barrier stops any condensation that forms from coming into contact with, and then penetrating, the ceiling beneath.
Roof coverings
There are quite a number of different flat roof coverings the more common of which are:
- built up mineral felt (two or three layers of felt bonded together with bitumen)
- single sheet, synthetic materials (plastic or rubber based) that are either heat sealed or are supplied with specialist adhesives
- asphalt which is, essentially, thick coatings of bitumen (usually two) that have been mixed with varieties of aggregates and are laid hot onto a separating layer of felt or paper, in turn, laid on top of the roof decking
Other flat roof coverings include solid, reinforced concrete slabs, lead, zinc or copper. Solid concrete slabs often become roofs over living areas when former outside lavatories or stores (often old coal sheds) are incorporated into extensions, usually of kitchens and can present considerable condensation problems resulting in severe discolouration to the ceilings.
Common problems
Problems with flat roofs arise through either poor original design, use of poor quality materials, poor original construction, damage (foot traffic, scaffolding, etc.), weathering, lack of maintenance or natural ageing.
Poor construction 
Where undersized timber joists have been used or they have been placed too far apart there will be deflections in the roof that will cause depressions on the roof surface that will hold rainwater. This is easily seen when puddles remain on a roof for some time after the rest of it has dried, following rain, known as "ponding". This can also occur when some inappropriate materials are used for the decking, for instance chipboard, which will quickly loose its integrity because, inevitably, it will absorb some moisture, albeit a small amount, from condensation. The chipboard gives a little, sag occurs and ponding takes place.
Additionally the dry section of the roof will behave differently from the wet section causing stresses where they meet that, in turn, can cause cracks to appear allowing water penetration that, if unchecked, will cause damage to roof timbers, ceilings and decorations. It is difficult to form most flat roof materials into bends because they aren't very flexible. Where they have to be dressed up around elements that pass through a roof such as pipes or have to be dressed up into walls or around skylights, problems can occur. A 90 degrees bend will more often than not create a crack. Inserting an angled fillet generates two, less severe, bends that help to overcome their inflexibility.
Drainage
A flat roof should never be flat! The design should always incorporate a fall so that water drains off. The better the draining of the roof the less likelihood that ponding will occur even when there are slight deflections.
The fall should always be towards the point where the roof is drained either by an eaves or parapet gutter or a drainage outlet. Blocked drainage can cause water to remain covering the roof or, in the case of a blocked eaves gutter, will cause water to cascade down the face of a wall beneath. A regular roof inspection is required to ensure that this and other problems can be dealt with before they become threatening to the roof covering or other elements of construction.
Where built up mineral felt is used, one of the commonest roof coverings, it has to be protected from harmful sun rays that, through ultra violet light, will quickly wear away at the covering and make it brittle and, thus, more prone to crack. Either chippings or solar reflective paint provide protection. Chippings are laid on top or are incorporated as part of the manufacturing process, painting can be used but the surface has to be totally free from debris and grease to be effective. Paint has a limited life and has to be replaced every few years. Chippings can be washed off parts of a roof if the roof fall is too steep or they can be blown off sections where exposed; yet another reason for regular inspection.
Other problems
There can be a build up of rubbish, soil and grit at the base of the pond that might just be enough to support weed or algae growth that can affect and weaken the roof covering.
Wind can also be a problem when the roof covering hasn't been fixed securely. It doesn't necessarily get under the covering but, when it passes over the roof, has the effect of sucking it up which exacerbates the poor fixing and can quickly lead to tearing of the covering.
Regular inspections
Many of the common problems with flat roofs can be prevented by regular inspection. Evidence of a problematic flat roof can be dampness defacing decorations in rooms below or ponding, blistering or cracking on the roof surface. It is only by regular inspection that some of these can be detected and dealt with before a problem really develops. Be careful that you don't create a problem (or make an existing one worse) when walking over a roof during an inspection, it's better to use crawling boards.
It is prudent to inspect all flat roofs at least on an annual basis - more often if there are factors that could cause faster deterioration - e.g. leaf build-up etc.
Further Information
Books and other references:
Understanding Housing Defects by Duncan Marshall, Derek Worthing & Roger Heath, 2nd edition.
Published by Estates Gazette Books, 2003. ISBN 0 7282 0417 7.
Tel: 01444 445335
The Repair and Maintenance of Houses by Ian A Melville & Ian a Gordon.
Published by Estates Gazette Books.
ISBN 0 900361 43 3
Tel: 01444 445335
Estates Gazette Books website:
www.propertybooks.co.uk
Drawings reproduced with the permission of EG Books