Gable wall restraint

The question of pressure on one end of the gable end of a house and suction on the other end was discussed in Chapter 6. Figure 8.17 illustrates this effect.

Above the wall plate the gable end is of course free standing brickwork and on steep pitched roofs this area of gable brickwork can be quite large, and the resulting wind loads quite significant. The Building Regulations require these external walls to be restrained and BS 8103/1 gives recommendations for the horizontal lateral

Wind pushes wall onto house

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Fig. 8.17 Wind loads on gables.

connection between masonry walls and the roof structure. NHBC, in its section 7.2 referred to earlier, covers this aspect under 'strapping', clause 7.2, S3. With the above ties and bracing as set out in BS 5268: Part 3, annex A, described in Chapter 6 of this book, the roof will be resistant to all lateral loads imposed upon it, provided the limiting criteria set out in the standards are not exceeded. Similar bracing and restraint will apply to all roofs, however constructed, and this must not be forgotten simply because the details which are now generally available invariably show only trussed rafter roofs. With a traditional roof the restraint of course can be provided more simply by ensuring good fixity for purlins, ridge, and other longitudinal members into the gable ends.

Restraint is generally provided in trussed rafter roofs by fixing galvanised steel straps across at least three trussed rafters into the gable end. These contain the suction loads holding the brickwork into the roof, whilst blocking pieces fixed between the wall and the first trussed rafter and between the first trussed rafter and the second trussed rafter guard against the gable being blown into the roof. The ties should be of 30 mm x 5 mm minimum in cross-section and are generally prepunched with nail holes to facilitate ease of fixing on site. Figure 8.18 illustrates a typical set of gable restraints.

It can be seen that these straps occur at both ceiling and rafter on gable ends, but on the ceiling they tie only on separating walls where they pass through them to form a continuous link between adjoining roof structures. The ties should be fixed with 3.35 mm diameter, 50mm long galvanised round wire nails.

In extremely exposed situations, care must be taken to check that the parameters laid out in the British Standard are not exceeded; if they are, then a separate strapping design will be required. The standard spacing for straps is 2 m on both rafter and ceiling tie, but this may have to be reduced to provide more straps if additional restraint is required. The straps may also need to be longer, being attached to four adjacent trussed rafters rather than the standard three.

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