The Householders Guide to Flat Roofing

Every effort has been made to ensure the accuracy of the information in this publication. Neither the FRA or the NFRC has itself verified the information by independent testing or has any control over the circumstances in which it will be used. They, their officers, employees or members cannot therefore accept any liability arising out of its use. 21140 FRA 20/9/04 10:00 am Page 3 INTRODUCTION This Guide has been produced by the Industry for the average Householder with minimal roofing knowledge. By providing information on the design, materials, construction and maintenance of successful flat roofs, we hope it will assist you to a satisfactory roof renewal. Most domestic flat roofs use two or more built-up layers of bituminous felt as their weatherproofing, each comprising a reinforcing base coated with oxidised or modified bitumen. The nature of the reinforcing base dominates the strength and fatigue resistance of the membrane. Traditional rag, wood-fibre or asbestos-based felts used since the 1950’s could give a serviceable life of 15 years or more, if correctly installed on a stable structural deck. Bad design, inadequate decking materials, and poor installation sometimes reduced their service life to 5 years or less. These rag/fibre-based products have now been withdrawn from British Standards, and are no longer recommended. Modern built-up roofing products incorporate more robust polyester bases, to provide far greater strength, durability and life expectancy. Also, the bitumens used can now be improved with polymers to increase flexibility, and further extend the roof’s life. The minimum quality of roofing recommended for habitable buildings is BS747 Type 5 or possibly Type 3 for shorter service life. Correctly installed on suitable decks Type 5 products will give a life expectancy in excess of 20 years. Although there are still some inexperienced contractors producing poor results in residential roofing, given good quality installation by reputable contractors with trained certificated operatives and simple maintenance, the householder can be assured of a dependable and long-lasting roof. 1. 21140 FRA 20/9/04 10:00 am Page 4 BASIC DESIGN In a flat roof, the waterproofing is always supported by a structural roof deck. This is usually a timber boarding of some type, which in turn is supported on joists. The ceiling, if any, is usually fixed directly to the underside of the joists. Garages may be un-insulated, but most roofs above the habitable part of the house will be insulated. In houses the insulation is often placed immediately above the ceiling. Alternatively, it may be placed above the roof deck before the application of the waterproofing (see below). Some specialised insulations can also be placed above the waterproofing but this sort of construction is normally confined to commercial and industrial buildings and has very seldom been used on domestic buildings. In almost all cases the surface of the waterproofing will need protection, both from ultra violet light and from fire from external sources. The Building Regulations lay down the degree of protection required in the case of fire. This can be achieved by adding a layer of mineral chippings bedded in compound to the surface of the roof, or alternatively by using a mineral surfaced layer of felt as the top layer, or ‘Capsheet’. WARM AND COLD ROOFS The construction of the roof deck and ceiling is has an important effect on the behaviour of the waterproofing material on top. The building industry uses the terms WARM ROOF and COLD ROOF to describe the two different types. In Britain as a whole, the Warm roof is strongly recommended. WARM ROOF This type of construction has the insulation above the roof deck, thus keeping the deck ‘WARM’ and is usually the most satisfactory construction for domestic properties as it avoids the need for ventilation of the roof structure. A typical warm roof is shown in sketch 1. 2. 21140 FRA 20/9/04 10:00 am Page 5 SKETCH 1 - WARM ROOF COLD ROOF Many residential flat roofs have the waterproofing laid directly onto the deck, and any insulation placed above the ceiling. This arrangement allows the deck to become ‘COLD’. In cold weather, such roofs are inherently prone to condensation, which can cause materials to decay and distort. To reduce this risk, through-ventilation has to be provided to each space above the insulation but this is often difficult to achieve, for example if the roof abuts the wall of a house. Cold roofs should not be endorsed for the climatic conditions in Scotland, according to the Building Standards (Scotland) Regulations. Although the Regs do not prevent the use of Cold roof constructions, Warm roof constructions are recommended. If you have an existing Cold roof there may be no need to change it. A typical Cold roof is shown in sketch 2. SKETCH 2 - COLD ROOF 21140 FRA 20/9/04 10:00 am Page 6 ESSENTIALS FOR A SUCCESSFUL FLAT ROOF Like any part of a building’s exterior, flat roofs should be constructed to withstand natural and human forces with the minimum of attention. PROTECTION FROM RAIN AND SNOW • The finished roof should have a slope of at least 1 in 80.