Figure 1b

Figure 1c 

Some design solutions are more suited than others to a particular category of use: for example, massive structures incorporating a concrete slab provide a more suitable base for roof gardens and terrace decks whereas long span structures are better suited to support the lesser load imposed by lightweight single ply membranes.

The performance requirements for any flat roof must be established by the designer, in consultation with the building owner, having regard to use, durability, initial cost and cost in use.

The service conditions which the flat roof system must meet are determined by the intended use, the climatic conditions for the particular site and the degree of care and maintenance the roof will receive. 

Remember, flat roofs are extremely likely to be used to accommodate future additional plant and equipment or to support extensions to the building envelope, all of which can change both the performance requirements and the service conditions of the roof.

3. STANDARDS

The attached bibliography lists publications which contain relevant basic data and advice which the designer of any flat roof should consider; most significant are:-

· CIBSE Guide : contains environmental data which define the climatic conditions the roof will experience if constructed in the British Isles. For roofs in other locations the Architect must obtain comparable data to allow proper assessment of the effects of solar radiation, temperature variation, precipitation and winds.

· BS 7543 : Guide to durability of buildings and building elements, products and components.

· BS EN 12056 : Gravity drainage systems inside buildings - 
Part 3: Roof drainage, layout and calculation.

· BS 6399 : Loading for buildings.

· BS 6229 : Code of practice for flat roofs with continuously supported coverings.

· BS 5250 : Code of Practice for control of condensation in buildings

In addition the designer must comply with the requirements of Building Regulations, in particular:-

Part A Structure (A1 - Loading)
Part B Fire safety (B4 - External fire spread)
Part C Site preparation and resistance to moisture (C4 - Resistance to weather and ground moisture)
Part F Ventilation (F2 - Condensation in roofs)
Part H Drainage and waste disposal (H3 - Rainwater drainage)
Part L Conservation of fuel and power (L1 in dwellings and L2 in buildings other than dwellings)
Part N Glazing -safety in relation to impact, opening and cleaning (N3 safe opening and closing of windows etc and N4 safe access for cleaning windows etc)
Regulation 7 Materials and workmanship


4. DESIGN CONSIDERATIONS

.1 General

If we are to achieve a successful flat roof we must adopt a holistic approach and begin from first principles. The most significant considerations are service conditions and durability together with whole life costs.

The main agents of ageing which affect a roof system are solar radiation, precipitation, and wind forces which cause ultra-violet degradation of materials and thermally induced movement; snow loading, water build up and impact damage by hail; and physical damage by wind uplift forces. A roof top is not a benign environment; the roof system is by far the most exposed element of the building envelope.

BS 7543 contains advice which helps us distinguish between design life for the building as a whole and life to renewal of various elements and components. Thus, whereas the basic structure of the roof, as a primary element, may be required to last the design life of the building without maintenance, the roof covering and insulation may be designed to be renewed after a shorter life.

A successful roof is one which meets the building owner’s specific requirements. We should not assume all clients want their roof to be “.... as cheap as possible, to last forever with absolutely no maintenance”. Always discuss, define and record the client’s requirements as part of the briefing process. Determine whole life costs as part of the design process by considering initial costs together with the costs of care and maintenance, periodic repair and essential renewal. If you are obliged subsequently to make changes to your solution you should file a disclaimer - or expect to be sued!

Contrary to popular opinion the most important design decisions are not about the choice of material for the waterproof covering; knowing the type of structure - lightweight/long span or massive - and knowing the extent of trafficking over the roof, you may then choose to develop a cold deck, or a warm deck or a protected membrane roof design but most important of all is consideration of how the roof will be drained. A flat roof should never be flat.

.2 Drainage

The primary function of a roof is to protect the building interior from the external elements, it must not allow water to penetrate. All parts of the roof - including gutter beds and parapet copings, should be designed to shed water so that they dry rapidly once rain stops. Un-drained surfaces which retain water are subjected to greatly increased stresses; temperature differences are high at the margin between wet and dry areas; damp surfaces attract dust and dirt in which lichens, moss and wind-blown seeds take root, adding significantly to dead load and creating an acidic environment. In extreme circumstances blocked outlets have caused the collapse of a flat roof. The slightest weakness in a waterproof membrane will be exploited by standing water which will find its way into the system and accelerate its deterioration.

 You should design the drainage pattern for a flat roof as if it were pitched, ensuring the drainage planes intersect at regular 45o mitres to form hips and valleys. The angle of inclination (pitch) should not be less than 1:80 on any part of the finished roof surface - including any internal gutters - 1:80 is the absolute minimum required to overcome surface tension and slight surface irregularities such as those which occur at laps in the membrane.

The most effective and economical way to achieve drainage falls is to design them into the supporting structure. The use of screeds to form falls cannot be recommended as they add dead load and moisture to the roof system. In refurbishment work, where it is often necessary to form falls where none existed before, pre-formed boards of cork, mineral fibre, polyurethane or polystyrene can play a useful role. Such boards must be mitre cut on pre-planned lines which form regular hips and valleys: to achieve a suitable layout it is often necessary to provide additional outlets and connections to drain especially if the plan form is irregular (fig 2).


Figure 2

The more directly rainwater is shed from a flat roof the better. An overhanging roof, discharging normally to external gutters and downpipes, is to be preferred over one which retains water behind parapets - effectively forming the whole roof into a reservoir with water discharging via outlets to downpipes within the building. Obviously this second alternative will fail unsafe, particularly as outlets are penetrations through the waterproof layer placed at the points where water is concentrated!

If internal draining is decided upon, pay particular attention to the siting, detailing and quality of outlets. When calculating the amount and intensity of rainfall on a flat roof ensure you take full account of the location of the roof, any adjacent surfaces which may discharge onto it, the design life of the roof system and the categories of risk involved. BS EN 12056-3 suggests a formula* for calculating the number of outlets and downpipes required to drain a given area of flat roof, based on the assumption that water can flow evenly over the entire rim of each outlet. That condition is not met when outlets are positioned behind a parapet or upstand (fig 3); an outlet placed in an angle between parapets - as all too often occurs - only allows water to flow over 25% of the rim so can drain only 25% of its maximum capacity. It is also common practice to place outlets close to the tops of columns because that is the easy way to support the downpipes. 


Figure 3

Unfortunately, because the deck/slab deflects under load, this results in outlets at the highest points of the roof. It is better to place outlets at the centre of structural bays and connect them to horizontal drainlines; the horizontal connections can accommodate movement without disturbing the outlets (fig 4). Outlets are critically important to long term performance of enclosed flat roofs - do not fail to select and detail them with care, ensuring they are fully supported and secured in place: the added cost of metal compared to plastics is marginal as a proportion of total cost and is a sound investment.


Figure 4

If roof drainage is directed to external gutters then any surcharge, resulting from a flash storm or blocked downpipes, will fall harmlessly outside the building. The same is not true for internally drained roofs and provision should therefore be made in the form of overflows or scuppers to cope with surcharge. An enclosed roof should never be provided with only a single outlet: the consequences of a single outlet becoming blocked can be very expensive!

.3 Thermal insulation and condensation risk

One should aim always to achieve a better elemental U-value than the minimum required by regulations. Software is available to enable you to do you own calculations on a PC; if you do so be sure always to use the aged lambda values of the insulant: most manufacturers of thermal insulation materials (including our sponsors) will prepare thermal and condensation risk calculations for flat roof constructions incorporating their products.

Building Regulations require provision to limit heat loss through a roof: the current elemental U-value of 0.25 W/m2K for a flat roof to any building is not onerous and can be readily achieved by the inclusion of various forms of thermal insulant.

In selecting an insulant it is not sufficient to consider its lambda value; you must also evaluate its compressive strength, its ability to resist water high temperatures.

In a cold deck roof the thermal insulation will normally consist of mineral fibre applied at ceiling line. Insulation of a warm deck roof is achieved with rigid boards formed from compressed cork, foamed glass, foamed polyurethane (PUR), polyisocyanurate (PIR) or polystyrene. Foamed polystryrene is produced in two forms - free foamed and cut to thickness (EPS) and foamed and extruded (XPS). Because of its lightweight and low cost EPS is commonly used to provide falls in refurbishment work. Polystyrene, with its low melt temperature (approx. 80oC), is not suitable for roofs with hot applied waterproofing such as asphalt and built-up bituminous felt; to overcome this it may be overlaid with compressed cork. The high working temperature (200oC+) of asphalt and of hot bitumen can also cause problems with PUR and PIR boards which release gases at high temperature; to overcome this problem these products are normally faced with a gas-tight aluminium foil or laminated to cork. For protected membrane roofs, where the insulant is exposed to both direct compressive load and to water, the only suitable insulation material is expanded extruded polystyrene (XPS).

Whatever the material, insulation boards formed with shiplap or tongued and grooved edges are to be preferred as they help to ensure continuous support for the waterproof layer and reduce the risk of thermal bridging at joints between boards.

Insulation boards are normally laid in brick bond pattern to promote a good fit. On profiled metal decking it may be advantageous to lay boards at 45o to the metal flutes to remove the risk of joints between boars occurring over a trough and so being unsupported.

 Spend time to plan the layout of insulation boards to minimise cutting and waste: give clear instructions and inspect the work to ensure the roofer achieves close fitting insulation.

The laws of building physics dictate that, to avoid interstitial condensation, the various layers of construction forming the external envelope of a (heated) building should offer progressively less resistance to the passage of water vapour as it migrates from inside to outside: failure to follow that basic principle is likely to result in condensation within the construction. Such interstitial condensation causes severe loss of insulation value in water sensitive materials, corrosion of metal deck and fixings and decay in organic materials such as timber; it has been responsible for countless flat roof failures.

For every flat roof a careful analysis of condensation risk should be undertaken at the design stage: if an unacceptable risk is predicted then the design must be modified. The avoidance of damaging condensation relies upon successful vapour control which requires careful design, material selection, specification and installation. There is no “golden solution” which will suit every project: selection of the most appropriate type of roof for a given set of circumstances is the important first step.

A cold deck roof assumes the provision of a vapour control layer between the internal finish - typically plasterboard or planking - and the thermal insulation. Because of the numerous perforations made by fixings, electric cables, pipes etc, such a VCL is likely to have a relatively low value (say 15 MNs/g) and vapour leakage into the roof void must be anticipated. To ensure that vapour can be safely removed, external air is allowed to circulate between insulation and deck in much the same way as happens with the loft space in a cold pitched roof. One major disadvantage with that type of construction is that external air at certain times of year will be very moist and can add to - rather than reduce - the total vapour load within the roof void. Moreover, uncontrolled air movement increases the rate of energy transfer through the insulant and greatly increases air leakage into the interior of the building.

In a conventional warm deck roof, calculations usually predict condensation beneath the waterproof layer. That is because the average VCL (such as 500 gauge polyethylene) has a lower vapour resistance value - approx 500 MNs/g - than the 100,000 MNs/g of the waterproof layer. In real terms the difference can be much greater when, for example, the VCL consists of low grade bituminous felt bonded to the crowns of profiled metal deck. If a high value VCL is required it is essential to apply not less than two layers of high grade felt over the decking or - better still - to fix a layer of plywood to the profiled metal in order to provide a continuous support to which a high value VCL can be then bonded.

The design codes (BS 6229 and BS 5250) recognise the likelihood that condensation will occur within a flat roof: the designer must consider the susceptibility to moisture of materials within the roof, in particular the risk of moisture damage to the insulation and to the building structure.

The protected membrane roof design avoids those problems: no vapour control layer is required because the waterproof layer performs that function and is kept above dew point temperature by the external insulation layer. It has been demonstrated that water draining through joints between insulation boards and across the waterproof layer accelerates heat loss through an inverted roof in wet weather. To mitigate that loss the increased thickness of insulation must be carefully calculated.

.4 The waterproof layer

A flat roof may be waterproofed with a wide range of different materials, most of which are covered by various British/European or American product standards. Materials commonly used include:-

· · lead
· copper
· zinc
· aluminium
· stainless steel
· asphalt
· bituminous felt
· single ply polymeric
· liquid applied polymers
· synthetic rubber (EPDM)

The application of most of those materials is covered by British Standard Codes of Practice CP143, BS 8217 and BS 8218 but for the application of EPDM and other polymeric plastics materials the Architect must rely on information provided by the individual manufacturer.

All of those materials have a good record of long term service provided they are properly selected, specified and installed. With the exception of asphalt and liquid applied polymers they all come in sheet form and must be seamed on site to form a continuous waterproof layer. Whilst one seldom sees a roof failure caused by a defective material, many failures occur as a result of poorly made joints in and between materials.

Different waterproofing materials have dramatically different physical characteristics and there is no one material which is appropriate for every application: the design process must include a detailed evaluation of weight, installation methods and cost, maintenance costs, life to renewal, availability of skilled applicators, appearance and environmental consequences.

Lead is relatively heavy (11.34 kgs/m2/mm thickness) compared to other metals (7.20 kgs/m2/mm thickness): asphalt is also heavy (2.4 kgs/m2/mm thickness) compared to bituminous felt. The weight of waterproofing is more critical on lightweight/long-span constructions than on concrete slabs.

I have already referred to the importance of assessing whole life cost rather than just initial cost: it is also worth bearing in mind the implications of having to renew a roof once the building is occupied. In addition to health and safety implications, access and protection of the Works can be difficult and disproportionately costly: many of our refurbishment projects have required a complete temporary over-roof, with temporary thermal protection to ensure the safety and security of the interior and of the Works themselves.

Do not assume the skills required to install long-established “traditional” materials such as lead, asphalt and bitumen are readily available or that those required to deal with more modern materials such as polymeric single ply membranes are not. All roof waterproofing requires skilled and experienced labour for its successful application and the manufacturers of “high tech” membranes go to great lengths training operatives in the techniques required to apply them. There is much to be said in favour of using materials whose manufacturers invest in such product support.

Except for the very smallest of roofs - and for fluid materials such as asphalt and liquid applied polymers - all waterproof layers will of necessity include joints or seams where the factory made pre-formed sheet is lapped and sealed. To assess the value, effectiveness and suitability of any waterproofing material it is essential, therefore, to consider not only the material itself but also the method of forming joints or seams and the means of attaching the seamed layer to the substrate.

The means of forming joints will be dictated by the materials used. Metal sheets may be interleaved/folded/welded; asphalt and built-up bituminous felt membranes are heat fused; thermoplastics such as PVC may be heat-fused or solvent welded; field seams in thermosets (vulcanised synthetic rubber) can only be made by means of adhesive. When it is new CPSE (generally known by the trade name Hypalon) can be fused by heat or solvent welding but after some 6 months of exposure to UV light becomes thermoset and can then only be jointed by the use of adhesive: the repair of an aged membrane can therefore be problematic.

.5 Attaching and securing the system

Wind uplift forces across a flat roof dictate the need for secure attachment of each and every layer of the system, starting from the deck and progressing right through to the protection layer. Each successive layer, and particularly the insulation should be independently and securely fixed in place even if the system relies on “through and through” mechanical fixings such as those used to attach a single ply membrane to a lightweight metal deck.

Wind loads should be calculated by reference to BS 6399; each site is unique and different parts of a flat roof will be subjected to different wind loads. The architect should ensure calculations are prepared for each project and should specify the type, number and disposition of fixings required to secure each area of the roof system against wind uplift.

The waterproof layer may be attached to the substrate by means of adhesive or by mechanical fixings: alternatively it may be loose laid and rely for resistance to wind uplift upon rigidity and self-weight (asphalt), or clips (sheet metals), or ballast (single ply).

High surface temperature (up to 80oC on a dark surface) acting on moisture trapped beneath a fully adhered membrane can result in blistering and loss of bond, such membranes are also prone to damage as a result of thermally induced stress in the substrate to which they are adhered. If an adhered system is selected it is essential to reduce the effects of differential movement between membrane and substrate by taping joints between insulation boards. Alternatively it may be better to adopt a system of partial bonding or of mechanical attachment, both of which can accommodate movements in the substrate without transferring stress to the membrane. In a conventional warm deck roof, mechanical fixings which secure the waterproof layer to the deck will also penetrate the thermal insulation and the vapour control layer. In high risk buildings, such as swimming pools, the resultant reduction in value of both thermal insulation and VCL may be critical and should be calculated (see BS 6229 and BS 5250: Appendix D). No architect should design a flat roof without assessing the risk of interstitial condensation causing damage to the roof system.

Perhaps the best illustrations of loose -laid and ballasted systems are the protected membrane roof and the green roof. Once applied, the waterproof layer is overlaid with XPS insulation boards which are then loaded/ballasted with gravel, paving slabs or growing medium applied over a filter fabric layer. The ballast layer not only secures the membrane against wind uplift but also prevents flotation of the insulant whilst protecting the whole system against UV degradation and mechanical damage. The protected membrane roof system has much to commend it and is my own system of first choice.

Whatever material is chosen as the waterproof layer and whatever form of attachment is adopted, long-term performance of the roof system will be enhanced if protection against UV light and mechanical impact is provided. Apart from ballast used on inverted warm deck roofs; other examples of a protection layer include:-

· patination oil - used on lead to delay/reduce surface oxidisation;
· mineral surfaced cap sheet - used as the top layer in built-up felt roofing;
· reflective paint - applied to asphalt to reduce heat gain and thermal movement.

Single ply polymerics (PVC, CPE, CPSE, HDPE, EPDM etc) and light gauge metal sheets are more susceptible to damage by physical impact than are the heavier thicker materials such as asphalt and built-up bituminous felt. Most thermoplastic sheet materials incorporate UV blockers; the large proportion of carbon black incorporated in synthetic rubbers performs the same function.




Protection should always be extended to include vertical upstands and the backs of parapets. Fig 5 shows sound detailing at a parapet which not only protects the vertical membrane but also reduces thermal bridging and avoids the risk of water penetration behind the cloak flashing.




Figure 5


 .6 Sitework

Roof top work is all too often split across several trades with different sub-contractors providing different parts of the system; sadly it is also prone to an “out of sight, out of mind” approach by the main contractor and the Contract Administrator! The deck must be inspected and checked for attachment, rigidity, falls, levels and surface cleanliness before it is entrusted to the roofer, who usually installs the VCL, insulation, waterproofing and protection as a sub-contract “package”. Do not assume his workforce are skilled and experienced.

The architect’s responsibility for site inspections means significant time and care must be devoted to roof top inspections at all stages: it is vitally important to inspect and monitor flat roof work. Each and every part of the system must be inspected, tested for conformity and formally accepted. Flood testing of the general roof area and pressure testing of outlets and drains are essential precautions before the protection layer is applied. Specify - and insist upon - written certification of operative training before work begins and of test results as soon as they are completed; do not issue your own Certificate until you have them!

A successful flat roof is unlikely to be achieved in bad working conditions so, during inspections, be particularly watchful for:-

· dirty site and cluttered work areas
· careless storage of materials
· careless handing of materials
· abuse of materials (overheating of bitumen, for example)
· working in adverse weather
· unsafe working practices (lack of roof edge protection)
· departures from specification.

.7 Care and maintenance

If you fail to maintain your motor car properly and have it serviced at intervals prescribed by the manufacturer, then the warranty is voided and so is your insurance policy! As with motor cars so with buildings, flat roofs require care and maintenance if they are to perform properly: they should be inspected at least once a year and the architect should provide the building owner with advice on when to inspect, what to look for and how to deal with minor defects revealed by inspection. If there is roof top equipment requiring inspection/maintenance then permanent access ladders and paved routes and work areas are essential.

When preparing the Maintenance Manual for the completed building, and/or the Health and Safety File required by the CONDAM Regulations, the Architect should include comprehensive information including:-

· the original user requirements and design criteria
· the design data and calculations relied upon
· the materials and products used and details of their manufacturers/suppliers
· records of tests completed
· who installed the roof and how to look after it
· safe access procedures
· the need for regular inspections
· a check list for inspections
· a pro forma for use during inspections.

Use of a standardised checklist during roof-top inspections is recommended: a good example is included as an Annex to BS 6229.
5. CAUSES OF FAILURE

As with most building defects, the cause of failure in a flat roof is invariably due to a combination of factors, commonly involving:-

· inappropriate design/specification
· bad workmanship
· material defect
· lack of maintenance
· abuse by users.

Time and again when investigating a defective flat roof we discover:-

· inadequate falls
· absence of overflows
· inadequate or badly sited outlets
· interstitial condensation
· inappropriate and/or incomplete insulation material
· gaps in substrate
· lack of attachment
· insufficient protection
· bad sitework
· lack of post-contract care and maintenance
· inappropriate adaptations by the building owner.

6. SUMMARY

In this paper I have rehearsed the basic principles of good flat roof design; if you follow those principles the system will perform satisfactorily for its design life. Of course, as part of the brief, it is essential to agree with the building owner how long the roof is required to last, how much he will pay for it and how he will use and maintain it. The design adopted must be a solution to the owner’s stated requirements in the context of specific conditions of climate and environment.

It was once suggested to me that to design a successful flat roof one would have to be a genius; but genius was once famously described as 10% inspiration, 90% perspiration.

Hopefully this presentation will provide you with the necessary inspiration, the perspiration will be yours as you apply your skills to:-

· analysis of the client’s requirements
· assessment of the climate conditions
· selection of materials
· design and detailing in 3D
· comprehensive specification
· careful choice of contractors
· ruthless site inspections
· regularised care and maintenance.

Good luck and success with your flat roofs!


Bibliography

1. CIBSE Guide: Volume A - Part A1: Environmental Criteria for Design,
Chartered Institution of Building Services Engineers 1978

2. BRE Digest 311: Wind scour of gravel ballast on roofs,
Building Research Establishment July 1986

3. BRE Information Paper IP2/89 Thermal performance of lightweight inverted warm deck flat roofs, 
Building Research Establishment January 1989

4. British Standards Institution

BS 5250: 1989 (1995) Code of practice for control of condensation in buildings

BS 6229:1982 Code of practice for flat roofs with continuously supported coverings

BS 6399 Loading for buildings

- Part 1: 1996 Code of practice for dead and imposed loads
- Part 2: 1997 Code of practice for wind loads
- Part 3: 1988 Code of practice for imposed roof loads

BS 7543: 1992 (1998) Guide to durability of buildings and building elements, products and components

BS 8210: 1986 Guide to building maintenance management

BS 8217: 1994 Code of practice for built-up felt roofing

BS 8218: 1998 Code of practice for mastic asphalt roofing

BS EN 12056-3: 2000 Roof drainage, layout and calculation
(replaces BS 6367: 1983)

CP143 Code of practice for sheet roof and wall coverings

- Part 1: 1958 Aluminium, corrugated and troughed
- Part 5: 1964 Zinc
- Part 10: 1973 Galvanised corrugated steel. Metric units
- Part 12: 1970 (1988) Copper. Metric units
- Part 15: 1973 (1986) Aluminium. Metric units
- Part 16: 1974 Semi-rigid asbestos bitumen sheet. Metric units


10 April 2002