What are flashings and are they important?
A component formed from rigid or flexible waterproof material that drains or deflects water back outside the cladding system NZ Building Code
A more specific definition for windows and doors would be:
A metal, plastic or waterproof material formed around a window or door at the cladding to window junction to ensure that any water that penetrates the critical junctions between the window and cladding is harmlessly dispersed to the exterior. If well designed and installed, the flashings will prevent external moisture from reaching the framing or any part of the interior of a building thus ensuring the health of the building users, lowering heating bills and preventing damage to the building.
Why are window and door flashings important?
Although window and door flashings are often visually unimportant in terms of aesthetics and value because they represent a tiny part of the overall build cost, they are the most important items in terms of the safety and weathertightness of the building.
If they are wrongly designed or installed, or both, they will be prove to be the most costly error of oversight in the entire construction apart from any major fault in the foundations or ground stability.
Foundation failure is very rare. However all buildings have windows and doors. How they are flashed is of paramount importance.
What constitutes a sound or good window or door flashing?
About 10 years ago an astute Canadian architect Don Hazeldean discovered the prime causes behind leaky home syndrome that occurred in Vancouver Canada were due primarily to bad flashing practice. Don looked at the essential purposes of flashings and identified these principles in a simple but complete conceptual phrase, the 4 D's.
The 4 D's clearly explain sound flashing design. The phrase and the principles behind the 4 D's have been adopted by the NZ Building Code, by weathertightness experts around the world and are now taught in building training establishments.
What are the 4 D's?
They stand for four design principles that should exist in each and every flashing.
- Deflection prevents water and moisture gaining entrance to the building structure as the first line of defence
- Drainage Should any moisture penetrate behind the flashing (occasionally) this water should have a pathway via a drainage system which will eventually and harmlessly disperse this unwanted water to the exterior.
- Drying To assist in the removal of any moisture that does ( occasionally) breach the first line of deflection defence.There should exist a suitable amount of fresh airflow to remove the moisture by evaporation
- Durability The flashing material and design should be tough enough to endure for the life of the cladding and therefore maintain the weathertightness protection.
Deflection is the top priority. If water cannot get into the building structure then there is no need for drainage (this is how we flashed buildings for over a century in NZ and overseas). Drainage is backup where a possible occasional leak might occur. If water did penetrate behind the flashing Drying will assist the removal of moisture but this will depend on the dampness in the air and the weather. If the weather is predominantly wet over long periods as in Vancouver and Auckland (which have similar rainfall patterns and volumes) then the drying aspect will be very low and even negligible.
Bad Flashing Practice
Misuse of sealants. Sealants have been around a long time in NZ and have proved in certain circumstance to be valuable but they cannot be compared performance wise to a robust,mechanical metal flashing system that has been properly designed in accordance with the 4 D's and in particular with excellent Deflective value.
Sealant manufacturers, Branz and weathertightness experts advise that sealant must only be applied in a 2:1 ratio of width to depth. In other words the sealant should be twice as wide as it is deep. What is commonly seen on site and which passes for flashing is sealant placed against the powder coated aluminium joinery and then smeared with a dirty finger to create a temporary seal between the cladding and the window junction. This is totally contrary to correct sealant application. Such poor but common practice will cause the joint to fail and leaks will occur. The sealant manufacturer will not warrant their products when incorrectly applied in this manner.
Sealant does not adhere well to powder coating and therefore the practice of "flashing" the window or door frame to the cladding with sealant represents bad building and flashing practice.
Another very common but totally incorrect practice is to smear sealant around window and door to cladding junctions without placing a Bond Breaker ( such as a PEF rod) at the back of the sealant. Sealant must not be held captive on three sides. Sealant only works where it is held on two sides and the back (or third side) is allowed to move freely to accommodate the inevitable movement in contraction and expansion between the materials it is in contact with. Clearly plaster, aluminium, wood and glass all move at different rates. If a thin smear of sealant is used against these different materials and is not allowed to move at the Bond Breaker, the seal will break and leaks occur.
Sadly, sealant applied in this manner is the normal flashing method on site for commercial and residential construction these days. These methods have failed badly in the past and will continue to fail. There will be no comeback against the sealant manufacturer as the product has been wrongly applied.
A complete integrated flashing system is required
NZBC flashings do not show flashing junctions. Most drawings do not show flashing junctions. This is because they are very hard to design, fabricate and install effectively. If the flashing junctions are not clearly shown, how can the flashings be fabricated and installed on site?
A window and door has cladding junctions at the head, jamb and bottom of all joinery.
It therefore makes perfect sense that there should be a specific flashing at each of these junctions between the aluminium joinery and the cladding?
More importantly the separate parts of a window and door flashing system should have clearly designed and precisely fabricated flashing junctions. Sadly this is not the case. The NZBC does not show flashing junctions in 3D so that the flashing fabricator, designer and builder can accurately create and install the window flashings. Instead smeared sealant is the common approach and represents poor flashing design.
Cladding manufacturers who employ NZBC E2/AS1 details also have disclaimers on their technical data stating that while flashing designs are provided they do not accept responsibility for these drawings and that this responsibility lies entirely with the designer and builder and not with the cladding company.
Why do they say this? Because they do not have faith in either the flashings or the installers of such flashings. Monolithic cladding manufacturers do not carry the responsibility for flashings on their proprietary systems but pass this responsibility to their licensed applicators.
A complete flashing system is needed
A sound flashing system for aluminum joinery should be virtually bullet proof. To achieve this it should have the following elements and credentials.
- Be designed exclusively for the unique requirements of aluminum joinery
- It should comply with all the 4 D's and especially Deflection
- Be made of strong extruded aluminum so it is compatible with the window or door
- Be a complete System comprising Head Jamb and Sill
- Have precise connections at the critical head to Jamb and Jamb to Sill junctions
- Be tested to E2/VM1 standard by an independent body such as Branz
- Be durable to last at least the life of the window or door
- Be installed by people who are specialists in the design and installation of window flashings
- Be warranted as to performance and installation
- Be able to withstand extremely high winds
- Suitable as a universal flashing solution for most if not all cladding types
- Accommodate window and door support
The NZBC sanctions the following materials to be used as flashings
uPVC, aluminium, galvanised steel, aluminium- zinc coated steel, stainless steel, copper, lead, Zinc sheet, Butyl rubber and EPDM, bituminous, flexible flashing tape.
Most of the metals mentioned above cannot be used in close conjunction with aluminium due to the effects of electrolysis which corrode one or other of the dissimilar metals when in contact with water or moisture rendering their use in conjunction with aluminum impractical.
There are really only two alternatives to satisfactorily flashing around aluminium windows and doors. They are aluminium and galvanized steel. However the steel has to be formed, fabricated, coated and installed. But who will do all this work and guarantee the design and the workmanship? Can a complete, tested and proven system together with sound and critical junction detailing be found?
uPVC does not have the credentials to last the required time frame when exposed to the harsh NZ ultra violet rays and soon breaks down becoming brittle and suspect. The differential expansion and contraction of PVC flashings has been well documented. Cracking is inevitable with plaster and PVC "Flashings" (actually moulding screeds) not flashings since the plaster is applied OVER the flashings trapping any water that penetrates the micron thin paint system that protects the thin porous plaster beneath.
The most obvious and compatible material to use around aluminium windows and doors is aluminium.
Where to get flashings
Flashings can be bent up by a sheet metal fabricator in accordance with the NZ Building Code (NZBC) and then installed usually by the builder. If the NZBC flashing details are followed precisely and then precisely installed they will meet the NZBC minimum requirements and will automatically be accepted as compliant by the Territorial Authority (Council)
The NZBC sets minimum standards for the industry. It can be argued that these minimal standards are actually minimal standards and not sufficiently robust or well policed to meet the requirements of modern building and reduced skills in the work force.
Best Practice is always a safer mode of building than minimum standards, especially when it comes to weathertightness and flashing practice around windows and doors.
The cavity system
The cavity was introduced into NZ building to combat the leaky home crisis and is a step in the right direction. However it is not a full proof solution. The cavity was never designed or meant to be a foil for poor flashing practice. The cavity which is essentially a 20mm thick full stud length timber or plastic or polystyrene batten creates a 20mm gap between the back of the cladding and the timber framing.
The purpose of cavity construction is to allow any occasional water leakage that penetrates the first line of defence (the deflection aspect of the flashing) to drain out harmlessly to the bottom of the wall but behind the cladding line to the exterior. Clearly it is far better if water never penetrates the cladding and there is NO water behind the cladding where it could be trapped by a blockage in the cavity or by the insulation which has been forced into place and has blocked the 20mm cavity.
Cavity construction is good. A flashing system that absolutely prevents and water entering the cavity is better. Unfortunately a common attitude to poor flashing practice is "The cavity will take care of it".
The cavity is not and internal downpipe! It was designed as an occasional back up until regular maintenance recognizes the fault in the flashing system and rectifies it.
The issue of window and door support has always been important but is more so now that the ventilated cavity has been introduced to NZ buildings and homes.
Windows and doors are now pushed out further from the supporting framing and need to be fully supported along their entire length to prevent stressing the mitre joints which can introduce unwanted water onto the framing and linings.
The Flashman Window and Door Flashing System addresses both the issue of window and door support with the Support Sill and accommodates leaking mitres should that occur for any reason, at the same time.
High Wind Zones
Aluminium Windows and glazing are subject to specific standards including NZS 4211 that must be adhered to by WANZ members. The window speed and suitability for purpose are always printed on the windows or doors supplied by WANZ members.
What is equally important is the performance of the window flashings in the various wind zones which are set out in NZS3604 and range from low wind to very high wind speeds of 114 – 180kph. Sometimes termed as metres per second or m/s which range from low wind at 32m/s – 50m/s for very high wind zones.
There are many places in NZ where this very high wind zone speed is exceeded. When this occurs a robust and proven flashing system is of paramount importance to all parties concerned, not the least being the local Territorial Authority. (Council)
The Flashman Flashing System
The patented Flashman Window and Door Flashing System is proudly NZ owned. The System was designed by New Zealanders for our unique and demanding climate. The flashings are factory fabricated to precisely fit each individual window or door and then installed exclusively by Flashman Distributor/Installers, providing an advanced flashing system far superior to the NZ Building Code Minimum standard requirements.
How the system works
See how the system works.