Holding down roofs

By - , Build 94

Securing roofs against wind uplift is a crucial step in the building process. Fortunately the cost of doing so adds very little to the overall cost of the building.

Figure 1: Unequal double-plate arrangements can be difficult to secure when the truss lands between studs.

There are a number of links in the chain between roof and ground – each one vital when the wind starts to howl. Where the roof passes over an opening in the wall, there are some extra links in the chain because of the lintel. NZS 3604 provides a complete set of solutions for the required fixings for all wind zones, although sometimes the information is a little hard to find. Tables 1 and 2 here list the relevant information. The main timber connector suppliers also provide illustrated installation and fixing guides, in a slightly more user-friendly format than NZS 3604.

In terms of durability, most of the links in the chain are in a closed environment and standard zinc-coated fixings are usually acceptable. However, in geothermal hot spots or sea spray zones, the fixings below floor level require more detailed consideration, covered by Section 4 of NZS 3604.

Connections between the roof and walls also deserve more detailed attention. Not only do the walls support the roof against gravity loads acting downwards, and wind loads acting up, but the roof also supports the tops of the walls against lateral wind loads. A failure here can have severe consequences, usually total destruction. ‘Z’ nails, cyclone ties and wind straps provide easy-to-use solutions for truss and rafter fixings, and wire dogs and a strap wrapped over the top plate provide good options for the plate/stud connection.

Unequal double top-plate arrangements, however, present a problem when the rafter does not land over a stud, as shown in Figure 1. The fixings described in Table 8.18 of NZS 3604 cannot easily be used in this situation, so alternative fixings of equivalent capacity must be used. Proprietary fixings can be used in this situation, but the manufacturers have not specifically addressed the issue with a fixing tested to NZS 3604 clause 2.4.7. Hence, this situation does give fixing manufacturers an ideal opportunity to provide innovative solutions to the market.

Figure 1: Unequal double-plate arrangements can be difficult to secure when the truss lands between studs.

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Figure 1: Unequal double-plate arrangements can be difficult to secure when the truss lands between studs.

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