Primary fasteners are fixings that attach flashings and cladding to the structural building frame and which should withstand all loads applied to the flashing, including expansion provisions. Primary fasteners are relied on for structural performance. Nails or screws are used for fixing into timber, and self-drilling or self-tapping screws or bolts for fixing into steel.
Crest fixing is the most common type of primary fastening of roof cladding in N.Z. The minimum embedment into a structural timber member should be 35 mm or 6 times the fastener diameter.
Flashings should be fastened at a point between 25 mm and 50 mm from the edge of the flashing and the maximum primary fastener spacing should be 600 mm. When this is not possible because there is no structural member provided, they should be fixed with secondary fasteners at a maximum of 200 mm centres.
If the sheet length exceeds 8 m, flashings should not be fixed rigidly to the structure without some provision for expansion. The allowance for movement will vary with the length of the material, its coefficient of expansion, its colour, and the temperature range likely to be encountered. See 6.2.2 Roof Cladding Expansion Provisions.
Both pull-out and pull-over resistance are critical elements in selecting the correct fasteners. See 3.11 Fastener Loads.
Pull-out resistance and the ability of a fastener to hold cladding in place is a function of the size of the fastener, the thread, type, and the density and strength of the substrate material. The pull-over resistance of the metal held in place where the fastener penetrates the sheet metal depends on the diameter of the head of the fastener, the washer size and the thickness, type, and grade of the material being secured.
Avoid wide, large, and flat areas of flashings, as these will buckle with thermal movement and require fixing at closer than normal centres. All flashings should be fixed to the structure using primary fasteners, and the design wind load determines the frequency of fastenings per lineal metre.
Most flashings are within the periphery of the building, which incurs an increased design wind load factor (kl) of a minimum of 1.5 times the design wind load for the main body of the roof.
For this reason, all flashings should be fixed on both edges and have a maximum flat surface width of 300 mm. Additional stiffening is required if the flashing width exceeds 300 mm.
Z or AZ coated, pre-painted steel roof, and wall cladding in severe and very severe environments should be fixed using screws complying with Class 5 of AS 3566.2 (under revision), which are compatible with the cladding materials. Otherwise, a minimum of Class 4 screws should be used. See 4.22.1 Fastener Durability.
All fasteners must be easily identified by a code stamped on the head to identify the manufacturer and the coating class. When using pre-coated steel and aluminium, only factory painted screws, washers, nails or rivets must be used.