The ability of a metal-clad roof to provide a waterproof envelope is dependent on the performance of the individual components of the chosen cladding system, i.e. profiled cladding, flashings, fixings, workmanship and, to a lesser degree, the supporting structure.
The main function of a flashing is to waterproof the junction between sections of cladding, an intersection between cladding and other parts of a building such as walls, plus items of plant.
The most vulnerable flashings are ridge (including hip), barge/gable, external corners and, to a lesser extent, back flashings. These flashings are subjected to the most severe wind loading, invariably applied as a suction, imposed on a building which can be as high as 2,8 times that of other surfaces.
The loss of one of these types of flashings can precipitate the loss of a considerable amount of adjacent cladding. Their ability to resist these forces, while simultaneously providing adequate cover to form a waterproof seal, is dependent on the fixings attaching them to the cladding, geometry and thickness of material.
Minimum thickness recommended for flashing
As a general guide, the minimum thickness recommended for flashing is as follows:
• Domestic residences: 0,50mm for steel and 0,70mm for aluminium.
• All other types of building: 0,58mm for steel and 0,80mm for aluminium.
• On very large or exposed buildings, these may need to be increased to 0,80mm and 1,0mm respectively.
Whereas reputable cladding manufacturers provide details of the geometry and thickness for flashing to be installed with their various cladding systems and most builders’ merchants provide scaled down and/or thinner products, often with less durable protective coatings.
The accompanying diagrams reflect the recommended minimum dimensions for pierce-fix systems. Lapped joints in flashings to be a minimum of 150mm suitably sealed at each end, with either a butyl-based sealer strip or bead of neutral-cure silicone.
Why flashings tear from buildings
The four most common reasons are:
• Insufficient number of fasteners.
• Undersized fasteners which fail under load (blind-rivets have 30% of the pull-out value of a flashing/stitching screw).
• Flashings made from under gauge material which tear from the fasteners when subjected to wind loading.
• Thermal movement.
A common error is the attachment of sidewall and headwall flashings directly to adjacent masonry, which results in the flashing tearing from the fasteners and/or pulling away from the masonry due to differential thermal movement between the cladding and masonry. An independent counter flash is required to overcome this problem. Under no circumstances are the head/sidewall and counter flashings to be connected together.
Using paint-on or stick-on bandages in lieu of two-part metal flashings or as a substitute for other types of flashing is a formula for disaster. Most of these products are susceptible to degradation from ultraviolet (UV) radiation and need to be recoated every two to three years, whereas an accurately installed good-quality flashing will last in excess of ten to fifteen years.
There is simply no substitute for well-designed and accurately fitted metal flashings in the long run. Whilst blind rivets are suitable for the attachment of flashings on domestic and small industrial buildings, the use of special flashing/stitching screws is recommended for large and exposed buildings.
Inferior imports seldom comply with regulations
The Southern African Metal Cladding and Roofing Association (SAMCRA) constantly receives complaints from aggrieved parties about the poor performance of products supplied to the cladding market, which has been flooded with inferior foreign products, sold at best by gullible vendors but mainly by rogues with price and a quick profit being the drivers.
The vast majority of these products do not comply with either the National Building Regulations, SANS National Standards or other internationally recognised standards. Fortunately, there are a few reputable suppliers that do supply quality and compliant products.
What is not generally known, is that there are four classes of coatings applied to fasteners. Classes 1 and 2 (usually a thin electroplated yellow coating) are not suitable for exterior applications. A further fact is that all the major manufacturers of coated coil specify a minimum of class 3 or class 4 for moderate and aggressive corrosive environments respectively, in order to not negate the terms of their warranties.
Why gaskets fail
Another area of concern is the weatherproofing gaskets on flanged head fasteners and bonded washers. These can be anything from sealed sponge rubber through various plastics to EVA and EPDM. Gaskets need to be capable of operating at temperatures between -10° to +80°C, be ultraviolet (UV) resistant and be free of active carbon.
No-sponge-based products and the majority of plastic gaskets meet these criteria. Their early failure leads to premature failure of the fastener and accelerated corrosion of the cladding. Based on historical performance, EPDM and EVA have proved to be the most suitable materials. The use of any form of nail or drive screw to secure the cladding to timber should be avoided at all costs.
Recommended thickness for washers
The basic purpose of washers is to provide a larger bearing surface when transferring wind forces from the cladding via the fasteners to the supporting structure. As with fasteners, washers need to have corrosion-resistant properties or coatings at least equal to those of the cladding. In addition to resisting the design wind loading, they have to be sufficiently robust to withstand the mechanical forces of installation.
The minimum recommended thickness for washers is 0,58mm. Preferably they should be 0,8mm thick. Currently, bonded washers available in the market range from aluminium alloy domes with hard plastic grommets (most hardware outlets) through 0,4mm slightly domed steel with a nominal galvanised coating and wafer-thin EVA or so-called EPDM gaskets (gaskets should be at least 3,0mm thick) which are easily dished during installation, to quality aluminium and stainless with electro-negative EPDM gaskets.
Washers made from too thin material provide virtually no benefit and have a tendency to dish when being installed, resulting in ponding around the head of the fastener and the formation of a crevice corrosion cell. Invariably the dish is transferred to the top of the rib of the cladding, thereby forming an additional larger pond and crevice corrosion cell, which will result in the premature failure of the protective coating on the cladding and thereafter perforation of the cladding.
Decline in workmanship
Within the construction industry, there has been an ever increasing decline in the standard of workmanship over the last two decades. This is resulting in unacceptable quality of installation, examples of which range from overtightened and skew fasteners, dragging materials and tools across stacked or installed cladding, thereby producing scratches and dents, and not cutting materials with special metal cutting disks or nibblers but rather with friction saws.
This leaves a jagged burr, which has a negative impact on the protective coatings and excessive amounts of swarf (metal particles) that are equally problematic if not swept from the roof on a daily basis, all of which can negate a warrantee through to improper site storage and stacking of materials.
Currently there is a core of highly skilled and competent roofing contractors, but unfortunately their numbers are diminishing. Remember, when it comes to cladding, you get what you paid for.
Full thanks and acknowledgement are given to www.samcra.co.za for the information included in this article.