McKenzie and Flynn - Built-up Felt Roofing


The concept of covering flat roofs with layers of "felt" laid in hot bitumen as a less-expensive alternative to mastic asphalt and metal roofing is a relatively recent phenomenon. With the expansion of the oil industry after the Second World War, the petrochemical by-product of bitumen was seen as an ideal waterproofing compound. With the increase in buildings being designed with flat roofs, coupled with the urgent need for new housing and factories (to replace pre-war slums and large areas of blitzed cities), the stage was set for a construction industry revolution. Buildings could now be built without the expense of pitched roofs and utilise further the concept of concrete roof decks. In addition, these new designs could be built more quickly, using less qualified and therefore less expensive labour. Unfortunately, the full effects of ultraviolet radiation from the sunlight,
	Pollock Halls of Residence, University of Edinburgh

the vagaries of the British climate and the dubious quality of the "felts" available at that time were not entirely understood within the industry. Within a short period of time, flat roofs covered with felt became despised by all because they were notoriously unreliable with a tendency to leak at the earliest opportunity. In an attempt to regulate the use of felt roofing, the British Standards Institution (BSI) investigated the various felts available and introduced British Standard 747 (BS747) as the industry standard. The felt types accepted as suitable were classed according to the bitumen carrier (felt) used: Type 1: Rag Fibre Based. With the exception of Type 1F (reinforced underslating felt) the trade now regards this material as suitable only for roofing the likes of garden sheds. Type 2: Asbestos Based. No longer manufactured because of the serious health problems associated with asbestos fibres. Type 3: Glass Fibre Based. Now regarded by the industry as a very cheap specification, suitable only for the likes of domestic garages. Type 4a(ii): Inodourous. Considered only for use as an underlay for lead roofing.
As a direct result of the problems with traditional felts, manufacturers invested in serious research to come up with alternatives which would counter the lack confidence held by the public at large. In the 1980s the BSI considered the use of Polyester based felts and approved their use as BS 747: Type 5: Polyester Based roofing felt. In addition to the 'roll and pour' felts mentioned, great strides were made in advancing torch-on felts. Torch-on felts can be laid by applying heat direct to the underside of the felt roll without the need for bitumen boilers and liquid bitumen. Their use, particularly on small contracts and those with restricted working areas is, therefore, invaluable. Nowadays, the quality of felts to waterproof roofs is such that many manufacturers now regularly issue 10, 15 and 20- year insurance backed warranties covering not only their materials, but also the workmanship of their approved contractors. The warranty applicable is dependant upon the felt specified and the method used for laying it. McKenzie and Flynn Ltd are proud to be Approved Contractors for the following felt manufacturers: Index Building Products Ltd Ruberoid Building Products Ltd Marley Waterproofing Ltd George Anderson Ltd MSP (Scotland) Ltd Chesterfelt Ltd
While replacing felt roof coverings it is a simple operation to include insulation within the felt layers. The thickness of felt required and the insulation used is dependent on many factors. McKenzie and Flynn Ltd are happy to advise and assist their clients on the most appropriate materials for the roof being refurbished, the following being routinely used during felting operations: Polyurethane (PU) Polyisocyanurate (PI) Expanded Polystyrene (EPS) Natural Cork Mineral Fibre (Rockwool)
The above insulants can be mixed to achieve composite boards dependant on the particular requirements of the client or his site. In addition, where existing roofs have particular problems with rainwater drainage or ponding, these can be overcome by using insulation boards specially cut to achieve falls and crossfalls.