Metal Cladding: How To Get The Best From A Metal Roof

Published courtesy SAMCRA

In making the decision to have a metal clad roof one must remember that the most important functions of a roof cladding system (profiled cladding plus ancillary items and fasteners) is to provide a weatherproof membrane followed by aesthetic appeal, however, aesthetic appeal is invariably the governing consideration.

PROFILE:

The first step is to choose a profile (geometric shape) from the two main categories, pierce fix or concealed (secret) fix that best suites the aesthetic and service performance requirements of the building.

 Pierce fix profiles are those where the cladding is anchored to the supporting structure by a fastener that passes through the cladding, the head of which is permanently exposed. Corrugated and box rib are the most common forms of pierce fix profiles. These profiles are not suitable for flat roofs (less than 5°) generally it is recommended that corrugated be limited to a minimum slope of 10° and 7,5° for box rib.

The length of individual sheets is limited to between 10 to 13m due to transport constraints. This, however, eliminates the need to provide for thermal movement. We strongly recommend that both side and end laps be sealed with a reinforced butyl based sealer strip and that side laps are stitched as per manufacturer’s recommendations.

Concealed fix profiles have unique anchoring devices which are contained within the profile and are therefore not exposed to the elements. They also allow for unrestricted thermal movement of the cladding. These profiles can be used on slopes as low as 2°, moreover, they can be rolled on site thereby eliminating the need for end laps. These profiles should never be end lapped which means they are not suitable for in-plane rooflights.

An often overlooked component of cladding systems are the flashings, most leaks emanate from undersized and/or poorly fitted flashings. Whilst the basic design of flashings is universal to all profiles their dimensional proportions vary considerably and we recommend the cladding manufacturer’s standard designs for the respective profiles be adhered to. The two flashings that cause the most problems are the valley and counter flashings. Valley flashings are to have a return, similar to that on a counter flashing, along the full length of their longitudinal outside edges which are overlapped by the cladding, this is necessary to eliminate the formation of a capillary siphon.

Counter flashings have to be independent from head and sidewall flashings, the reason for this is the differential thermal movement between the flashing attached to the cladding and the counter flashing which is anchored into masonry. Under no circumstances is the counter flashing to be mechanically connected to any other flashing.

A similar situation arises with gable and barge flashings on concealed fix systems where sliding connections are required for the attachment of the flashing to the roof cladding. Sliding connections are also required for the attachment of headwall and ridge flashings. Most manufacturers of concealed fix cladding systems supply matching sliding connectors. Under no circumstances are paint-on membranes to be used as a substitute for metal flashings.

Fasteners are to be in accordance with the cladding system manufacturer’s specification and the requirements of SANS 1273 which requires that the durability of the coating on the fasteners and washers, together with that of the sealing gasket, is equal to or better than that of the cladding.

BASE MATERIAL:

The second step is to choose the material from which the cladding is to be roll-formed together with the finish, ie metallic or colour coated. Traditionally metal cladding is formed from metallic coated steel and less frequently from aluminium, stainless steel, titanium zinc or copper, all of which perform differently in a given environment. With coated steel there is a choice between galvanised and 55% aluminium / zinc (ZincAL, Zincalume, etc) both with a colour-coated option with finishes ranging from 30% gloss to matt and textured. Aluminium is also available with a 30% gloss painted finish.

Whilst galvanised is the cheapest option, it is, with a few exceptions, the most vulnerable to corrosion both from the environment and rainwater runoff from other metallic, painted or glazed (including glass and plastics) surfaces, whereas the 55% aluminium / zinc is vulnerable in highly acidic and alkaline environments (pH less than 4 and greater than 9), plus areas of intensive animal farming together with runoff from copper or lead.

All coated steel products must not be in direct contact with stainless steel, copper or lead. Please note that metallic coatings are available in a range of thicknesses and that in general durability is proportional to thickness, ie the thicker the coating, the longer the corrosion protection. When selecting the base material it is vitally important to consider both the macro (area or region as a whole) and micro (adjacent area surrounding a building) environments. Under no circumstances are solar heaters and PV panels to be installed on uncoated galvanised surfaces.

MAINTENANCE:

An important aspect that is regularly overlooked is maintenance, particularly in coastal and highly polluted areas where the regular washing, on a quarterly basis, of surfaces not washed by rainfall is necessary to preserve the protective coatings and compliance with warrantee conditions. We recommend an annual inspection of roofs to check the condition of the surface, attachment of flashings and in the case of pierce fix systems, the condition of the fasteners and their sealing gaskets. In all cases, the regular removal of debris is important as plastics and vegetative matter block drainage systems. In addition, accumulated vegetative matter accelerates corrosion of surface coatings, including painted ones.

INSTALLATION:

In order to ensure the best performance from a cladding system, we strongly recommend that it is installed by a competent, well-trained and experienced roofing contractor. A good starting point is to establish if a prospective contractor is approved by the manufacturer of the chosen system.

Posted in Construction materials, Roofing.

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