COP:installation; fixing

13.5 Fixing 

13.5.1 Crest Fixing 

Crest fixing is the most common type of fastening of roof cladding in New Zealand.
For fixing into timber a Type 17 self-drilling fastener that has a pierce point slot thread is used, which penetrates the roofing metal and then threads itself into timber.
The embedment into soft wood must be a minimum of 35 mm and greater than 6 times the screw thread diameter.
For fixing into steel, a self-drilling fastener with a drill point of a diameter suitable for the tapping size of the fastener is used. Extended drill points are available for drilling up to 12 mm thick steel and the pitch spacing of threads should be increased as the steel thickness decreases. See 13.3.3B Screw Thread Types
Steel embedment should be 5 to 6 mm beyond the drill point with a minimum of 3 threads beyond the drill point.
Screws are specified by gauge, TPI (threads per inch), and length.
Self-threading screws require a pre-drilled hole in the cladding and structure. The tapping size drill diameter is determined by the TPI and gauge of the fastener.
Both self-drilling and self-tapping screws can have different points, TPI, and heads to suit the particular requirements of the connection.

 

 

13.5.2 Pan Fixing 

Pan or trough fixing is commonly used for wall fixing although it is not widely used for fixing roof cladding in New Zealand. Pan fixing is common practice in Europe and offers an economical alternative to rib fixing.

Pan fixing can be used for roof as well as wall fixing provided certain fixing conditions are met:

  1. Light coloured roofs are limited to 12 m, thereafter with sliding washers and crest fixed.
  2. Dark coloured roofs are limited to 8 m, thereafter with sliding washers and crest fixed.
  3. If the insulation is laid close to the roof cladding, deduct 4 m of pan fixing from the above.
  4. If the ceiling is insulated, the roof cavity should be vented by a ridge ventilator or other permanent vents.
  5. Metal roofs that are not post painted should be regarded as dark coloured due to the oxidation and change in colour and surface over time.
  6. Translucent fibreglass sheets can be pan fixed for 8 m then crest fixed with sliding washers for 4 m, limited to a maximum of 12 m.
  7. Fasteners should be a minimum of 12 x 20 mm self-drilling for steel purlins and 12 x 40 mm for timber, both using a 25 mm metal and sealing washer.
  8. All fasteners should be driven snug tight with a torque driver or depth locator.
  9. Fasteners should be placed within 50 mm of the rib of the sheeting but allowing 25 mm clearance for water egress.

N.B. The pullover design values established by testing for pan fixing are more than twice those for crest fixing.

All sheets should have full bearing on all purlins that they cross to ensure a positive seal. Care should be taken to fix at right angles to the roof, and purlin flange alignment is critical if purlin creasing is to be avoided.

Corrugate and symmetrical profiles should not be pan fixed for roof cladding.

Pan fixing of roof cladding, is used in USA and Europe because of its cost and efficiency; the pullover values are increased, and it provides a much more effective shear diaphragm for cladding lengths up to 8 m.

13.5.3 Expansion Fixings 

A limited amount of movement between the fastener and the cladding can be provided with an oversized hole for the fastener. When a greater amount of expansion is required, sliding fixings are necessary to allow for expansion and contraction. (see table 4.1.6)

There are two main types of sliding fixings.

  • Sliding Washers
  • Sliding Roofs

 

13.5.3A Sliding Washers

Sliding washers are used where the sheet and the fastener are separated by a material with a low friction coefficient, such as Teflon, enabling them to move independently in elongated holes. N.B. The seal should be made between the roof cladding and the washer by placing the sliding face (Teflon) upwards and the sealing face down onto the cladding.

Where an oversized or elongated hole is used, the sealing washer must be wide enough to seal the hole.

13.5.3B Sliding Roof

Sliding Roof is where a clip is securely fastened to the structure, and a longitudinal groove cladding profile provides clearance for expansion and contraction.

13.5.4 Lap Stitching 

All roof or wall cladding, except self-locking and fully supported profiles, must be lap stitched at midspan when purlin spacings exceed 1.2 m.
Lap-fixings must be coarse thread stitching screws with a neoprene washer or 4.8 mm diameter aluminium Bulb-Tite rivets.

Without lap stitching the wind and point load performance will not comply with the load span graphs in 3.14.5 Wind Load Span Graphs