COP v3.0:flashings; flashing-materials

7.2 Flashing Materials 

To ensure equal durability flashings, ridge cappings, and accessories should be manufactured from the same material and have the same coating system as the material used for the roof and wall cladding. When it is not possible, or if different materials are intended or specified, the alternative materials should be compatible for both contact and run-off. (See 4.10.3 Compatibility Table

All flashings must be fabricated from a ductile grade of metal and designed for lateral strength by folding, stiffening or ribbing on external edges, with a maximum unstiffened width of 300 mm.

Metallic coated steel flashings must have a bend radius complying with the minimum radii prescribed in 4.11.8 Microcracking to avoid visible microcracking.

Where a flashing is hidden or is otherwise difficult to access or replace, it must have a durability of the life of the building or not less than 50 years.

In some corrosive environments, coated steel is unlikely to achieve such durability in an enclosed environment that lacks ventilation, such as cappings to parapet walls or flashings in contact with concrete.

In those situations, non-ferrous metals or stainless steel should be used.

The use of other metals for flashings requires an understanding of their strengths and weaknesses as any metal will suffer corrosion if used inappropriately.

The minimum thickness for metal flashings must be:

  • Coated steel — 0.55 mm.
  • Aluminium — 0.90 mm.
  • Copper — 0.60 mm
  • Zinc — .070 mm
  • Stainless steel — 0.45 mm

When using pre-coated and AZ coated products and where the requirement is for a profiled shaped flashing or metal closure, such as a ridging or apron, a soft aluminium or proprietary strip can be used. Soft sheet zinc or aluminium can also be worked and shaped by hand to form flashings. Proprietary EPDM penetration flashings are detailed in 8.4 Collar Or Boot Flashings

7.2.1 Compatibility 

Materials must be used in accordance with the 4.10.3 Compatibility Table.

When two dissimilar metals are in contact and moisture is present, one metal is relatively protected while the other suffers accelerated corrosion. Galvanic or bimetallic corrosion can also happen when water flows over dissimilar metals.

The design of flashings has a direct relationship to corrosion, as the underlying design principle is to keep the metal dry, to avoid any retention of moisture by debris, and to provide all flashings with a minimum fall of 1 ½ ° to prevent ponding.

The designer and roofing contractor should be aware of compatibility issues when combining materials in a roof system or placing assemblies such as air-conditioning units, solar collectors, and walkways on top of roof cladding. If materials are combined incorrectly, severe localised corrosion may occur.

Unpainted galvanised steel is incompatible with most inert materials and is subject to inert catchment corrosion. (See 4.12 Inert Catchment)

7.2.2 Design 

All roof cladding edges located other than at the gutter, must have a cover flashing fastened on both faces.

Flashings at the periphery of roof or wall cladding are subject to high suction or negative wind loads that often exceed all other positive imposed loads. As a result , the wind design load can be up to twice that of the main roof area and extra fixings are required to fix flashings.

The design wind load specific to any particular building will determine the number and the spacing of flashing fasteners. There is, however, a minimum number of fastenings that should be provided to avoid flexing, oil canning or fatigue cracking of metal cladding under fluctuating loads and also to prevent noise or flutter. (See 11.1 Roof Noise.)

In very high wind design load areas and where the pitch is less than 10°, mating flashings must be sealed and fastened with sealant at each end of the lap to prevent the ingress of dirt or water.

A hook or a hem must be used to terminate the vertical edge of hidden flashings to avoid capillary action or to prevent moisture ingress. Where a hook is not suitable, the flashing height given in 7.3A Minimum dimension 'C' flashing cover must be increased by a minimum of 25 mm.

A hook is used into an open space when the depth of the hook will not interfere with any fastening. A hem is a flattened hook, but should not be completely closed because then it will act as a bridge and will not fulfil the intended purpose of avoiding capillary action.

A hem is also termed a safety edge as it will protect a person from injury by contact with a cut edge. As all flashings are used in areas that are likely to suffer from extreme macro and micro-climates, the folded edges of any flashings should not have visible microcracking. (See 4.11.8 Microcracking.) 

The minimum edge that can be provided by most long length folding machines is 10 mm, but for material thicker than 0.55 mm or longer than 3 m, the minimum is 12 mm.

Long length folding machines have the folding beam angled at 45° which limits the dimensions that can be folded after making due allowance for springback. (See 7.2.2A Folding Beam.) 

 

7.2.2A Folding Beam

 

The design load and the fixing frequency of all flashings must be determined by the loadings criteria in 16.5 Wind load Span Graphs.

Screws are preferred to rivets for fastening flashings because the larger diameter shank gives a greater shear capacity, and as they have a bigger head a washer can be used to increase the pull over resistance per fastener.

The ingress of rain into the roof or wall cavity via the flashings is predominantly caused by the pressure differential between the air outside and that inside the roof or wall cavity. This pressure differential caused by wind gusting is a dynamic one and fluctuates greatly, which means that a cyclic or pumping action can occur when water is sucked into the joint or cavity that a flashing is covering.

The design wind load is determined not only from the wind speed of the geographical area, but by a number of weighted factors. (See 3.7 Wind Load.) An area with a low wind speed may have a high design wind load and, therefore, any categorical reference to flashing width depending on wind speed alone is invalid.

AS/NZS 1170.2 tables a wind speed of 45 m/s for all of New Zealand for a return period of 50 years, with the exception of the Cook Strait area, which is 51 m/s, although greater consideration is given to other conditions, such as topography, terrain and the micro climate which make up the basic wind speed.

The designer should take these factors into account when determining the flashing design and fastener frequency.

An anti-capillary offset fold, a clearance gap of up to 5 mm or a sealant should be provided on all flashing edges to prevent capillary action where flashings are in close contact with the roof or wall cladding. (See 4.11.6 Capillary Action.)

Flashings, other than standard ridging, are produced to specific order and are designed and manufactured from flat sheet or coil. If these flashings are required to match the colour of the profiled cladding, it is necessary for the pre-painted flat sheet or coil to be made by the same manufacturer using the same process in order to avoid colour mismatching or fading.

The preferred maximum length of flashing is 6 m, with expansion joints provided after a maximum of two lengths of flashing (12 m) have been fixed together, as any lap secured by rivets or screws effectively becomes one length. Flashings are restricted in length in the same manner as roof and wall cladding sheets and are subject to the same requirements and expansion provisions. (See 6.2.2 Roof Cladding Expansion Provisions.)

The edges of all flashings must be spaced at a minimum distance of 2 – 5 mm away from an adjacent horizontal surface to avoid moisture retention and deterioration at the cut edge of coated steel flashings. This clearance applies particularly when a cut edge is very close to concrete, plaster or butyl rubber. When scribed flashings are used, the cut edge must not touch the pan. That causes corrion from abrasion, as well as, from water retension. (See drawing 5.1.2.B.)

Soft edge flashings, also, should be spaced away from the pan to avoid the accumulation of dirt and so that the bituminous backing does not adhere to the sheeting.