aterproof membranes

must be suitable both for the

is Digest helps the designer to up bituminous felts, single-ply bituminous sheets; mastic g those incorporating glass-

tic conditions. There is a wide

# * - F

I

roofs.

~ ~ a ~ ~ t i o n a l l y , the,,choice of waterproof membranes Far flat.i&fs was between mastic asphalt and various grades 'of bitumen felts, bonded'with hot bitumen'into a three-layer built-up membrane. Mastic asphalt was generally used over heavy concrete decks,

itumen felt prpducts were widely used on roofs of lighter construction. The cheapest felts based on xganic fibres were unsatisfactory for roof coverings because they degraded when exposed to water. More durable products, based on asbestos or glass fibre, were more satisfactory and had better dimensional stability. BS 747 covered these three types of bitumen Felt but is being revised to cover only glass fibre and polyester materials.

In the 1960s, the requirement for bet& thermal insulation of buildings led to the replacement of insulation materials (such as low density fibre boards) by rigid foam-plastic boards. Foremost was extruded polystyrene board which has good thermal insulation and water resistance properties but a high thermal expansion coefficient.

At the time, it was not appreciated that the repeated thermal movements occurring at the joins in the thermal insulation layer of a flat roof, as a result of temperature variation on the roof above the

insulation, were sufficiently large to cause fatigue failures 'and cracking of the built-up felt layer. The result was often premature failure of insulated flat roofs of the warm-deck (sandwich) design, in which the built-up membrane is fully bonded with hot bitumen to the insulant immediately below it.

The importance of good fatigue resistance and of flexibility, particularly at low temperature, was recognised; new, more durable products were introduced to the market. They included improved formulations of bitumen-based felts, as well as a range of highly durable synthetic polymeric materials. These are discussed later in this Digest, together with liquid- applied coatings which are principally used for remedial work on existing roofs with failed or inadequate waterproof coverings.

Liquid resin coatings have appeared recently on the UK market and have been advertised widely as an effective solution to the 'problem' of sealing flat roofs against water ingress; some incorporate a mat or fleece of glass fibre or polyester fibres when laid on the roof.

A choice between the numerous options is iarely- possible on the basis of simple comparisons, since many factors must be considered. -,

c'

Buikling Research Establishment Garston, Watford, W02 7JR - . _ _ ~~ - - - _ _ -

372

1 STANDARDS FOR ROOFING MEMBRANES 1 (* ,Jew flexible sheetings are covered by British Stan- /---

dards. BS 747 covers felts based on organic, glass and polyester fibres, covered with oxidised bitumen. For all other types of bitumen-based felts and polymeric sheetings, Agrkment Certificates can be used for assuring quality and for advice on suitability.

In order to implement the single market, product standards (ENS) will be progressively published and transposed into national standards together with stan- dards to enable compliance with the design standards to be verified, These European (CEN) standards will be transposed into national standards and will replace the British Standards over a period of some years.

In so far as the European product standards concern health and safety, they will be harmonised standards for the purposes of the Construction Products Directive (CPD). Provided compliance with them has

been verified by a procedure to be chosen by the Commission, they will enable products to bear the CE mark confirming that they are fit for their intended use as defined in the CPD.

European technical approvals, similar to Agrkment Certificates, will be available for products which are innovative or not covered by a European standard. These approvals will also enable products to bear the CE mark confirming that they are fit for their intended use as defined in the CPD.

In due course, European design standards will also be published and transposed into national standards to enable compliance with the design standards to be verified. In the meantime, British Standards concerning design and installation will remain, subject to their being reviewed to take account of the European product standards.

The three practicable and technically acceptable arrangements of the functional parts of a flat roof are described in Digest 312. In the cold-deck and warm- deck sandwich constructions, the membrane is exposed to solar radiation, unless it is protected by chippings or other means. There is usually a layer of insulation immediately below it. The membrane can be exposed to a range of temperatures as low as -20" C in extreme winter conditions and as high as 80" C with direct exposure to sun in summer.

the inverted roof, the membrane is located below the insulation layer where it is protected from solar radiation and is subject to smaller variation in temperature. However, rain falling on the roof, or melting snow, can percolate down through gaps between the sheets of insulant and flow over the surface of the membrane. The membrane is therefore likely to be kept damp for long periods and must be fully-resistant to moisture. There is also a risk of penetration by fine particles carried down from the layer of ballast above the insulation. Slight thermal movement of the insulant can cause damage to a thin membrane. To overcome this risk, with single-ply membranes, it is usual to use a sheet at least 1.2 mm thick and to place a fine fleece between the ballast layer and the insulant to trap any fine particles.

There are three ways of attaching the membrane:

loose laid, ballasted;

o mechanically fixed.

With bituminous built-up membranes, the first layer of the felt is usually bonded to the substrate using hot

' adhered (fully or partially-bonded);

bitumen but, if it is laid directly on a timber deck, it must be screwed or nailed. The choice of insulant may be affected if the felt is to be hot-bonded to it - see Digest 324.

Partial bonding of bitumen-based membranes is achieved by a first layer of perforated felt (BS 747: Type 3G); hot bitumen then bonds the second layer to the substrate through the perforations. It is now recommended that partial bonding is used where bituminous membranes are used over foam plastics insulation.

Flat roofs can be ballasted by a layer of mineral aggregate or by laying paving slabs on the membrane. This protects against the effects of solar radiation but the membrane must be fully resistant to long-term exposure to moisture.

Several membrane systems use mechanical fixings to secure the waterproof sheet to the roof. Most use polymeric single-ply membranes, usually PVC, but elastomeric membranes can also be fixed in this way. A recent introduction to the UK market is the mechanically-fixed bituminous single-ply membrane, usually consisting of heavy-duty SBS (styrene- butadiene-styrene) modified sheet with specially - - designed mechanical fixings.

Access to the roof must be taken into account in the choice of a membrane. Generally, only occasional pedestrian access is required, when flat roofs are inspected as part of regular maintenance. But more onerous traffic by pedestrians, or Yven by vehicles, may be required and provision must be made for this.

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372

Osidised-bitumen felts BS 747 felts are based on fibrous mats of organic fibre, glass-fibre or polyester fibre, which are impregnated with hot bitumen before being coated on both sides by oxidised (or blown) bitumen. Organic fibre (or rag- based) felts are rarely used f m flat roofing membranes in the UK because of inadequate durability in the wet climate.

I BITUMEN FELTS

There is a wide range of performance and quality in different products. Durability is loosely related to price, but it must be recognised that it is the performance and service life of the built-up felt system which is important. Clearly, many combinations of felt sheets are available in a built-up membrane of up to three layers. In practice, however, only a few are recommended, based on satisfactory performance. These are mainly either glass fibre or polyester-based felts to BS 747; since membranes based on glass-fibre felts alone do not give adequate performance, glass fibre and polyester-based oxidised bitumen sheetings are sometimes used in combination, Their use is covered by CP 144: Part 3.

__ ._.-.I___._-

Felts incorporating polymer-modified bitumens, which are not covered by BS 747, are now being specified. Expectation of their superior durability is based on laboratory tests (eg as part of Agrkment certification) and more than 20 years experience in France, Italy and Scandinavia.

Most bituminous felts are laid on flat roofs as built-up membranes, comprising two or three sheets bonded with hot bitumen. Bonding of one or more of the layers by torching or hot-air gun is also possible.

The first layer of felt may be fully bonded to the substrate: usually the deck or the insulation layer. Partial bonding is recommended for foam plastics insulants with relatively high thermal expansion/ contraction characteristics. This effectively reduces the stresses imposed on the membrane. This is done by first laying a perforated layer of felt, so that the second layer is bonded to the substrate only through these perforations: usually about 10 per cent of the total roof area. Alternatively, the first layer can be partially bonded to the substrate by applying bonding bitumen only over discrete areas of the roof membrane. Subsequent layers of the built-up roofing are fully bonded with hot bitumen.

The top layer may be a cap sheet with a protective layer of mineral granules embedded and bonded to the upper surface during the manufacturing process. If the top sheet lacks this layer, protection against the effects of solar radiation (UV and heat) can be provided by a layer of mineral aggregate bonded to the surface with a bitumen dressing compound. Another method is to use a cap sheet incorporating a thin layer of metal foil bonded to the upper surface. Upstands are normally protected by using mineral surfaced felts. Solar protective paints and similar finishes have low durability, with only one to five years' useful life. If ponding occurs, rapid deterioration often follows at the edges of the ponded areas as a result of exposure to UV radiation

e 3B felt has fine granules on both sides c3 prevent the felt sticking during manufacture and in use. It is suitable for the lower layers of built-up membranes and for the top layer if it is to be covered with a protective layer of bitumen dressing compound and mineral aggregate surface finish.

e 3E felt is covered with a protective layer of mineral granules on the upper surface. This also gives decorative value for use as the top layer (cap sheet). No additional surface treatment is needed.

e 3G felt is used as a venting base layer; it has a coarse granular finish on the lower surface. It is perforated with 25 mm diameter holes, about 80 mm apart, with a margin at the edges to allow the joints to overlap.

BS 747 was amended in 1986 to include Class 5 felts with a polyester base. There are three types:

e 5B felt is impregnated and coated with oxidised bitumen; the polyester base has a nominal weight of 350 g/m2. It is used as the top layer of a membrane with a surface protection of bitumen dressing compound with mineral aggregate. 5E has a layer of firmly bonded mineral granules. It can be used as the cap sheet of a polyester built-up felt membrane without further protection to the exposed surface. Weight is a nominal 350 dm2 for the polyester base.

two-layer specification or an intermediate layer in a partially bonded system with 3G. It is also used as the first layer where nailing is required. It is covered with surfacing material on both sides to prevent sticking during manufacture and in the rolls. 5U felt has a polyester base of nominal weight 125 dm2.

a 5U felt is an underlayer, used as a first layer in a

When fully bonded, built-up polyester felt membranes generally consist of only two layers; the cap sheet, 5B or 5E felt, is tough and gives durability to the weatherproof covering. When partial bonding or nailing is used, an additional first layer is required.

- By virtue of their improved strength and toughness, built-up polyester membranes have performed satisfactorily on flat roofs in the UK for up to 20 years. In contrast to the older types of felt to BS 747, there i s no evidence of failures caused by fatigue cracking of the membranes when fully-bonded to foam-plastics insu1;ltion boards - ie in a warm-deck (sandwich) design - that have resulted in water penetration.

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372

Polymer-rnodificd bituminous felts The performance and durability of bituminous sheets

ise but also on the properties of the bitumen used to , ~~ depend . not only on the mechanical strength of the fibre

impregnate and coat the base. Oxidised bitumen is essentially a durable material but it is gradually embrittled by prolonged exposure to solar radiation; it becomes stiffer and less resistant to mechanical fatigue at low temperatures. Adding small proportions of various polymers to the bitumen enhances flexibility.

The two main types of polymer-modified bitumen felts use a plastic, atactic polypropylene (APP) or an elast- omeric (rubbery) polymer, styrene-butadiene-styrene (SBS), to modify the performance of the bitumen.

APP-modified bitumen felts have good creep resistance at high temperatures. They are applied by torching or hot-air welding. Most have a polyester base; flexural fatigue resistance is significantly superior to the BS747 felts. They have been used widely for qome years in Europe (and more recently in the

mthern states of the USA) where high surface temperatures are common.

SBS-modified bitumen felts The addition of SBS polymers gives a rubbery character; the polyestez: base gives excellent low temperature flexibility. Fatigue resistance is generally superior to that of the APP- modified felts. A few products are suitable for applic- ation by hot-air welding on to the roof but they are less suited to this method than are APP-modified felts. They are usually applied by bonding with hot bitumen in a two-layer built-up system. As with Class 5 felts, the under-layer is usually of light weight with a more robust and heavy cap sheet; this gives a durable system.

There are two other types. One has a glass fibre base (some makes are strengthened by adding polyester). Fatigue endurance and low temperature flexibility is between BS 747 felts and polyester-based modified

itumen products. The other type is produced by calendering a mixture of polymers with bitumen or pitch, without using a base. This forms durable, elastic and flexible sheets; they are sometimes called ‘pitch/polymer’ or ‘bitumen/polymer’ roofing. Most of these materials are prone to excessive shrinkage and they have largely been replaced by SBS-modified felts.

Single-ply bituminous membranes One major disadvantage of built-up bituminous membranes is the time needed to apply the separate layers to the flat roof. Bituminous membranes which are applied to the roof as a single, robust sheet have been available on the European market; some are now available in UK. They can be fully bonded to the deck or insulation layer but most are fixed mechanically using specially designed devices, screwed down into a metal or timber deck, along the edges of the roofing -heets. A waterproof joint is then formed, by hot-air ,delding an overlapping section of the adjacent sheet over the mechanically fixed section. Typically, the rolls are 1 m wide and overlap by about 100 mm. Single-ply bituminous roofings are generally at least 4 mm thick and are manufactured from SBS-modified bitumen felt 4

with a polyester base (one product uses a blend of APP and SBS to modify the bitumen).

There is little evidence of performance, or laboratory test data, of these systems to enable judgement to be made on their long-term durability. Wind uplift tests have been carried out successfully on large assemblies incorporating typical mechanical fixings. The materials themselves are similar to the high-performance polymer modifi‘ecl built-up systems and may therefore be expected to have equally good durability, subject to acceptable performance of the mechanical fixings.

POLYMERIC (SINGLE-PLY)’ ’ ’

. MEMBRANES . I .

Since the early 1960s, membranes have been developed using a wide range of synthetic polymeric materials. They still represent only about 10% of the total market for flexible membranes in the UK but have a much larger share in other markets (over 50% in the USA). They are particularly suitable for large commercial and industrial buildings, and where rapid construction and light weight are needed. It is useful to classify them according to the type of polymer, the method of joining the sheets, and the method of fixing, rather than by the specific generic polymers from which they are made. The properties of polymeric materials depend on a number of factors: the length of the polymer chains, the degree of cross-linking of these chains, and the presence of minor components such as fillers and plasticizers. The properties of the manufactured polymeric sheeting may be further modified by including a fibrous reinforcing layer or by applying a polyester fleece backing to the lower surface when the membrane is laid on surfaces which are uneven or incompatible with the polymer sheeting.

Polymer character Synthetic single-ply sheets are elastomeric or thermoplastic in character. Elastomeric materials show some elasticity owing to the high degree of cross-linking between adjacent polymer chains. Some slippage between chains can occur with lower cross-linking and the material does not completely recover its shape when the distorting force is removed. Elastomers tend to retain their elastic properties, even at sub-zero temperatures, and are generally insensitive to temperature over normal ranges of service temperatures. Weather resistance is excellent; resistance to chemical attack is good. Thermoplastic materials have a polymer structure of entangled chains with little cross-linking. The chains are relatively free to move and soften as the temperature rises. At low temperature, the materials become less flexible. Exposure to high temperatures -

and ultra-violet radiation degrades the polymer structure; this effect is reduced by the use of appropriate additives to roof sheeting products. Once degraded. the temperature at which the brittle condition is attained may be significantly raised, and there is an increased risk of the membrane cracking if it subsequently becomes exposed to sub-zero temperatures or to tensile stresses.

'I'he principal polymer types available i n thc UK are:

Elnstonzeric o Ethylene propylene diene monomer (EPDM); Q Butyl rubber (Polyisobutylene/Isoprene) (IIR); e Polyisobutylene (PIB) (this is a non-vulcanised

rubber, with some plastic characteristics).

Thermoplastic e Plasticised PVC; e Chlorinated polyethylene (CPE) e Chlorosulphonated polyethyleqe (CSM). These

products tend to form cross-links on exposure to heat and ultra-violet radiation. As a result they acquire some elasticity and become tougher.

e Vinyl ethylene terpolymer (VET).

Forming joints All the synthetic polymeric sheetings arc. extremely resistant, as materials, to weather, movement and thermal stresses. The integrity of the waterproof layer depends on the joints in the membrane; they are usually made on site. Some sheeting can be factory- formed into very large complete membranes to obviate the problems of making satisfactory joints on site.

Highly cross-linked or vulcanised elastomers (and thermoplastic rubbers, such as CSM, which become cross-linked as a result of exposure to weather) can only be welded by heat under site conditions when new.

Thermoplastics can be joined by heat or solvent welding. Table 1 summarises site techniques. A few products may be classed as thermoplastic elastomers; they have elastic properties comparable with vulcanised EPDM and allow on-site seam welding by hot-air techniques.

Method of attachment The method of attaching the waterproofing layer to the roof is an important factor when selecting a synthetic single-ply membrane. There are three basic methods:

adhesive bonding to the deck or insulant; loose laid, with ballast to prevent wind uplift;

@ mechanical fixing, with special fasteners fixed to the roof deck.

The first two are generally used for cold-deck and inverted flat roofs; mechanical fixing is used in warm-

Table 1 Methods of jointing single-ply membranes on site

372 deck (sandwich) roofs, with the fixing securing bot13 the membrane and the insulation layer to the deck.

Major suppliers of mechanically-fixed single-ply systems have specially designed fasteners for their membranes; they recommend spacings of fasteners depending on service conditions, particularly wind loading. The joint must overlap at the junctions between adjacent membrane sheets; penetrations of the membrane by fasteners are covered by the overlap to provide a completely watertight joint.

Non-reinforced polymeric sheetings can be used where the membrane is adhered or loose-laid over the deck or insulation; some are used in mechanically fixed systems.

Uplift forces on the roof created by the wind subject mechanically-fixed membranes to fluctuating stresses. The membrane system must be designed as a whole to resist these stresses; this imposes special demands on the fixings, the deck and the membrane, particularly at the points of attachment.

The ability of the system to resist wind-induced stresses and to remain watertight can be assessed properly only by testing full-scale models. The area tested must be big enough to include sufficient fixings to allow an accurate assessment of edge-effects. Wind forces do not act uniformly over a flat roof; areas of high stress can be induced by strong winds, particularly around the edges of the roof. It is here that failure of mechanical fixings or the membrane is likely to begin.

BRERWULF (BRE Realtime Wind Uniform Load Follower) was developed to carry out such assessments. It enables a recording of real fluctuating wind pressures obtained from measurements in full- scale or in the wind tunnel to be applied to the roof assembly at increasing levels of uplift force. By reproducing the variety of level and frequency of wind pressure fluctuations in the proportions and sequence that occurs in nature, BRERWULF overcomes important shortcomings in the methods of repeated cycle testing which, among other failings, do not distribute the wind loads correctly through multi- layered systems. Tests on BRERWULF and similar facilities have been carried out only on assemblies incorporating new membranes. Yet to be overcome are the problems associated with accelerated ageing of large areas of roof sheeting in a test assembly.

Ballasted systems, which include inverted roofs, do not need fixings and protect the membrane against damage by traffic and exposure to the weather. Ballast can be a layer of aggregate, paving slabs, or a combination of both if pedestrian paths are to be provided. - - Interlocking insulation boards are available for lightweight, non-ballasted systems; they have a layer of glass-reinforced plastics, or similar material, bonded to the top surface. t

4

5

I 1

up of products. They are thetic formulations and are rk, to cure leaks, or to g waterproof membrane

s mat is laid over the roof bituminous dressing

ings, they may be used over embrane provided the two

s of a glass fibre and/or on the roof and bonded to of resin (usually is seamless and can be laid ers, it is claimed it

ovides a solution to the problems of constructing a ,liable flat roof. After the resin topcoat is applied, the

,suiting membrane cures, forming a flexible covering, permeable to water, which should resist thermal and

inor structural movements. There is little published information regarding long-term performance and, in view of some past failures of specific proprietary systems, it is advisable to choose products which have been granted an Agrkment Certificate.

1 'MASTIC ASPHALT I 1 .I____

Mastic asphalt has been used successfully for roofing in the UK for more than 60 years. Applied hot, it is a mixture of asphaltic cement, consisting of bitumen derived from crude oil or of naturally occurring asphalts, and graded mineral aggregate. It is usually

laid about 20 mm thick, in two layers, but the exact specification depends on whether or not people or vehicles will be allowed on the roof.

I

Mastic asphalt is laid as a seamless, waterproof layer and can be dressed around details and at upstands. Being thermoplastic, it sets to form a relatively hard layer which becomes brittle and inflexible at low temperatures. It is important, therefore, to isolate the mastic asphalt layer from thermal or other movements occurring in the underlaying roof deck or substrate. This can be done by laying special low bitumen sheathing felt between the substrate and the asphalt to avoid thermal bonding. Because of its vulnerability to movements, mastic asphalt is suitable only where there is a heavyweight deck to support it. More recently, the incorporation of rigid foam plastics as insulants in flat roofs has imposed higher stresses on mastic asphalt; this is mainly due to the wider range of temperatures to which the material is subjected when thermally insulated from the deck and the building below. As a result, the mastic asphalt may be subjected to prolonged stress in exceptionally cold winters owing to its embrittlement and loss of flexibility. This is the likely explanation for the incidence of increased failures by splitting of mastic asphalt-covered flat roofs after some severe winters in the 1980s. These failures occurred mainly on roofs without solar protection, such as reflective chippings or loose-laid pebbles.

A proposed solution was to modify the bituminous component of the mastic asphalt by adding sma.11 amounts of suitable polymers, in a way similar to the polymer modification of bitumen felts. Mastic asphalts of this type are now available in the UK. BRE evaluated one of these products in 1988 and concluded that polymer-modified mastic asphalt could be expected to have greatly enhanced performance at temperatures down to -20 "C. There is insufficient experience of the use of these materials to confirm this. Tests also revealed that polymer-modified asphalt was less vulnerable to overheating during laying.

372

p , > ,:7-

2 . J Whatever membrane is selected for a particular job, one question is invariably posed: how long will it last?

Two kinds of information can help e the service record of a well-established product or

type of membrane; laboratory tests which simulate various service con- ditions and measure their effects on key properties.

Agrement Certificates use such dsta to predict likely service life subject to correct use.

It is not possible here to present a comprehensive review of all the available data for the wide range of roofing membranes. However, laboratory tests on bitumen felts fully-bonded to rigid foam plastics insulation boards in warm-deck roofs have shown that fatigue resistance has a direct relationship with durability. Subsequent studies were made of flexural fatigue resistance of individual roofing sheets and of built-up assemblies of two or three layers bonded to a rigid substrate.

Fatigue resistance has been measured also on single- ply polymeric sheeting; values given in Table 2 show the considerable improvements which have been achieved in newer materials, since widespread failures in the 1960s of BS 747 felts with organic, asbestos or glass fibre bases. For polymeric sheets, values for Table 2 Fatigue resistance of flexible sheeting

fatigue resistance are given at two temperatures: 23 oc and -20 "C. Of course the latter is rarely reached in the UK but the loss of fatigue resistance at this temper- ature may well reflect a loss of flexibility as the temperature falls below zero.

Data are given for aged and unaged samples. The period of 28 days at 80°C does not necessarily equate to a known period of weather exposure. It is, however, frequently used as a simple means of comparing materials, and indicates the possible loss of performance-related properties with time.

The fatigue resistance has been chosen to characterise the types of roof sheeting as materials in respect of performance (and also, to some degree, of durability). However, it is important to bear in mind the performance and durability of the membrane system; with a built-up bituminous membrane, each of the felt layers will contribute to the performance; with single- ply sheets, the integrity and durability of the joins can have a major influence on long-term performance.

There is no direct relationship between fatigue resistance of the individual sheets (in Table 2) and the service life of the total membrane system. Tests have been made on assemblies of built-up bituminous felt and on single-ply membranes incorporating typical jointing features (welds or bonds with suitable overlaps) but the results are complex. With built-up bituminous membranes, improvernents in fatigue resistance shown by individual sheets of 'improved' felts (eg polyester-based sheets) are generally reflected in the membranes which incorporate them. However, the fatigue resistance of each built-up specification (and hence its probable durability) will depend on the particular combination of bottom, intermediate and top sheets. For single-ply polymeric systems, the reliability with which the seams or overlaps can be fabricated on site is most important. Water leakage through single-ply membranes has been due usually to inadequate joints in the membranes.

When assessing durability of polymeric membranes, therefore, the specifier should first determine whether the product has an Agrement Certificate. This will usually be based on experience of the system over a period of years and on laboratory assessments of the membrane and the fixings (for mechanically-fixed systems); these may include wind-uplift tests.

Second, the specifier must recognise the importance of high standards of workmanship in the installation of polymeric roof membrane systems. A well-trained workforce, with good understanding of the materials, is indispensable if required standards are to be achieved.

7

372 _ _ _ ~ _ _ _ ~. .~.. .II---

i-ll_-_l I SURVEYS OF FLAT ROOFS I-_____ --

* 8 . ,<-,

c:-iRE has carried out surveys of flat roofs since the early 1970's, when an investigation into the condition of 323 flat roofs in the possession of Crown Estates was made. The results are reported in Digest 144. The 200 roofs covered with bituminous felt showed a higher rate of failure, after shorter periods of service, than the 123 mastic asphalt coverings. The general conclusion was that asphalt roofing, when properly designed and laid, was capable of lasting 50 to 60 years whereas the natural ag;ing of bitumen felt would limit its life to 20 years. It must be remembered that this latter prediction referred to the oxidised bitumen felts available at the time the roofs were constructed.

Later, more limited, surveys by BRE suggested that BS 747 felts (mainly glass fibre-based), laid on flat roofs fully-bonded to extruded polystyrene insulation, tended to fail as soon as three years after construction although some lasted up to eight years. Specifications

., If this type have not been used for 15 to 20 years.

A more recent BRE survey, for the British Flat Roofing Council, was conducted of flat roofs incorpor- ating polyester-based oxidised bitumen felts,(BS 747 Class 5) . All the roofs were at least eight years old. None had failed because of poor performance of the membrane: the few cases of water penetration were due to poor detailing. The oldest roofs using Class 5 polyester-based felts are now about 20 years old and there have been no reports of failures of the membranes.

Evidence for the durability of improved polymer- modified bituminous and single-ply polymeric sheetings is more tentative, at least as far as the UK is concerned. because they are comparatively new. Agrkment Certificates will give service life predictions of 20 - 25 years based on experience and laboratory testing. On the basis of experience in countries such as Switzerland, France, Germany and Scandinavia, these estimates are probably conservative.

FURTHER READING

BEECH, JC an>.DINWOODIE, JMW. Preventing water penetratian.k'fiatand low-pitched roofs Budding Technology and Management, 1982,20, .(10),21-.27.

_ _ _ , I . -

., . ,. . , ti':-' ~ i. +':,,; -.* .--. >. . . ' ' / - - . ' . * ^ .'.I I.'>.

..,s,. - I. . . I .. . , < .. % I 1 PROPERTYSERVICES AGENCY. Technical Guide to Flat Roofing: Directorate

I of Building and Quantity Surveying Services. Two volumes. March 1987: / , . i

British Standare Institution BS,747: 1977 (1986) Specification for roofing felts Cl? 144:- Rqof qiiverings '

" Part 3:.1970 Built-up bitumen felt Part 4: 1970 Mastic asphalt

. I

BRE Digests' 144 Asphalt and built-up felt roofing: durability 312 Flat roof design: the technical options 324 Fiat roof design: thermal insulation

. - . - . . . . - . . . . . . ._.. ~

Printed in the UK and published by Building Research Establishment. Price Group 3. Also available by subscription. Current prices from: BRE Bookshop, Building Research Establishment, Garston, Watford, WD2 7JR (Tel: 0923 664444). Full details of all recent issues of BRE publications are given in BRE News sent free to subscribers.

..._ ._ ~ -

Crown copyright 1992

ISBN 0 85125 841 8 R