Flat roofing

Although a traditional material and with a proven life expectancy of up to 60 years in some cases, asphalt roof finishes, particularly in new-build commercial buildings, are giving way to liquid membranes. It is now rare to find asphalt being specified as a primary membrane in new build.

When installed upon an uninsulated rigid concrete deck, asphalt performs very well (assuming that the detailing and workmanship are correct), but its long-term performance with insulated decks (warm roof construction) is less satisfactory. A properly installed mastic asphalt roof should have a life expectancy of at least 25 years, but if inadequate attention is paid to solar control, heat dissipation and maintenance it can be significantly less than this. The durability of asphalt is reflected not only in terms of high temperatures but also low temperatures, where the same insulation layer can prevent or at least significantly reduce the amount of heat taken up from the structure.

BRE Information Paper IP8/91, Mastic asphalt for flat roofs: testing for quality assurance, considers the durability of different grades of asphalt. Following testing of various samples, BRE concluded that major failures in asphalt were more likely to be the result of contraction and embrittlement at low temperatures. Thermal shock (rapid change from high to low temperatures) was found not to be a significant factor in the performance of the samples.

The material

Asphalt is essentially a formless blend of asphaltic cement with graded fine and coarse aggregates. The material is liquid when heated but cools to a dense, cohesive and voidless mass on cooling.

Asphaltic cement is a mixture of bitumen or a mixture of refined lake asphalt (a natural product) with asphaltite (a material allied to bitumen but with a higher softening point and greater strength and stability). Bitumen is either taken from natural sources or distilled from petroleum; it is a thick viscous liquid, mainly non-volatile. Without aggregates, asphalt does not perform very well, so it is usually combined with graded coarse and fine aggregates made from crushed limestone, siliceous, igneous or calcareous rock.

During the 1950s and 1960s asphalt roofing was common on flat uninsulated decks. The gradual increase in thermal insulation during the 1970s meant that, although until then it had performed well as a strong and durable material, asphalt's life was becoming much shorter. At that time warm roof construction was common; it was not unusual to specify asphalt on top of cork, polystyrene or rigid foam board with a layer of chippings or solar reflective paint as protection. While chippings of limestone, granite, feldspar, etc. offered a good level of protection initially, these materials could be subject to wind scour leaving larger areas of roof unprotected. Solar reflective paint systems were thought to be a better alternative, but they lose effectiveness fairly quickly and offer little benefit to flat horizontal surfaces. (Solar reflective paint is however the usual choice for upstands.)

Asphalt is relatively inflexible at normal UK daytime temperatures but inevitably the asphalt roof surface is subject to solar radiation. In older construction some of this heat is dissipated away into the structure, but if the asphalt is placed on top of insulation, the heat cannot dissipate to the same degree. As a result the asphalt softens and is prone to elastic deformation. Slumping of upstands or the gradual sinking of plates or bearers into the surface of the asphalt are typical indicators of plastic deformation. Repeated cycles of cooling and heating lead to cracking, particularly if the asphalt is restrained in some way.

Solar reflective paint

Thermal cycling can cause problems with the solar reflective paint as well. The paint can apply significant stress to the surface of the asphalt leading to a pattern of regular cracking.

Various different methods of protection of roof surfaces have been attempted over the years, for example:

• silver reflective paint;
• white reflective paint;
• asbestos cement promenade tiles (and their non-asbestos counterparts);
• capping sheets;
• chippings; and
• inverted roof insulation systems.

The solar reflective paint applied to this upstand has shrunk upon ageing, resulting in severe surface cracking of the asphalt. In turn the cracked areas absorb more radiation, exacerbating the problem.

Until the late 1970s the predominant finish was chippings of one variety or another, often with asbestos-based promenade tiles ('Spartan' tiles) in walkways or balcony areas. Towards the end of the 1970s greater reliance was placed on solar reflective coatings. These gradually became more popular because they could be added to vertical or sloping areas. However, solar reflective paints were not without problems and if the incorrect materials were used their performance could be disastrous.

All roofs will absorb a measure of solar radiation, most of which is converted into heat energy. Since solar radiation contains a variety of different waveforms in the spectrum, some UV light can also be absorbed. UV light causes organic materials to degrade. Asphalt hardens following exposure to UV light, reducing its tensile strength and making it susceptible to movements in the deck or flashings, for example.

A highly reflective surface prevents or certainly reduces the amount of radiation absorbed, while a black or dark surface will absorb more. In terms of roof coatings, few (if any) materials prevent absorption of radiation, although they can mitigate the effects. Conversely, a coating can serve to reduce (or increase) the amount of heat radiated from a roof at long wavelengths, thus affecting surface temperatures at night. During a cold, clear night, heat is radiated to the night sky (see cold night sky condensation) resulting in a reduction in surface temperature to several degrees below ambient air temperature. This can render the asphalt brittle, while the change back to a warm temperature during the day can permit the formation of longitudinal cracking unless the effects are mitigated by a solar coating.

Asphalt hardens following exposure to UV light, rendering it less flexible. Expansion and contraction of the lead flashing to this abutment detail has resulted in tensile failure of the asphalt. Ideally, joints in the flashing should be welted rather than lapped, and the lead securely fixed to a suitable timber edge batten bolted to the structure. (Defects in buildings, HMSO, London, 1989)

Asphalt is thermoplastic, it softens on heating and exhibits creep characteristics that are not reversed upon cooling. Below a critical temperature (or glass transition temperature, Tg) the material changes from being relatively plastic to hard and brittle - and at greater risk of damage from impact loads. Solar reflective paint can extend the time during which the asphalt is below its transition temperature.

The glass transition temperature (Tg) The glass transition temperature provides a useful means of comparing the cold weather characteristics of a variety of polymers. The chains of molecules that form a polymer can be likened to a plate of spaghetti. When warm, the individual strands slide over one another smoothly; this is described as an amorphous state. As the spaghetti cools it gets stiffer, the strands are less likely to move easily and eventually reach a stage when no movement occurs at all. The temperature at which this occurs is called the glass transition temperature - at this point the spaghetti reaches its glassy state. The transition from amorphous to glassy state is critical in evaluating the performance of roofing materials.

The glass transition temperature can be lowered by introducing small molecules called plasticisers. Plasticisers increase the distance between the larger polymer molecules.

The beneficial effects of solar reflective paint can tail off quite quickly as a result of weathering, soiling or ponding on the roof. Initially, a high degree of visible light is reflected, but this gradually diminishes. Cleaning the surface in spring may restore some of the benefits, but regular re-treatment, perhaps every 5 years or so, is needed for adequate long-term protection.

Some paints are not tolerant of surface contamination or moisture; application to an unclean surface or to one that is damp following overnight dew will probably result in failure and large areas of paint flaking off within a short time. In some cases performance improves if the asphalt has been sanded prior to coating: this serves to give the paint a better mechanical key.

Research by BRE in the late 1970s sought to identify if there were any particular benefits in aluminium paints over white paints. Aluminium, which tended to be based upon a bitumen binder rather than a water emulsion, was a common finish but is less so now. BRE found that aluminium was successful in blocking infrared and UV radiation to about 65% when new, but that reflectance dropped off after a short period of time. The paint served to reduce emissivity but not by enough to compensate for absorption. BRE were unable to identify clear advantages of aluminium over white paints although they noted that in some cases, presumably as a result of low permeability, blistering was more prevalent with aluminium paint. (Solar reflective paints, Information Paper 26/81, BRE,Watford, 1981)

For a solar reflective paint to be effective, it must be compatible with the roof material and should be designed specifically for the purpose. Paints that are not specially formulated can cause more harm than good, especially as they shrink with age. The shrinkage can cause the surface of the asphalt to crack and in turn the cracked area absorbs more radiation than the surrounding area. The localised increase in temperature results in localised softening and expansion of the asphalt - an effect that is not reversed upon cooling. Gradually, the problem increases in severity, the cracking is extended and the asphalt deteriorates.

Blistering

Blistering of asphalt is a common problem, often due to:

• poorly detailed upstands;
• a failure to provide effective vapour control; or
• water ingress into a roof deck.

The trapped moisture vaporises and forms a blister in the asphalt. Eventually, repeated cycles of vaporisation result in the perforation of a blister and further water ingress. See also text on blistering in Built-up felt roofing.

Upstands typically suffer from blistering as a result of poor detailing. Good construction practice dictates the use of raked joints in brickwork to provide a key to the asphalt and or the use of high bond primer to prevent the asphalt slumping. Ideally, the tuck at the top of a skirting should be protected by a lead or metal flashing. Frequently this is neglected, and a mortar joint substituted instead. Such a detail leaves the tuck vulnerable to water penetration. If a lead flashing is used, it is essential that the cavity tray is dressed over the top of the lead and not beneath it, otherwise water leaching from the cavity tray could be directed into the asphalt tuck and possibly behind the skirting. Similarly, the cavity tray must always project far enough to lap over the lead and not stop short of it. This problem is illustrated below.

Large vapour-filled blisters in an asphalt roof covering

The use of asphalt in warm roof situations has become far less prevalent and is best avoided. As an alternative, inverted roof construction has proved far more successful in maintaining the long-term performance of the roof membrane. The membrane is protected against mechanical damage as well as thermal cycling: being below the insulation, the asphalt is subjected to a much lower range of temperature variation.

Gap between insulation and wall causes cold bridge and allows asphalt to slump.

Cavitation

Asbestos cement promenade tiles were a common finish to asphalt, offering good levels of solar as well as mechanical protection. However, smooth finish tiles tend to promote the growth of algae, in some conditions, making them very slippery and sometimes dangerous.

With the gradual restrictions on asbestos, cement fibre or GRC products became widely available. An early problem with these materials was their low permeability, meaning that water that became trapped beneath the tiles could not necessarily escape.

During the day, the surface temperature of the tiles could cause the trapped moisture to vaporise, a problem that was accompanied by the softening of the asphalt or bitumen bedding compound. Gradually, tiles could lift out of place, ultimately ratcheting upwards to present a trip hazard. However, while ratcheting was a problem, the formation and subsequent collapse of vapour bubbles gives rise to a problem of the gradual deterioration of the asphalt - in effect a process of cavitation. Cavitation is the formation of local cavities in a liquid, as a result of the reduction of pressure below a critical value. It frequently causes the pitting or wearing away of a solid surface.

Cavitation damage below impervious promenade tiles

To deal with the problem, promenade tiles were subsequently manufactured to be porous or vapour permeable.

Cracking

Asphalt is susceptible to deck movements, particularly following ageing and exposure to UV light. At low temperatures, the material is very brittle and can fracture under impact loads, or misguided attempts at opening up using a bolster. Similarly, embrittlement can result from overheating during laying.

Cracking is often found around junctions with alternative construction, perimeter details and projections as a result of differential movements in the background materials. If asphalt is laid on a lightweight or flexible deck (such as woodwool), movement between the deck and the enclosing walls must be accommodated by isolated upstands, otherwise failure occurs at the transition between horizontal and vertical work. Cracking at upstands can sometimes be caused not only by failure to provide for movement, but by poor workmanship during installation. For example, when the asphalter places the skirting, he/she will form a triangular fillet detail. If the asphalt is insufficiently cleaned before forming the fillet, poor adhesion permits cracking to form along its edge.

Another reason for fillet failure could be a lack of support at the edge of the insulation in warm roof construction. If a gap exists between the insulation and the enclosing wall, the asphalt can slump into the void over time, compromising the asphalt at the perimeter.

Deflection of the roof structure or collapse of a roofing screed due to poor compaction could create similar tensile crack patterns in the base of an upstand.

Examination of the crack patterns in asphalt will give some indications as to their likely cause. For example, cyclic movements in the deck arising from thermal or moisture changes are likely to produce compressive rippling type defects, while tensile cracking can occur as a result of the structural arrangement of the deck or background materials.

Asphalt skirting has been taken across a parapet movement joint without provision for movement: it has cracked as a result.

Ponding

Ideally a flat roof should never be totally flat. CIRIA (Flat roofing: design and good practice, CIRIA/BFRC, London, 1993) recommends a minimum fall of 1:80 or greater, although this is often not attained. Deflection of the roof deck can lead to the reversal of falls away from outlets and to ponding.

There is some debate as to whether or not ponding is a bad thing for asphalt roofs. The general consensus is that as far as asphalt is concerned, ponding is not necessarily harmful, although of course if a leak does occur there is a greater reservoir of water available to penetrate the building.

Rigid foam insulation

Since polystyrene (EPS) is a thermoplastic material it is easily damaged during the placing of asphalt. Therefore, its use as an insulant in conjunction with asphalt is generally limited unless suitably protected (at least one manufacturer supplies a felt faced material for this purpose). Furthermore, it can be dimensionally unstable over changes in temperature and this can lead to the opening and closing of the boards and problems of rucking or splitting in the waterproof layer.

Rigid foam boarding, usually polyisocyanurate (PIR), is a more popular choice because of its higher temperature resistance and good insulation properties. PIR is a thermosetting material, and rather than melting in the way EPS would, it tends to char. The material begins to degrade at temperatures above 150°C and so still requires protection in the form (usually) of bitumen-impregnated fibreboard.

Until the early 1990s almost all commercial rigid foam products used CFC-11 as a blowing agent. CFC-11 was held to be an ozone-depleting substance. Under various protocols it was phased out of use and substituted with, for example, HCFC 141b. The transition was thought to have led to various manufacturing problems and product failures, although it is probably more the case that poor mixing of the component parts was to blame.

PIR foams are generally stable, but roofing failures involving these materials are not unheard of and can lead to expensive remedial work, often involving re-covering. Typical defects are:

• formation of depressions in the roof after laying;
• shrinkage of boards creating board outlines in the asphalt;
• poor compressive strength; and
• cupping of boards giving rise to reflective undulations in the asphalt.

Most of the above problems can be attributed to manufacturing faults rather than issues of poor workmanship, save that improper storage of materials on site can give rise to performance difficulties.

The production of PIR and PUR foams involves the mixing of 2 component parts, part A and part B, to form a chemical reaction. Part A is usually Methylene Diphenyl di-Isocyanate (MDI). The catalyst, part B, is a mix of polyol, various additives and blowing agents. The mixed components then react exothermally (producing heat) to form a rigid thermosetting polymer. The maximum strength of the product is determined by the cell structure of the foam; spherical or slightly egg-shaped cells are needed and these are attained by mixing very precise quantities of A and B chemicals. If the ratio of chemicals is allowed to deviate during manufacture, there is a risk that the cell structure will be more elongated rather than spherical. Elongated cells are much weaker, with the result that the crushing resistance of the foam is less; it becomes brittle and is easily damaged.

Incorrect mixing ratios therefore account for the subsequent deterioration of the insulation, particularly in areas that are regularly trafficked during installation or afterwards. Poor compressive strength leads to crushing damage under foot, which may become manifest over a period of time after laying in the form of a series of depressions in the roof surface.

If severe enough, the deterioration of the boards results in the delamination of the fibreglass outer covering.

Edge cavitation

Edge cavitation of the boards is a defect where the edges gradually become concave, creating gaps and reducing thermal insulation. The problem is usually associated with poor strength as well. Again a manufacturing problem, the root cause appears to be the introduction of small quantities of water into the B chemicals to produce CO2 as a joint blowing agent. However, the water can dissolve some of the HCFC blowing agent resulting in an imbalance in the mixing ratio and subsequent weakness.

During mixing, the foam reaches temperatures of around 149°C. If CO2 is present it will diffuse out of the cells at a rate that is much faster than air at ambient temperature can replace it. This leads to an imbalance of pressure which, coupled with the weakened cells, creates an elongated cell structure. The problem occurs most rapidly at the board's edges, resulting in poor foam structure and board-edge cavitation.

Cupping and bowing of the boards can result either from manufacturing faults or from poor storage of the boards on site. The latter is easily explained if the boards are maintained in damp conditions prior to laying. Since they are usually supplied in bundles, the top or bottom sheet is at most risk if its outer surface is allowed to become saturated. If one face is wetter than the reverse face, differential expansion occurs causing the panel to cup. The problem is likely to be randomly spread over a roof and may be reflected in the asphalt layer.

Case study: Art Deco building, South coast The building comprised a large L-shaped structure with an asphalt covered flat roof originally constructed in the 1930s. The roof had performed well in its time, but with no thermal insulation it gave rise to large heat losses. The designer of the re-roofing scheme concluded that the most appropriate form of repair would be to treat the existing asphalt as a vapour barrier and to provide thermal insulation in the form of polyisocyanurate boards, a layer of fibreboard and a new covering of polymer modified asphalt finished with solar reflective paint. Within a few months of laying, depressions formed in the roof surface leading to ponding. The asphalt appeared to be performing well, but the freeholder expressed concerns over its long-term performance. The depressions were random in nature, but were more prevalent in areas of the roof where the asphalt boiler had been located - areas that would have received more foot traffic during installation. A series of trial holes were cut into the asphalt. These revealed that while insulation was present it comprised 2 different types - a mixture of pink (PIR) and cream (PUR) types. Generally, the depressions occurred where the PUR had been used. In a few cases, moisture was found trapped within the roof construction. Since PIR boards had been specified, the contractor had clearly used the incorrect material and was in breach of contract. However, theoretically there was no reason why the insulation should not have performed properly as the fibreboard layer should have been of sufficient thickness to have protected the insulation (which is thermosetting) from heat damage during the laying of the asphalt. Further core tests revealed a manufacturing problem with the boards leading to a lower than expected compressive strength. The cells of the board were elongated rather than spherical, which gave them a much weaker structure, easily damaged under load. Eventually the roof covering was replaced at the cost of the main contractor, the subcontractor having gone into receivership in the intervening period.

Alternatively, poor mixing, or poor mixing ratios, can lead to an uneven distribution of cells (in terms of size and shape), which create dimensional instability and different properties throughout the thickness of the board. When placed under load, the different properties react in different ways, with the result that cupping or bowing can occur. Such problems lead to the formation of regular ridges or depressions on the outline of the boards.

Case study: office building, west London The building was concrete framed with in-situ concrete roof decks constructed in the early 1980s. At top floor level were some residential flats, each with a large balcony enclosed by concrete parapet walls. The roofs were of asphalt laid upon an expanded clay insulation system, 'Insulscreed' - small balls of expanded clay coated with bitumen. The insulation layer was around 150mm thick, with the roof draining towards a shallow gutter along its front edge. To protect the asphalt, precast concrete pavings had been laid on plastic pedestals; there was no separate insulation or geotextile membrane. At the perimeter of the roof, a shingle margin had been provided. One of the residents placed a series of wooden planters around the roof, planted with bamboo for screening purposes. After 5 years or so a small leak occurred in the roof and the roof was eventually exposed to reveal that the roots from the bamboo had broken through the lining to the planters and had entered the shingle margin. From here they had penetrated the asphalt and grown out again through the asphalt upstand. Shoots of bamboo sprouting through the asphalt upstand Bamboo roots had first penetrated the asphalt then spread through the expanded clay screed and exited out through skirtings. On examination, the roots had spread through the screed to affect nearly the whole roof. The black 'pebbles' visible above are the insulating screed, exposed after removal of the asphalt. At first sight, the remedial work appeared simple: remove the planters, cut out the localised area of damage and make good the asphalt. However, on exposure it became evident that the insulating screed had created an ideal growing medium for the roots, being both warm and damp (from the leaks), and the roots had developed into a tangled mass that spread throughout the screed. Given the propensity of types of bamboo to propagate from roots, the risk was that even if the damaged areas were to be repaired, further plant growth could occur again within a period of months or years. The only safe solution would be to remove the roof and screed in its entirety and replace with new.

Case study: listed building, Kensington The building comprised a terrace of 3 listed grade 2* large former houses in Kensington, converted for use as the headquarters for a large professional body. The building backed on to a typical London garden square. At basement and ground floor levels the building extended rearwards to form a large terrace at first floor level. The owner of the building used the terrace for corporate events and had for many years maintained a quarry tiled covering on its surface. The terrace had fallen into disrepair and so it was decided to replace the asphalt covering with new and to provide a decorative finish of ceramic tiles. The specification included for the provision of foamed glass insulation cut to falls and laid over the original brick and filler joist vaulting. The roof was thus turned into a warm deck. The terrace extended over the rear of 3 houses, with a step at each party wall line. Asphalt had been dressed vertically up the steps to create a seamless membrane. The tiles were laid using a flexible ceramic tile adhesive formulated for exterior use. Vertical tiles were placed at each upstand. Within a few months of completion, tiles began to lift off and the asphalt began to slump at upstands and changes of level. Tiles covering the steps failed rapidly and became dangerous. Eventually the problem became so bad that the entire finish was removed and replaced. The problems were due mainly to the tendency for plastic deformation within the asphalt. The layer of insulation beneath the membrane prevented heat that built up in the surface from dissipating. This resulted in higher working temperatures than would have been the case previously. Thus, the asphalt would soften and under gravity gradually tend to 'flow' over steps in the roof. Movement in the asphalt was greater than that which could be accommodated by the tile adhesive. Rigid grouted joints created compression stresses in the tile layer, which could only be relived by shear failure on the tile bed and 'popping' up of the tiles. The difficulty for the designer of the remedial works was that in the intervening period the adhesive manufacturer withdrew its recommendation that the adhesives would be suitable for use on asphalt. In addition, if ceramic tiles were refixed, the problem would reoccur. The tiles were replaced with purpose made cement fibre tiles manufactured to resemble the original quarry tiles. Joints were not grouted but left open and compatible bedding adhesive was used. The solution was very much a compromise and while not matching the original in terms of quality did return the terrace to its original use.

Mastic asphalt performs well as a roof covering if applied properly and if the design of the roofing system is such that the effects of temperature are reduced as far as possible.

However, annual inspections are a sensible precaution. This is probably best carried out in the autumn so that leaves and other detritus can be cleared at the same time. In areas that are heavily affected by trees or other sources of debris more frequent inspections may be necessary. The early signs of failure can then be identified and corrective action taken.

The areas most likely at risk are abutment details, penetrations, movement joints, gutters and outlets. Blistering and slumping of asphalt, particularly to upstands, usually requires fairly urgent work to arrest deterioration and more water ingress into the substrate.

Retraction of asphalt around rainwater outlet details or edge flashings can also lead to problems of water ingress, and thermal expansion of flashings often leads to cracking at joint positions.