There is a continuing demand for bespoke timber windows, either to meet aesthetic requirements, or where the sizes or timber species required cannot be met by standard units.
Architectural requirements for sizes and appearance are commonly placed with the joinery manufacturer with an expectation that these will be translated into durable windows with high levels of resistance to rain and wind penetration. This requires the window manufacturer to provide the timber details and features which, along with the glass units, hinges, sealants, finishes, fixings and fittings, come together to meet these expectations.
Window manufacturers and designers may follow practices which they regard as traditional and “which work”, although there may be an absence of formal design procedures and test evidence. However, these practices often fall short when changes in materials or performance requirements occur. These may be initiated from many outside sources such as environmental legislation or even just consumer preference.
Early British Standards in the 1950s and 1960s gave considerable guidance on design features, largely based on specifications provided by the English Joinery Manufacturers’ Association (EJMA).
However, the subsequent British and the more recent European Standards have moved away from showing details that are assumed to provide a particular level of performance, towards a specification for the performance only, leaving it open to the manufacturer to provide the details which will achieve this. This provides greater scope for innovation and adoption of new materials and products, but it is less helpful in providing guidance on how the wide range of options can be brought together to provide a useful product.
A glance at the early EJMA sections shows that all of the external horizontal surfaces are sloped outward to help shed the rain. A worrying detail, commonly seen on some current window installations, is the presence of flat horizontal surfaces which do not shed rain and which may hold puddles of water long after the shower is over. This will encourage greater risk of timber decay and accelerate the breakdown of decorative finishes and glazing sealants.
A simple check for good design should be the absence of any horizontal flat timber surfaces on any of the outer exposed components, including glazing beads and fixed frames.
Despite the earlier comments, the current BS 644: 2003 ‘Timber windows – Factory assembled windows of various types’ does provide guidance on this detail and advises that the minimum slope on the top surfaces of horizontal members should be 1 in 8 (7O). In some more vulnerable situations, such as timber glazing beads, or where the timber surface is to be left bare of decorative finish, TRADA Technology recommends increasing the slope to not less than 1 in 6 (11O).
Under most circumstances the window joinery will be installed with insulated glass units which were not in use when the early profiles were developed.
To meet the current thermal requirements of Building Regulations the glass units will be at least 20mm thickness if low energy glass coatings and gas filling are used and 24mm where plain glass and cavities are used. If laminated safety glass is required, the glass unit may increase to 28mm thickness or more. The window must be of sufficient thickness to accommodate these units without compromising the installation of the glazing system or the rigidity of the window.
Customer expectations require that most windows should provide higher levels of draught-proofing and watertightness than those previously achieved and this makes the use of flexible weatherseals a common and almost obligatory feature of a modern window.
The detailing of the junction between the window casement and the frame should incorporate a pressure relief groove, which can be formed in either the frame or the casement
or shared between them. These pressure relief grooves, in some form, should be present on all edges.
The grooves are external to the weatherstrip and provide a break in which water blown in by gusts of wind or held by capillary attraction can be drained before it wets the weatherseal. Weatherseals that are not protected by this sort of device can become iced in winter and damaged if attempts are then made to open the window.
The importance of proper site installation and the fitting and sealing of the joinery unit within the building opening should not be overlooked. The bespoke joinery manufacturer should provide appropriate guidance for the installation to ensure that the window is not damaged and is effectively sealed by the installation process.
Last but not least, the manufacturer who will be uniquely familiar with the materials used in the window construction should provide guidance to the client as to how the windows should be maintained in service. This may include the choice of finish for the timber components and likely maintenance intervals, the avoidance of applying such coatings to the weatherstripping, the maintenance of the glazing system and lubrication of hinges and hardware. y
• Further guidance is provided in the TRADA Technology publication High performance wood windows by Patrick Hislop RIBA
