COP v3.0:installation; fasteners

14.3 Fasteners 

A fastener is a mechanical device for securing cladding and components to a structure or to another component, this definition includes nails screws, clips, and bolts.
Fasteners are also commonly known as fastenings and fixings, which is confusing because a fixing describes the result achieved by the use of a fastener by a fixer, and the verb to fix and to fasten are interchangeable.
The strength of a fastener relates to the mechanical properties of the fastener alone, whereas the strength of a fixing relates to the minimum value of pullout, pull-over, washer inversion and strip-out capacities. These values are obtained from the strength of the member (e.g., purlin) into which the fastener is secured and also of the component (e.g., cladding) that is being restrained.

The performance data published by the fastener manufacturer and that described in AS 3566 relate to the product only, and should not be used for design purposes. Values assessed by the manufacturer do not necessarily follow the method required to obtain applicable design load values.

The design load required for metal cladding and accessories is calculated for the joint and not the fastener.

Because the durability of a fastener will determine the long-term performance of the metal cladding, the fastener and its coating should be compatible and suitable for the environment.

 

14.3.1 Primary Fasteners 

Primary fasteners are those that attach roof or wall cladding to the building frame. Because they are relied on for structural performance, they should be capable of withstanding the design withdrawal load specific to the site and their location on the building. They should be able to withstand all loads applied to the sheeting, including those due to expansion, and remain watertight.

Fasteners should not be overdriven as that can damage the sealing washer, distort the roof cladding, and cause leaks. Screw fasteners should not be overdriven as this can affect the pullout values and cause strip-out when fastening to light gauge purlins or battens.

The shank diameter and thread type of the fastener will determine the withdrawal loads into either timber or steel, so the designer or contractor should ensure that the fastener complies with the requirements of 3.12 Fastener Performance.

The number of fasteners per square metre required to resist all wind or other loads on a building depends on the type of fastening.

Primary fasteners for timber consist of nails and screws with varying diameters , but their performance should be individually assessed.

When using light gauge metal roof and wall cladding the size of the head will determine the performance of the sheeting under load, as the calculation of pull-over strength assume that either the screw or the nail washer has a minimum head size of 12 mm. See 3.12.2 Pull-over Values.

Fasteners with heads smaller than 12 mm, such as wafer heads and proprietary screws for fixing miniature profiles used without metal or sealing washers, should not be regarded as a substitute for primary fixings for metal claddings. These fixings can be used with different fastening patterns to those in 3.14.4 Fastener Patterns, but the pull-over load should be determined from tests and should not be confused with the pull-out values provided by the manufacturer.

The fastener to connect battens or purlins to the structure is a primary fastener, and if the roofing contractor is responsible for attaching these members, the wind uplift load should be determined. See 3.7 Wind Load and 3.14.1 Purlin-Rafter Connections.

14.3.2 Nails 

Only enhanced shank nails with metal and sealing washers must be used as primary fasteners to secure metal roof and wall cladding. Smooth shank nails must not be used.
Nails are usually hand driven except when used for fastening clips for secret-fix or fully supported roof or wall cladding, when air or gas operated nail guns may be used.

The holding power of a nail in timber depends on the type of timber and its moisture content, not only when nailed but also in service. Nails can back out during its service life if the moisture content is more than 18%; screws should be used instead of nails if it is higher.

The traditional New Zealand roof fastener was until 1980 the "Lead Head", a flat-headed smooth shank steel nail with a lead head cast on it. Lead heads often 'backed out' causing leaks, which has led to the mistaken belief that nails have to be "hammered home".
Spiral Shank nail fasteners rely on the frictional grip of the spiral shank to stay in the timber. The holding power of a spiral shank exceeds that of a smooth shank by two to three times. The seal produced by a resilient washer bonded to a metal washer requires only 50% compression to produce a seal.

Spiral shank nails should not be over-driven or the washer seal will be impaired. Spiral Shank nails used to fix metal roof and wall cladding should have a hot dipped galvanised coating of 70 µ in thickness. Potentially rust-causing damage to the head from hammering can be minimised by using a nylon- faced hammer.

Only factory paint coated nails and washers should be used with pre-painted cladding. Galvanised nails should only be used with zinc or AZ coated sheeting.

Painted stainless nails can be used for fastening:

  • non-ferrous and plastic roof and wall cladding in severe and very severe environments;
  • when other exotic coatings are used.; or
  • when unusual corrosion conditions exist.

Only paint coated stainless nails and washers may be used with paint-coated aluminium roof and wall cladding. Factory coated stainless steel nails and washers are available to colour match cladding for environmental conditions where zinc coated fasteners would not perform their function for the 15 years required by the NZBC, or where aesthetic appearance is paramount.

Acrylic paint should not be applied directly to galvanised nails or washers without a suitable primer.

The nail should always be hammered in at right angles to the roof and if the purlin is missed no attempt should be made to "skew" the nail. The hole should be sealed with a rivet and neutral cure sealant, or the sheet should be replaced.

Minimum penetration of nails into timber must be 40 mm.

 

14.3.3 Screws 

Self-drilling screws with a piercing point and a thread suitable for drilling into timber with a hexagon head are designated as Type 17. A screw gun can provide sufficient torque for a type 17 screw to pierce metal roof cladding profiles and thread themselves into soft wood, but hardwoods may require a pilot hole.

14.3.3B Screw Thread Types

GaugeDiameters
Nominal
Threads Per Inch
TPI
Head
ShankThread TPIThread Type
mmmmABC
62.53.5 2032Pozidrive/Philips
834.2 18321/4" HEX
103.54.8 16245/16"HEX
124.15.51114245/16" HEX
144.86.31014203/8" HEX

 

Coarser threads (less than 17 TPI) are used to fix roof and wall cladding. However, a finer thread, known as metal thread, is used when fixing to steel thicker than 2.4 mm. Different thread configurations are available for special applications and some shanks are provided with a gripping thread at the top of the shank.

 

 

All fasteners must be easily identified by a code stamped on the head to identify the manufacturer and the coating class. The size, type, length, head type and the standard to which the fastener is manufactured must be identified on the packaging and in the manufacturer's literature.

When counter battens are used, and the primary cladding fastener does not always penetrate the structure, the fastener used to secure the counter batten to the structure also becomes a primary fixing.

When solid sarking, insulation or decking is interposed between the cladding and the structure, the design uplift force on the decking should be resisted by the total number of fasteners per square metre, multiplied by their individual capacity. (See 3.12 Fastener Performance)

Screws that are to be used as a structural fastener, such as in a stressed skin roof design or structural purlin/rafter connection, should be made using a screw gun with an adjustable torque setting to ensure consistency of fastening.

Stainless screws should be driven with a new driving socket so that steel smear from a used or worn socket head does not contaminate the stainless steel screw head. Alternatively, a stainless socket should be used.

While the common roofing screw is a Class 4 coated steel screw, stainless steel and aluminium are alternatives. (Also see 8.7.3 Secret Fixing Clips)

The 14 gauge aluminium screws available are suitable for rib fastening of profile metal cladding into timber purlins and 12 gauge are suitable for pan fixing. They require pre-drilling into timber and are not suitable for fixing into steel. Care should be taken not to over-drive aluminium screws as their shear strength is less than steel.

Stainless screws are available in Grades 304 and 316, but only 304 should be used for fixing aluminium cladding. Grade 304 is suitable for very severe environments, provided the screw shank is isolated by an oversized hole and a load spreading washer made from 445M2 stainless steel is used with an EPDM sealing washer.