COP v3.0:flashings;

External Moisture Flashings 

This section should be read in conjunction with 6 External Moisture Overview and  9 External Moisture Penetrations.

The purpose of a flashing is to divert water away from any point of entry and to make a building weatherproof.

Flashings are not only required to weather the many junctions on a roof or wall structure but are often a highly visible part of the roof and wall cladding design. Therefore, they perform an important role in the aesthetic appearance of the building.

It takes longer to make and install flashings than fixing roof or wall cladding, so designers should be aware of the cost effects of design complexity.

8.1 Flashing Materials 

Exposed flashings are typically manufactured using the same base metal material as used for the roof and wall cladding. If 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. When it is not possible, or if different materials are intended or specified, the alternative materials should be compatible considering both contact and runoff. (See 4.9.4 Compatibility Table)

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

Vertical faces of flashings such as barges and fascia can exhibit oil canning. This can be minimised by using heavier gauge material, by forming stiffeners, or by providing clip fastened attachment systems which allow for thermal movement. In critical visual areas, the COP recommends a maximum unstiffened depth of 200 mm on the vertical face.

Metal flashings must have a bend radius complying with the minimum radii prescribed in 4.10.8 Microcracking.
The minimum thickness for metal flashings must be:
  • Coated steel — 0.55 mm,
  • Aluminium — 0.70 mm,
  • Copper — 0.60 mm,
  • Zinc — 0.70 mm,
  • Stainless steel — 0.45 mm.

Proprietary EPDM penetration flashings are detailed in 9.5 Boot Flashings.

8.1.1 Flashing Durability 

Where a flashing is hidden or is otherwise difficult to access for maintenance or replacement, or it requires cladding to be removed to replace the flashing, it must have durability for the life of the building or not less than 50 years. All flashings should be made from materials with a durability expectation not less than that of the cladding.

In some corrosive environments, stainless steel or non-ferrous metals should be used for flashings. All metals must be checked for compatibility with the roof material. (See 4.9 Compatibility and 4.9.4A Interactive Material Compatibility Tool)

8.2 Flashing Design 

Transverse flashings run across the roof slope at right angles to the ribs of the roof and longitudinal flashings run down the roof slope.

Ingress of rain into the roof or wall cavity via the flashings can be caused by the pressure differential between the air outside and that inside the roof or wall cavity. The pressure differential caused by wind gusting fluctuates greatly, so a gap should be created behind the outer edge of a flashing to provide a pressure cushion. Longitudinal flashings are best designed with a pressure equalisation gap to balance varying pressures and prevent capillary action. (See 4.10.6 Capillary Action.)

The preferred maximum production length of flashing is 6–8 m, depending on profile strength. As any sealed lap secured by rivets or screws effectively becomes one length, provide expansion joints where required. Flashings are similarly restricted in length as roof and wall cladding sheets and are subject to the same requirements and expansion provisions. (See 7.3.2 Roof Cladding Expansion Provisions.) Inadequate provision for flashing expansion can also cause roof noise

Avoid wet contact between the edges of flashings and concrete, plaster or butyl rubber (See 4.9 Compatibility.) When notched flashings are used, the cut edge must not touch the pan, as that can cause corrosion from abrasion.

 

 

8.3 Flashing Cover 

The weathertightness of a flashing is determined by the flashing cover, not the flashing dimension.

The minimum dimension for flashing cover (C) over profiled metal roof and wall cladding must correlate with 8.3A Minimum Dimension 'C' Flashing Cover.
There are two categories of exposure or pitch that determine the flashing cover width. Extra High Wind Zone can be extended to include wind speeds of up to 60 m/s.
Category A:
  • Low, Medium or High Wind Zones, and
  • where the pitch is no less than 10°.
 Category B:
  • Very High and Extra High Wind Zones, or
  • where the pitch is less than 10°.

NOTE:

This chart is relevant to trapezoidal profiles with a height of 25 – 34 mm and standard corrugate. Barge and apron flashings over profiles with a minimum 35 mm may have one rib overlap.

8.3A Minimum Dimension 'C' Flashing Cover

TYPEDirectionCategory ACategory BDrawing
RIDGETransverse over sheeting130 mm200 mm8.3B Ridge Flashing
BARGE

Longitudinal trapezoidal

& tray

one ribone rib, two ribs (<20 mm) 
 Longitudinal corrugate2 corrugations3 corrugations 
 Vertically down face (smooth)50 mm75 mm 
 Vertically down face (profiled)75 mm100 mm 
APRONTransverse over sheeting150 mm200 mm8.3C Transverse Apron
 

Longitudinal trapezoidal

& tray

one ribtwo ribs * 
 Longitudinal corrugate2 corrugations3 corrugations 
 Vertically up face (smooth)50 mm+ hem or 75 mm75 mm + hem or 100 mm8.3C Transverse Apron
 Vertically up face (profiled)75 mm + hem or 100 mm100 mm + hem or 125 mm8.3C Transverse Apron
PARAPETVertically down face (smooth)50 mm75 mm8.3D Parapet Cap
 Vertically down face (profiled)75 mm100 mm8.3D Parapet Cap
CHANGE OF PITCHTransverse over sheet150 mm2008.4.4.2A Change of Pitch Junction Flashing
Transverse under sheet250250 

 

 
* Not to exceed 300mm.

The end of wall cladding should have a minimum clearance of 25 mm from an apron flashing or another horizontal surface. Where vegetation is present, or debris may accumulate, greater clearances or regular maintenance is required to achieve optimum durability. (See 4 Durability)

The dressed soft edging or the downturn of a notched flashing acts as a baffle to wind and rain at the outside edge of a transverse flashing, as does the stop-end of the cladding at the upper end. The void in between these two barriers acts as a pressure equalisation gap.

Pull-up stop ends must be provided for all pitches in all wind zones

In extreme circumstances, a profiled or notched closure can be used between these two baffles if required. When closures or filler blocks are used, they must be adhered to the profile or secured by the cladding fasteners to ensure they remain in position. Ventilation requirements must be considered when using profiled closures.

8.3E Barge Flashing Over One Trapezoidal Rib, Smooth Vertical Cladding

Cover two ribs with low rib trapezoidal on residential when the pitch is less than 10° and the wind zone is very high or extremely high. The face edge of a barge flashing must be attached to the fascia or wall cladding with pierce fasteners or sliding clips

8.3M Ridging and Other Transverse Flashings

A soft edge or notched flashing must be provided to transverse flashings. The positioning of the top purlin must be adjusted with the pitch and the ridge girth to ensure that primary fastening is provided between 15 mm and 50 mm from the leading edge of the flashing.

8.3N Apron Upstands

The upstand of an apron flashing varies depending on whether the upstand is hemmed or unhemmed, and the covering cladding is smooth or profiled. Profiled claddings require more coverage because they allow vertical air movement through the voids of the profile.

8.3.1 Flashing Laps 

Laps in flashings should comply with the following criteria:

  • Water must flow over a lap, not into it,
  • a lap must not rely solely on sealant,
  • sealed laps must be mechanically fixed at 50 mm centres, and
  • end laps must be a minimum of 150 mm for unsealed laps and 100 mm for sealed laps.

8.4 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.

8.4.1 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.

8.4.1C 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  8.4.1C 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 Ridging End-Closure )

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.

8.4.2 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.

8.4.3 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 8.4.4B 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.

 

8.4.3A 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.

 

 

 

8.4.4 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 8.4.4F 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.

 

 

8.4.4A Chased Apron and 8.4.4C Angle Diverter are not the preferred installation methods for apron flashings. 8.4.4B 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 (8.4.4G 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.

8.4.4.1 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 8.4.4.1A Parapet/Apron Ridge Cap.
  2. By making a separate saddle flashing as shown in 8.4.4.1B Separate Saddle Flashing.

 

 

 

Parapet cappings should have a separate cap at the apex or be joined as shown in 8.4.4.1D One-piece Parapet Ridge.

 

 

 

8.4.4.2 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 8.4.4.2A 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.

 

8.4.4.3 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. 

 

 

8.4.4.4 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, however, serve a purpose in many applications of improving the durability of roofing at the eaves.

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.

Eaves flashings are recommended in Marine Environments to improve the durability of the roof cladding, and are required for pre-painted aluminium roofs (see 14.9). Eaves flashings may be pre-painted steel or aluminium (either pre-painted or uncoated). To minimise the build-up of salt deposits at the eaves, a high fronted gutter fully covering the roof crests, attached closely to the fascia, is recommended in Severe and Very Severe Marine environments.

 

8.4.5 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.

8.4.5.1 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 14.22 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”. 

8.4.5.2 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.

 

 

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 14.13 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.

8.6 Sealants 

Sealants play an important part in cladding because AZ or paint coatings cannot be sealed using solder.

The purpose of sealants used in the joints between metal roof and wall cladding and flashings is to prevent the entry of rain or snow and to exclude dirt, debris, and moisture by capillary action. They may also have varying degrees of adhesive action.

Sealants are generally Neutral Cure silicone or Modified Silicone (MS) types. The latter is generally paintable. Proprietary self-adhesive tape sealants are also entering the market and give excellent performance in certain conditions. Acid cure sealant, designed to bond to inert surfaces such as glass, must not be used.

Designs using sealants to seal sheet components must always include a mechanical fastening such as a screw, rivet or a mechanical seam. 

Generally, the function of a sealant is carried out by that sealant being sandwiched in the joint. In most cases, the post application of sealant to the outside of a joint is both futile and unacceptable.

The sealant does not require significant total adhesive strength, but it should bond positively to both surfaces as a continuous layer. To ensure complete sealant cure, the width of sealant in a lap should not exceed 25 mm when compressed

8.6.1 Methods Of Sealing 

There are two main methods of sealing metal sheets — gun applied sealants and self-adhesive tapes.

Sealants should have a low resistance to compression so that metal sheets can be brought closely together by their fasteners and not held apart by the sealant.

8.6.2 Surface Preparation 

For adequate bonding, all surfaces must be clean, dry and free from contaminants, such as rolling oils, dirt or dust. 

8.6.3 Sealant Extrusion And Placement 

Sealant should be gun-applied in a continuous flow to provide a joint without gaps or voids. This will prevent the entry of both condensation from above and water drawn up by capillary action, either of which can cause metal corrosion.

After placing the sealant cartridge in the gun, the tapered nozzle should be cut with a sharp knife at an angle of 45 ° at a position which would provide a bead of sealant approximately 6 mm in diameter.
 

Sealed rivets are the preferred fastener. The stem holes of unsealed blind rivets must be sealed with silicone, which can attract dust and look unsightly.

Sealant should be placed in a continuous bead close to the weather end, covering any pre-drilled rivet holes. If a complete ring of sealant is applied around the fastener, an air pocket forms which compresses during tightening of fasteners. It can blow a channel, thus preventing an effective seal.

The best method for lap fabrication is.

  • Assemble and drill the components. Rivets should be positioned at 50 mm spacings.
  • Separate the components and remove any drilling swarf.
  • Remove any strippable film.
  • Clean the joint.
  • Apply a continuous sealant bead as described above.
  • Relocate the components, align the holes and fasten.
  • Externally seal the centre of all rivets if required.
  • Remove surplus sealant after it has cured (not while uncured), to avoid smearing.

When placing the overlapping sheet, care should be taken to avoid disturbing the sealant when fastening.

Excess sealant should be removed with a plastic spatula or purpose made plastic scoop as excess sealant collects dirt that is unsightly.

Visible excess of sealant is not an acceptable trade practice.

8.7 Profile Closures 

Profiled closures are available for all profiles. They are available in narrow tip (for going under the profile) or wide tip (for fitting over).

As they restrict the free movement of air, their use should be restricted to where they are required for weatherproofing and appearance (such as scribers under window jamb flashings) or for maintaining positive internal pressures, such as in building with a managed internal environment. Profiled closures should be of closed cell type, rather than bitumen impregnated.

In marine areas, corrosion is likely to initiate under apex flashings and under the eaves overhang, regardless of whether profiled foam is used. A high fronted spouting is the best protection against the eaves corrosion. In very severe cases, or where there is a gap between the back of the gutter and the fascia, an eaves flashing of durable material (e.g., aluminium or plastic) can be used to prevent much of the salt build-up on the underside of the roof cladding.  (See 8.4.4.4 Eaves Flashing)

8.7E Eaves Comb

For vermin control, a perforated scribed aluminium flashing, or an eaves comb, may be used.

In marine areas, corrosion is likely to initiate under apex flashings and under the eaves overhang, regardless of whether profiled foam is used. A high fronted spouting is the best protection against the eaves corrosion. In very severe cases, or where there is a gap between the back of the gutter and the fascia, an eaves flashing of durable material (e.g., aluminium or plastic) can be used to prevent much of the salt build-up on the underside of the roof cladding.  (See Eaves Flashing) 

8.7F Eaves Corrosion due to Low Fronted Gutters

Low fronted gutters in severe conditions will often lead to eaves corrosion

8.7.1 Soft Edging 

To prevent the ingress of weather, dirt, or birds, it is common practice in New Zealand to close the openings created by the pan and ribs of profiled sheets using notched metal flashings or soft edging. Lead was the traditional material for this application, but as it is incompatible with AZ coatings, soft aluminium edging or composite materials are offered as an alternative.

Proprietary soft edging is available for corrugated and low trapezoidal profiles, consisting of an expandable aluminium mesh with a flexible backer.

All soft-edged flashings should be primed and colour painted before installation — but the preferred option is for the paint finish on soft edges to be factory painted under controlled conditions.

Soft-edged flashings are available in a partially-backed form to facilitate passive ventilation at the apex. (See  Internal Moisture)

 

8.7.2 Notched Edging 

Where the profile height exceeds 30 mm, a notched flashing is the preferred option.

All scribed edges should have clearance to avoid damaging the coating of the roof sheet. The scribed metal edge of any flashing must have a clearance of 2 mm to 5 mm.

8.8 Flashing Buckling 

 

 

 

Before any transverse flashings are fixed, the framing timber must have a maximum moisture content of 12%.

Transverse flashings, such as ridging, are sometimes prone to compressive buckling which is blamed on metal expansion but is usually due to timber shrinkage, and a phenomenon known as compression timber.

 

8.8B Compressed Timber

Compression wood has a darker appearance in the early wood (summer) growth rings; to visually distinguish such severity levels in sawn timber accurately requires microscopic identification. (Source: Rooflink)

Abnormal growth causes this defect in timber, which can cause shrinkage ten times more shrinkage than normal.

It is not easy to recognise compression timber visually and roofers are advised to measure the moisture content, particularly of ridge purlins, before fixing. Framing timber must have a maximum moisture content of 12% before any transverse flashings are fixed.

Alternatively, use steel top hat purlins.

8.9 Flashing Expansion Details 

Expansion should be considered at the design stage, and the flashing details should be included in the working drawings and tender documents. As flashings cannot move in the lateral direction without stress, they should have some provision for longitudinal expansion when their length exceeds 18 m, or for lesser distances for visual flashings such as fascias and facades.

When using an expansion joint, independence of movement should be assured by the omission of any fixing through both sides of the joint. Preferred and acceptable slip joint expansion details are shown in 8.9A Apron Slip-joint and 8.9B Ridge Slip-joint. Soaker expansion joints relying on sealant should not be used.

Slip joints require sealants, not to make them weatherproof, but to exclude dust and dirt from two close fitting surfaces, which can retain moisture by capillary action and cause corrosion. Rivets and other fasteners must not prevent movement of the slip joint.

 

 

 

8.9.1 Flashing Expansion 

Before any transverse flashings are fixed the framing timber must have a maximum moisture content of 18%.

Transverse flashings, such as ridging, are sometimes prone to excessive buckling which is blamed on metal expansion but is usually due to timber shrinkage, and a phenomenon known as compression timber. Abnormal growth causes this defect in timber, and it can shrink up to 10 times more than normal. It is not easy to recognise compression timber and roofers are advised to measure the moisture content, particularly of ridge purlins, before fixing.

An alternative is to use steel top hat purlins.

Fixing roof cladding should be treated in the same way as internal linings, i.e., do not fix transverse flashings when the moisture content of any timber is more than 18%. The thickness of flashings should always comply as specified in 8.1 Flashing Materials

If flashings are positively fixed, framing timber that does not meet this requirement can cause failure of ridging and flashings due to timber shrinkage when drying.

The metal expansion allowances quoted in many publications can be misleading because the information is based on theoretical metal expansion values and is not related to real-world conditions.

Figures published for metal expansion rates are given linearly per degree, but it does not take into account the many other factors that mitigate the theoretical figure.  (See 7.3 Thermal Expansion And Contraction.) 

It is necessary to make provision for cladding and flashing movement; when long lengths are used and positively screwed or riveted together, they should be regarded as one length.

The maximum length before expansion provision should be made for either cladding or flashings will vary according to colour, micro-climate, ventilation and fixing spacings. It is , however, possible to provide indicative figures based on a study of empirical data over time. The maximum recommended flashing length without any expansion provision is similar to that of roof cladding. (See 7.3.2 Roof Cladding Expansion Provisions.)

Aluminium rivets, which have a low shear value, will fail when there is no provision made for expansion in flashing lengths of over 12 m. Using aluminium joints is only acceptable if they are used at the prescribed distances, and are not used to replace expansion joints.

Lengths of coated steel ridging, cappings, and apron flashings over 12 m should have a slip joint as described in 8.9 Flashing Expansion Details.

Inadequate provision for expansion can also cause Roofnoise.

8.9.2 Building Expansion Joints 

Expansion joints should be designed to accommodate contraction and expansion. Expansion joints should be detailed and constructed to a minimum height of 100 mm above the roof cladding, and curb-type expansion joints should be designed and installed to ensure drainage of the roof and to prevent any damming of water.

Wood curbing secured to the substrate on both sides of an expansion joint should be flashed with a metal capping capable of acting as an expansion joint cover.

Expansion joints have two main designs.

1. A 8.9.2A One-piece Expansion Joint to accommodate movement using a central bellows or roll that allows the flashing to be positively fixed on both sides.

2. A two-piece design to accommodate movement by the use of hemmed edges, with sufficient clearance for the expected movement.

Both of these designs are shown as a parapet following the pitch of the roof. Where this is not the case, the top of flashing should have a 10˚ slope as for all other parapet flashings. (See 8.4.3 Parapet Cappings).

Metal wall construction joint flashings that are embedded in the wall should be made with a bellows or other means of accommodating movement without fatigue and have a durability of 50 years.

 

8.10 Metal Wall Cladding Flashings 

Flashing details for cladding using profiled metals should:

  • have two lines of defence at any point,
  • not rely entirely on sealant,
  • not unnecessarily bridge the cavity, and
  • give water a means of escape when it does get in.

Details from WANZ, cladding suppliers, or bespoke solutions may be used. In the case of bespoke solutions, it is critical that the intersection of the different elements is considered holistically, particularly at the critical head/jamb flashing intersection. Changing one element of a proprietary solution may have unconsidered ramifications on the performance of other elements.