The eaves of a building vary greatly in design throughout the UK, and to some extent are an architectural detail rather than a structural requirement. The function of the eaves is of course to close off the ends of the rafters and, where a generous overhang is provided, to protect the building to a certain degree below. The traditional large overhang associated with most thatched roofs provided excellent protection to the heads of doors and windows below.
The functions to be considered in the design of eaves are therefore as follows:
(1) To effectively close off the space between the rafter feet;
(2) To provide a means of ventilation for the roof;
(3) To provide protection for the building below if required;
(4) To provide support for the rainwater drainage system;
(5) To provide support for the tile underfelt;
(6) To provide support for the soffit if required.
One of the most important features mentioned above is the support of the tile underfelt. Figure 8.23a shows the problems of underfelting being unsupported, being allowed to sag without support between the rafters and thus allowing ponding, with the resulting degradation of both fascia and soffit and possibly the top of the wall structure itself. Adequate support must be given at the bottom of the roof slope for the felt to avoid this ponding, this being achieved in the form of a thin sheet material applied to the top of the rafter feet or sprockets if provided, or in the form of a continuous triangular fillet fixed to the top of the rafter feet. This detail (Fig. 8.23b) allows any water which may have penetrated through the tile to run down the roof slope into the gutter in the normal manner.
The next important aspect is to detail the eaves allowing adequate ventilation, and simple methods to achieve this are indicated in Figs. 8.24a-e.
Figure 8.24a shows a detail with no overhang, care being taken not to fix the fascia tight to the wall, although with a ventilation system shown in Fig. 8.24d the fascia could be fitted directly to the wall if necessary. Figure 8.24b shows a typical overhang with fascia and soffit, this particular detail indicating a timber framed structure, care having been taken in this instance to show a gap between the soffit and the top of the brickwork to allow for the differential movement between it and the timber structure. Figure 8.24c shows a corbel eaves detail with no soffit and with the junction between rafter and ceiling tie taken beyond the outside of the wall. This particular detail would impose certain structural problems for all types of roof construction, and may require a blocked heel or additional top chord, should trussed rafters be specified. This particular detail indicates a loose fill insulation with a timber board controller to prevent the insulation spilling through into the cavity, or across the cavity blocking the ventilation space.
Figure 8.24d shows one of the proprietary combined ventilators and insulation controllers fitted on top of the fascia. This particular detail also indicates a sloping soffit fitted directly to the underside of the rafters. Figure 8.24e indicates exposed rafter feet with ventilation provided by slots between the infills between rafters.
The above illustrations show only a few of the many variations on design imparting individuality to any building. The only two details likely to give any structural problems are those indicated in Fig. 8.24c because of its cantilevering effect for the
Fig. 8.24b Eaves ventilation - with soffit.
Fig. 8.24b Eaves ventilation - with soffit.
Fig. 8.24c Eaves ventilation - corbel soffit.
truss, and any of the details where the rafter overhang is excessively long. In general this would mean beyond 700 or 800 mm, depending on the rafter depth. The use of the triangular sprocket piece on top of the rafter foot will not aid its strength in this respect, unless of course it is carried up the rafter well beyond the wall plate position.
Fascias and barge boards should always be preservative treated in accordance with building regulations and NHBC requirements and should be given one coat of either paint or stain prior to fixing.
The soffit boards need not be preservative treated for they are generally not exposed to the weather, although in the writer's opinion it is desirable to do so if softwood tongued and grooved boarding is used in an exposed eaves detail, such as Fig. 8.24e. It is normal to support the soffit at the fascia by fitting it into a groove in the back of the fascia, and on light timber softwood framings on the wall side of the building. Figure 8.20 illustrates a well framed soffit support system.
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