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.)
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.