There are a host of factors that need to be considered when selecting and specifying roofing products - modern versus traditional, natural versus man-made, as well as cost and aesthetics. Here we look at the latest developments in flat and pitched roofing, and the opportunities, and potential problems, for roof refurbishment Flat roofs are an essential part of the building industry but they have suffered in the past from some bad press following failures during the 1970s and '80s. The majority of these failures were the result of the industry's lack of knowledge about the overall effect of adding thermal insulation to flat roofs. This thermal insulation, normally in the form of glass fibre, was added to the ceiling void between the joists without providing any ventilation. As a result, condensation developed on the underside of the waterproof covering during the winter months. Since much of the roof decking was constructed from moisture-sensitive materials, such as chipboard or strawboard, many decking failures occurred which then lead to the failure of the waterproofing.
The roofing industry now has a much better understanding of how an insulated flat roof performs in the UK climate, and the problems of the past should not return if designers and specifiers follow current advice in the codes of practice.
The main guidance to modern flat roofing design is the British Standard BS 6229 Code of Practice for Flat Roofs with continuously supported coverings, which was first published in 1982. The British Standards Technical Committee has recently completed a total revision of the code, which should be available in early 2003. BS 6229 is a head code. This means that it covers the design and application of all types of waterproof coverings - from traditional lead to modern single-layer membranes - so long as they are continuously supported by a structural deck or thermal insulation. The code provides the designer with advice on each of the components that will be required to produce a modern flat roof from the structural deck through to any applied surface finishes. The head code is also referenced by the materialspecific codes of practice, which provide information on the installation of the waterproof layer.
With the introduction of the revised Part L1 and L2 of the Building Regulations in England and Wales, and Part J in Scotland, the thickness of thermal insulation has increased to a level where a cold roof constructed in accordance with the code of practice is impractical. The roof void does not have sufficient depth to contain both the insulation and the necessary cross ventilation. I would therefore recommend that a warm roof or an inverted roof be used in all cases.
In both forms of construction, the insulation is above the structural decking, thus avoiding the need to ventilate the voids. In the warm roof, an effective vapour-control layer is essential below the thermal insulation, while in the inverted roof the thermal insulation is placed on top of the waterproof membrane, thus avoiding the need for a vapour-control layer. The only insulation that can be employed in an inverted roof is extruded expanded polystyrene (XPS) since the other forms of insulation are not waterresistant. The inverted roof does suffer from some loss of performance due to rainwater running below the insulation but this is normally taken into account by an increase in the insulation thickness.
One of the major developments in flat roofing has been the increase in the range of waterproofing materials available to the specifier. Metal coverings can be formed from lead, zinc, stainless steel, copper and aluminium, which can be laid in the traditional manner with standing seams or roll edges. Several proprietary systems use a metal finish that is often pre-fixed to a suitable board. Mastic asphalt is also available and, in recent years, the grades of asphalt have been modified to make them more suitable forthe better-insulated roofing systems. Any asphalt laid over thermal insulation needs to be able to withstand greater and faster temperature changes than asphalt laid on to a dense structure such as a concrete slab.
The most commonly used waterproofing for flat roofs in the UK is almost certainly flexible bitumen sheeting. Developments in bitumen materials have been extensive during the past 10 years. Polyester-based felt has improved the long-term performance of waterproofing systems dramatically.
Although glass-based felts are still made to BS 747, their use is normally confined to the preparation layer or for use on temporary buildings. Polyester-based felts are available in many forms, including oxidised bitumen, elastomeric sheets and plastomeric sheets.
These various types have been developed to suit particular application techniques and to provide different performance levels. There are now several methods of installation, including application by pour-and-roll techniques in hot bitumen, using torch-on felts, by cold adhesive, as a self-adhesive and by mechanical fastening. Although many methods of application have been developed, it is important to employ the correct type recommended for any particular felt.
You should always obtain the advice of the manufacturer when specifying these materials. The British Standard code of practice for flexible bitumen sheeting roofing BS 8217 is currently being updated to cover all the modern felts and new methods of application. The draft code will be available for public comment in the spring of 2003.
Another major change in the flat-roofing industry has been the introduction of singleply membranes. There is a wide range of man-made materials available as roof coverings. These are normally loose-laid and mechanically fastened against wind uplift unless used in inverted-roof applications where the weight of the insulation and ballast will provide the required resistance against wind uplift. Although many of the single-ply membranes are robust, care is required in the choice of materials since mechanical damage after installation could have disastrous consequences for the occupiers. The membranes can be patch-repaired but it is better to ensure that surface protection is provided where regular access is required to motor rooms and to any plant on the roof. Flag (UK) has recently introduced a new non-slip granular walkway material that can be bonded to the roof surface. This will reduce the risk of damage while providing a safe means of access to any equipment installed on the roof. The single membranes are not currently covered by any British Standard, although many have product approval certification. The installation of single membranes is not covered by a British Standard although the Single Ply Roofing Association (SPRA) does have its own code of practice.
Another solution for covering a flat roof is to use a liquid waterproofing system that can be either hot- or cold-applied. These liquid systems are constructed on site, using various components that are mixed together to form the flexible waterproofing membrane that is fully bonded to the roof surface. These types of membranes should only be laid by trained operatives who are familiar with the special application techniques required.
Information about liquid waterproofing systems can be obtained from the European Liquid Roofing Association (ELRA) which has prepared a detailed code of practice covering the design and installation of the products produced by their members.
Pitched roofing Although concrete interlocking tiles have dominated the UK market for pitch-roofing products, particularly in the speculative house-building field, the more traditional roofing materials are starting to make a comeback. The most traditional products are thatch, clay tiles and natural slate.
Problems of acceptability under the Building Regulations and its high installation cost have prevented the return of thatching to the mainstream roof market, but clay tiles and natural slate are increasingly being seen on buildings.
One of the main advantages to the designer in using both clay tiles and slates is that they are better suited to the more complicated roofs. Detailing of valleys, hips, verges and changes of roofline is easier to achieve with simple overlapping tiles and slates. The concrete interlocking tile is ideal for a basic duo-pitched roof with no penetrations such as dormers. The speed of installation makes the roof covering economic, but the introduction of complicated details slows the rate of laying and can produce details that look untidy and cumbersome. Concrete-tile manufacturers will supply full details for all their products but this can never overcome the fact that they do not always suit a complicated roof. When it comes to a curved roof shape, traditional clay and slate products outshine the modern upstarts.
There is a wide range of traditional materials from which the designer can choose.
Hand-made clay tiles are available from companies such as Keymer Tiles, which offers a full range of very traditional clay tiles complete with all the specials that are necessary to form a sound roof. Work in conservation areas or on listed properties will require these high-quality, hand-made products to meet the demands of the planners. In less sensitive areas, matching or blending in with the surrounding buildings can be achieved by using machine-made tiles such as the plain tile range available from Redland Roofing Systems. It has four product ranges, which include a machinemade clay tile, the Rosemary, and three concrete tiles that give the appearance of a traditional clay material. The machinemade products may not have the quirky variations of the hand-made tile, but they still provide an economic alternative where the appearance of the surrounding buildings has to be maintained.
Another method of achieving the appearance of a clay tile is to use one of the 'look-alike' products offered by the roofing industry. Many of the concrete-tile manufacturers now offer a concrete interlocking tile that provides a roof that could easily be taken for a clay-tiled roof. The reduced weight and the faster laying time are the main advantages of using these concrete tiles in place of the more traditional plain tile. An alternative approach to the problem of how to increase the speed of installation of clay tiles has been taken by Sandtoft Roof Tiles which has introduced an interlocking clay tile. The Sandtoft Clay range now offers a choice of four colours, and provides a very economic roof covering due to the fact that it requires only 20 tiles per square metre compared with the normal 60 required in traditional clay installations. As these tiles interlock, they will suffer from the disadvantage of not being suitable for curved roofs and their detailing will be non-traditional at valleys, hips, etc.
When considering natural slate, the range of possibilities for the designer is even wider.
Natural slate of various grades and qualities is being imported into the UK from many areas of the world. My advice to any specifier is to check very carefully that the product they are being offered is suitable for the UK climate and that the slate quality is constant and if possible meets the current British Standard for slates. One supplier of slates, Eternit, has recently redefined its range of slates to assist the specifier when choosing the slate. The natural slate products are now distributed over four ranges - Heritage, Classic, Cabrera and Navia, and each range is aimed at a different market sector, from the prestige building to the repair market.
The alternative to a natural slate is to use a man-made product. Fibre-cement products have been available for many years and offer a more economic roof covering than natural slates. The alternative is to use plastic-resin products, which often incorporate a large percentage of natural state as the aggregate core. The big advantage of all these products, in addition to their lower initial cost, is that the method of installation is virtually identical to natural slate, so the roofer does not have to understand different installation techniques for each product. The designer should, however, check that the product is suitable for the particular location, since the reduced weight of the man-made product may require extra fixings or the use of other methods for overcoming a high wind load.
The other warning I would give is that some of these man-made slates do suffer from colour loss or changes in colour with age, so check that these changes are acceptable to the planner and the client. Equally, do not allow man-made products to be substituted for natural slate in conservation areas or on listed buildings without the prior consent of the local planning authority.
The concrete-tile industry also offers alternatives to natural slate, which use interlocking systems. These, like the equivalent clay-tile replacement products, are quick to install but suffer from detailing difficulties at valleys, hips etc. The designer has a wide choice of products from which to select the roof covering on any building but it is important that the final selection takes into account all the factors, not just colour and minimum cost. It is important to check if the product is acceptable to the planners and that the roofer can achieve acceptable details at junctions with the chosen tile or slate.
Problems of roof refurbishment Refurbishment of a pitched roof is generally straightforward in that the full range of roofing tiles and slates can often be employed.
The main caveat is that the weight of the roof covering should not exceed that of the original unless the roof structure is checked for the increased loading and strengthened where necessary. During any refurbishment, it would be normal practice to introduce a suitable sarking material, which almost certainly means that some form of roof ventilation will have to be added to prevent condensation forming in the winter. This can be a very serious problem where extra thermal insulation is added, as the resulting lower roof temperature can often produce adverse conditions in a roof which was previously free of problems.
The refurbishment of any pitched roof gives the opportunity of adding extra components to roofs to improve the quality of life in the rooms below. On larger properties or in buildings that have been converted to flats, some areas of the building receive little or no natural light. One solution is to introduce roof lights during any refurbishment but this is often impractical above small rooms or passageways. An alternative solution has recently been introduced into the Glidevale range of products. It is called the Sunscoop, and is a double-skin polycarbonate dome that can be installed in any type of roof covering. The dome is connected to a highly reflective tube that carries the daylight to the room below and hence makes the area more inviting. The Sunscoop conforms fully to Part L of the Building Regulations and roof refurbishment is the ideal time to add them to the building.
Similar products are also available from other manufacturers.
Refurbishment of an old flat roof is a common problem when dealing with commercial and public buildings. The roof finish has often reached the end of its effective life and is probably leaking. The falls on the roof and the drainage will often be inadequate, producing ponding of rainwater, and the thermal insulation in the roof is either nonexistent or else totally inadequate for a modern building. The first option to consider is whether the roof can be improved simply by replacing the waterproof layer or overlaying it with a new material. I would suggest that this solution is probably acceptable when the roof already has a reasonable level of insulation and the surface has adequate drainage.
If this is the situation, almost any of the flat roof systems considered earlier could be used. A specialist coating system such as Decothane Delta 25, which is manufactured by Liquid Plastics, is often used for this purpose as it can provide an effective surface both quickly and without disturbing the occupants.
The more common situation is where the existing falls are inadequate and thermal insulation has to be improved. In this situation it is recommended that a warm roof be constructed and that tapered insulation be added below the waterproof membrane to form better drainage falls. Pittsburgh Corning UK now offers a tapered roof system service using its Foamglas cellular glass insulation which employs cut-to-shape slabs that are then installed in accordance with a layout drawing provide by the manufacturer. Although tapered insulation is an ideal solution, there are many circumstances where the increased thickness of insulation cannot be accommodated due to factors such as the height of the parapet wall or the need to revise the method of water drainage from the roof. Under these conditions it may be advisable to consider replacing the flat roof with some form of pitched roof draining to external gutters. Kelsey Roofing Industries recently carried out such a scheme at a Somerset school where it used Disadeck constructed on a lightweight steel pitched-roof structure that drains to outside gutters. A similar lightweight system was used on a two-storey residential home in Watford as a replacement for a centrally draining flat roof. In this case the system used was supplied by Tufftile and the resulting pitch roof has a 30-year guarantee. The methods of refurbishing roofs are numerous and the architect should always be able to find a system that meets their particular requirements.
Trevor Higgs is a building consultant who specialises in roofing Alwitra 1400 CA Building Products 1401 Callenders Vulcanite 1402 Decra Roof Systems 1403 Eternit 1404 Flag (UK) 1405 Glidevale 1406 Kelsey Roofing Industries 1407 Keymer Tiles 1408 Kestrel 1409 Kingspan 1410 Liquid Plastics 1411 Marley Roofing Products 1412 Pittsburgh Corning UK 1413 Redland Roofing Systems 1414 Sandtoft Roof Tiles 1415 Tufftile 1416