COP v3.0:flashings; flashing-types

8.5 Flashing Types 

Different types of flashings include:

  • Ridge and Hip.
  • Barge and Verge.
  • Parapet Cappings.
  • Apron.
  • Valley.
  • Curved.

See 9 External Moisture Penetrations for penetration flashings.

8.5.1 Ridge And Hip 

Ridge and hip roll top flashings are roll-formed as a standard pitch flashing with a soft edge. When the angle of the flashing is not custom-made to suit the roof, it can result in visible distortion and stress around the fastening if used on pitches steeper than 35°.

Custom-made square top ridging is available for any pitch and width and is available in lengths of up to 8 m. The roll top or square top of a ridging helps accommodate expansion of the roof sheeting and strengthens the ridge.

A vee ridge is not able to support walking traffic and is vulnerable to buckling caused by point load and lateral or longitudinal thermal movement.

A soft-edged flashing can be used for corrugated and low rib trapezoidal profiles, and notched ridging can be used on all trapezoidal, trough or standing seam profiles.

 

 

8.5.1C Ridge End

 The transition of the ridge and the apex of a hip requires skill to make a neat and weathertight finish and relies on sealant for prolonged weather tightness. An under-flashing such as soft aluminium, waterproof tape, or EPDM membrane is recommended as a secondary means of making this joint waterproof in conjunction with the ridge-hip intersection design. (See  8.5.1C Ridge End)

The gable end termination of roll ridging must be made vermin proof by cutting the ridging back 25 mm and closing it. (See 7.4.1.1C Roll-top End Cap )

As an alternative to continuous ridging on ribbed profiled sheets, use individual rib or ridge caps fitted to each rib after the ribs have been cut and the roof cladding bent over the ridge. Each cap should be accurately fitted and sealed. This method does not allow for free expansion at the ridge of long length roof cladding and restricts natural ventilation of the ceiling cavity. These caps rely on sealant to be made watertight and are only recommended for use at the apex of a roof and when there is sufficient ventilation.

 

 

 

8.5.2 Barge And Verge 

Barge or verge flashings assist in holding the roof cladding in place under high wind loads at the periphery.

Where barge flashings are omitted, the roof must be designed to withstand the upwards loads imposed on both surfaces of the roof cladding edges.

A transition flashing must be provided when the roof cladding weathers the barge at an internal angle.

 

8.5.3 Parapet Cappings 

Cappings are flashings used to cover the top of a parapet wall to protect the wall from the ingress of moisture. Instead of chasing into the wall and using step flashings, the preferred detail is to cover the inside of the parapet wall with vertical ribbed metal or other cladding material, used in conjunction with an apron flashing. (See 8.5.4B Vertical Cladding (Parapet Flashing and Detail).

Capping corners and expansion joints located at the corners must be sealed within 2 m from each direction of the corner measured on the interior side.

The corner of the wall is a fixed point for the capping, and as thermal movement can only occur in one direction away from the corner, an expansion joint should be provided as shown.

 

8.5.3A Parapet Corner Capping

A slope is required to drain water from the top surface and avoid ponding

Parapet cappings must have a positive slope of 5˚ onto the roof side, but parapet cappings that follow the slope of the roof do not require any fall across its width. Parapet cappings wider than 300 mm can fall both ways, with the 5˚ drainage angle provided on both sides.

Cappings must not be fixed to the structure through the top of the capping, but can be fixed on the vertical leg by one of three methods. (see drawing 5.3.4. B, C, & D).

  1. A continuous cleat is fastened on the exterior face of the wall with an open hem at the drip for ease of application. After the exterior face of the capping is hooked to the cleat, the capping on the interior side is secured to the parapet using washered fasteners through oversized holes.
  2. Intermittent clips are fastened to the exterior face of the wall at 600 mm centres. All cleats and clips should be accurately aligned and clinched after fixing, while still allowing for expansion without chatter or vibration.
  3. The capping is fastened to the parapet on both sides using screws with sealing washers through oversized holes.

 

 

 

8.5.4 Apron Flashings 

Cladding should not fit tightly onto the horizontal surface of any apron flashing; as it could retain moisture and collect dirt and debris. It should have a minimum clearance of 25 mm to allow for cleaning and maintenance.

Apron flashings should be in position before any cladding is installed. Where the finish above the apron is plaster or the durability of the cladding is 50 years, the hidden apron flashing is also required to have a 50-year durability. This durability can be achieved by using a non-ferrous material for the flashing. Alternatively, a two-piece apron should be used where the over flashing has 50-year durability, and the apron flashing can be renewed independently. (See 8.5.4F Two-piece Apron.)

When the parapet walls will be plastered or when other trades are likely to follow the roof cladding installation, the roof cladding and flashings should be protected from damage. Provision should also be made for working and walking on the roof cladding during this time.

Where an apron abuts a block, brick, or concrete wall without any cladding above it, the apron flashing can be weathered by a step or a continuous chased flashing. These flashings can be made from aluminium, stainless steel, or zinc when used with metallic-coated steels and should extend 25 mm into the wall. They should be hooked or mechanically wedged and should be sealed using a compatible, flexible mortar or sealant. Over-clad flashings give better weather protection than chase-cut flashings.

 

 

8.5.4A Chased Apron and 8.5.4C Angle Diverter are not the preferred installation methods for apron flashings. 8.5.4B Vertical Cladding (Parapet Flashing and Detail)shows a better method that provides more positive weathering by covering the wall with vertical metal cladding up to the capping.

It is not possible to ensure that all the water discharged from the apron flashing will be collected by the spouting, unless special provisions are made including:

Spouting should be fitted after the wall has been finished and should be clear of the wall cladding.

Aprons should be stop-ended and turned down to weather and bird-proof the end of the apron at this junction.

When flashing a cavity parapet, the apron should be in place before the cavity batten as it is not possible to retrofit the apron. The apron material should have a 50-year durability unless the parapet cladding is easy to replace.

When a chase or rebate (8.5.4G Chase-fixed Flashing)  is not provided, a pressure bar flashing can be used to weather an apron flashing into a concrete tilt-up slab wall. 

Over-clad apron flashings are always preferred to chase-cut alternatives in structures such as residences where the occurrence of chase-cut weatherproofing failure could have severe consequences.

8.5.4.1 Parapet And Apron Capping Terminations 

A junction where and apron flashing meets a ridge can be weathered by two methods.

  1. By carrying the apron over the ridge and covering the joint with the ridging as shown in 8.5.4.1A Parapet/Apron Ridge Cap.
  2. By making a separate saddle flashing as shown in 8.5.4.1B Separate Saddle Flashing.

 

 

 

Parapet cappings should have a separate cap at the apex or be joined as shown in 8.5.4.1D One-piece Parapet Ridge.

 

 

 

8.5.4.2 Change Of Pitch 

Differential movement will happen at any change of roof cladding pitch where the sheets are overlapped and butted together. This movement causes noise and deterioration of the coating. A separate apron flashing as shown in 8.5.4.2A Change of Pitch Junction Flashing is required to prevent that.

A junction flashing must be used where sheeting is cut at a change of pitch. The junction must be hooked and have the minimum coverage as required according to the exposure category in 8.3.1 Flashing Laps. The underlay must overlap the flashing as shown.

 

8.5.4.3 Step Apron 

A step apron or waterfall flashing must be used where the length of roof cladding is more than can be transported, or exceeds the recommended length for expansion as shown in 7.3.2 Roof Cladding Expansion Provisions.

The designer should allow a minimum 20 mm step in the purlin height to accommodate a step apron, giving a total change of height equal to 20 mm plus the profile height. The purlin height can be adjusted at the purlin cleat or by using a different size purlin. 

 

 

8.5.4.4 Eaves Flashing 

Gutter eaves flashings are not required for weatherproofing unless the building is in a very exposed location and the eaves are not protected by spouting. They can serve a purpose in many applications of improving the durability of roofing at the eaves (see 4 Durability).

The flashing should extend into the gutter, and the underlay finishes on the down-slope of the flashing. If there is no over flashing to the gutter, the underlay should be extended into the gutter by a maximum of 20 mm.

Unwashed flashings should be made of durable materials such as organic coated steel, aluminium, or PVC.

Eaves flashings as referred to in E1/AS1 are not required for weatherproofing unless the eaves are unprotected by spouting, but they can make a contribution to durability. (See 3. Durability)

 

8.5.5 Curved Flashings 

Drape curved and crimp curved metal roof cladding requires curved barge and apron flashings which may not always have an even radius.

There are two different ways to form curved flashings, all of which require specialist equipment.

  • lock-forming or lock seaming; or
  • crimping.

Manufacturing curved flashings is a highly skilled operation and should be done in a factory by skilled workers.

8.5.5.1 Lockseamed Flashings 

All lock-seamed flashings should be custom-made to suit the profile shape and the rib distance from the fascia with sufficient downturn on both sides to weather the junction. Lock-seamed flashings are made in two parts, the horizontal (part a) and the vertical (part b). Edges can be either crimped or lock-seamed, as shown in these drawings.

 

The vertical barge component (part b) can be cut to the curve and lock-seamed, with the horizontal component (part a) hooked and seamed jointed. (See 13.8 Tools Of The Trade.)  The outside edge can be crimped as shown here, or hemmed.

Shallowly curved flashings can be folded straight in one piece and crimped on both edges. Fix the turn-down at the curved rib by “mini-crimping” or “gear-crimping”. 

8.5.5.2 Crimped Flashings 

Crimped flashings are suitable for use on radii of more than 2 m, with the crimp spaced and deformed at regularly spaced intervals. A flat strip should be placed on the template and crimped until the strip fits the template.

Because the flashings are seen from much closer, domestic clients may find this type of flashing aesthetically unacceptable.