COP v3.0:flashings; fastening-flashings

8.5 Fastening Flashings 

 

Flashings must be fastened in one of three ways.

 

  • By primary fastening – fastening into the main structure.
  • By secondary fastening — fastening 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 14.12 Fasteners.

 

8.5.1 Primary Fasteners 

Primary fasteners are fixings that attach flashings to the structural building frame and which should withstand all the loads applied to the cladding, including expansion provisions. Primary fasteners are relied on for structural performance. Flashings should be fastened at a point within 25 mm from the exposed edges of the flashing, and the primary fastener spacing should be to each structural member it crosses.

Flashings should be fixed on both edges.

Putting bonded or embossed washers under all primary fasteners through the horizontal upper surface of flashings improves weathertightness.

8.5.2 Secondary Fasteners 

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

Sealing washers are required on all secondary fasteners, except under rivets which should themselves be sealed or self-sealing.

8.5.3 Flashing Cleats 

 

A flashing 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.

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.

 

8.5.4 Seams 

Flashings can be joined together by various 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.