Installing Flashing

General

The purpose of a flashing is to divert water away from any point of entry, and to make any building weatherproof. Flashings and cappings are strips of metal formed to weatherproof the edges of roofing and walling.

Similar methods of flashing are used for different cladding profiles. You can adapt the principles to suit your application. In all cases it is important to have ample cover provided by the flashing and proper turn-up of the cladding underneath. We can supply a range of standard flashings. We can also supply custom flashings to your requirements – please download the custom flashing form here.

Flashings are not only required to provide weather-resistance to the many junctions on a roof or wall structure, but are also a very visible part of the roof and wall cladding design, and perform an important role in the aesthetic appearance of the building.

Experience is required to design, cut and fasten flashings that will last the lifetime of the roof or wall cladding, and specialist assistance should be sought where this experience is not available. All flashings must be designed to prevent ponding of water or build-up of debris.

Flashings must be designed to provide weather-resistance for the roof or wall cladding without reliance on sealant as the prime means of providing weather resistance. Unpainted galvanised steel is incompatible with most inert materials and is subject to inert catchment corrosion.

 

Materials & finishes

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. (*see below) When this is not possible, or if different materials are intended or specified, the alternative materials should be compatible for both contact and runoff. The most widely used cladding profiles are listed below. They are available in colorbond® prepainted steel, or in unpainted zincalume®  aluminium/zinc alloy-coated steel.

 

¨      Apron Flashing

¨      Valley Flashing

¨      Roll Top Ridge Flashing

¨      Fascia Flashing

¨      Flat Ridge Flashing

¨      Barge Capping Flashing

¨      Box Guttering Flashing

¨      Parapet Flashing

¨      Pipe Flashing

¨      Barge Roll Capping

 

*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 occur 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 11 or 12° to avoid ponding.

 

Low roof pitches

Unless there is adequate positive fall in a roof, there is danger of ponding, which can lead to a reduced service life, particularly in coastal areas. At low slopes, say around 1 in 50 (1°) slope, all roof supports

must be in the one plane because slight variations can result in zero or negative fall. This may occur even after completion of the building as the result of settlement, timber warping or shrinking, or extra loadings (such as air conditioning units).

 

Maximum Length Of Roofing

The valleys (or pans) of roofing have to carry water to the gutters. If the valleys overfill in heavy rain, water can flow into the roof through the side-laps and flashings. Factors affecting waterproof and drainage capacity of the laps of a profile include:

¨      The width and depth of the valleys or pans;

¨      The pitch of the roof—rain flows faster on a steeper pitch;

¨      Rainfall intensity for the geographical area;

¨      The length of the roof from ridge to gutter; and

¨      Penetrations that cause nearby valleys to carry extra rain diverted from valleys obstructed by the penetration.

 

Wind Forces On Roofs

Winds create considerable forces on both the topside and the underside of roof cladding, and you must consider these forces in the design and fixing of any roof. The forces are:

¨      Inward forces tending to collapse the roof cladding inwards, caused by wind acting directly on the windward side; and

¨      Outward forces tending to lift the roof cladding from its framing, and the entire roof structure from the rest of the building. Outward forces can be caused both by uplift

from negative wind pressures, outside the building; and by positive wind pressure inside the building.

Generally the greatest wind forces imposed on roofs are due to the outward forces. Because the dead weight of roofing materials is relatively small, the outward forces must be resisted by the roof fasteners.

It is very important that the battens and roof framing are adequately fixed to the rafters and walls, and that claddings and flashings also be fixed to withstand these pressures.

 

Additional Information

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. It also to prevent noise or flutter. 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 outside air 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. An anti-capillary offset fold, a clearance gap of up to 5mm or a sealant should be provided on all flashing edges to avoid capillary action where flashings are in close contact with the roof or wall cladding.

 

Fastening information

Roof flashings & cappings must be fastened to withstand wind pressures and thermal movements. In addition, they should be tightly notched and fastened as per the table below:

 

ROOF TYPE

FREQUENCY

FASTENER TYPE

Concealed fixed roofs

No more than 300mm centres

rivets

Pierced fixed roofs

No more than 300mm centres

Self tapping screws or rivets

Corrugated

Every 4th rib

Self tapping screws or rivets

 

Joints in flashings and cappings other than straight joints should be accurately mitred, fastened and sealed. All joints should be 50mm and fastened at intervals no more than 40mm. Joints should lap in the direction of fall.

 

Box Gutters - for information on Box Gutters click here