COP:flashings; fastening

7.8 Fastening 

 

Flashings must be fastened in one of three ways .

 

  • By primary fastening – fastening into main structure .
  • By secondary fastening — f astening into the cladding .
  • By means of clips, cleats or seams to allow for differential movement of the roof cladding and the flashings .

For full details on fixings, see 13.3 Fasteners.

 

7.8.1 Primary Fasteners 

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.

7.8.2 Secondary Fasteners 

Secondary fasteners are fixings that attach flashings to sheets and one another to transfer loads and provide side lap sealing. Rivets and stitching screws are secondary fasteners used to fasten cladding side laps. They are subject to shear loading due to expansion and differential movement.

Sealing washers are required on all secondary fasteners, except rivets which should be sealed.

Where the purlin or girt spacing exceeds 1.2 m, secondary side lap fastening must be provided at 600 mm centres to fix longitudinal barge or apron flashings.
All rivets used as fasteners at sealed laps must have a minimum diameter of 4 mm and fixed at centres of not more than 50 mm.

Except for self-locking profiles, flashings should be fastened to the roof or wall cladding using coarse thread stitching screws with a neoprene washer, or self-sealing bulbtite 4.8 mm aluminium blind rivets. Stitching screws, where the thread stops before the head, can be used to prevent stripping-out.

Screws or clouts fastening the vertical leg of a barge flashing should be at no greater spacing than 600 mm.

7.8.3 Secret Fixing Clips 

A secret-fix clip is an individual metal component installed at predetermined locations along the edge of a metal roof panel or behind the leading edge of a flashing.

Clips, cleats, and fasteners are the components that secure metal panels, accessories and flashings to the substrate or another metal component.

They should be compatible with the cladding metal and be capable of resisting the design wind loads; particularly around the periphery of the building where the uplift loads can be up to twice as much as at the body of the roof. To allow for expansion on long length flashings, the clip fixing which secures the flashing should not restrain it lengthwise.

Pull-out resistance and the ability of a fastener to hold a clip in place is a function of the type, length, and shank diameter of the fastener, and of the density or strength of the substrate material. The pull-over resistance of the metal clip depends on the diameter of the head of the fastener, the thickness of the clip, and the grade and type of the metal being secured.

Both pull-out and pull-over resistance are critical values in selecting correct fasteners for clips. Clips should be made from a grade and thickness of metal at least equal to the cladding thickness, but the COP recommends they are made from heavier metal than the cladding.

All fixing clips should be a minimum width of 30 mm, spaced at distances derived from 14.4.4A Load/Clip Spacing and fastened with two fasteners. Smooth shank nails should not be used and are not acceptable. Only enhanced shank nails should be used.

Clips that are used with long spanning profiles can be interlocked in a run sheets, thereby controlling the sheet creep and avoiding an increase in cover width that can occur using single or double clips.

Screw fasteners used to fix self-supporting cladding clips should comply with the design wind load requirements.

Clips that in very severe environments should be coated with a corrosion resistant paint system

 

7.8.4 Cleats 

 

A cleat is a continuous metal under-flashing installed behind the leading edge of a metal capping or flashing. Cleats secure cladding or flashings to the substrate or structure using a slip joint or by crimping the leading edge of the flashing to the cleat.
Cleats and clips should be accurately aligned and clinched after fixing to avoid vibration or chatter, but should still allow for expansion of the flashing.

Cleats are fastened to the substrate using mechanical fasteners and should be made from the same metal as the flashing or sheeting.

Cleats used at the eave and barge should be made from metal of an equal or heavier gauge than the flashings or cladding because of the increased wind load at the roof perimeter. To allow for differential expansion and contraction, the flashing should be securely hooked to the drip edge of the cleat but should not be attached directly to it.

 

 

 

7.8.5 Seams 

 

Flashings can be joined together by a number of different types of seam to avoid a plain lap joint without sealant. If the joint is likely to retain moisture and it is required to be sealed, the sealant should be introduced into the joint before it is completed.