LWRA VIEWPOINT Waterproofing metal roofs: What to look out for Sarah Spink, chief executive officer of the Liquid Roofing and Waterproofing Association, signposts key advice from its guidance note when waterproofing metal profiled sheet roofing and cladding In many circumstances, refurbishing metal profiled sheet roofing and cladding is the preferred option to full replacement, and has been within the realms of the liquid roofing and waterproofing industry for many years. Yet, there are factors to take into consideration before going ahead, which are detailed in the Liquid Roofing and Waterproofing Association’s (LRWA’s) ‘Guidance Note 10: Waterproofing of Metal Profiled Sheet Roofing and Cladding’. Before a decision is made whether to refurbish or replace, a full roof survey should be undertaken. Some common issues that arise include cut end corrosion of metal sheets, which can be a result of insufficient coating during the manufacturing stage, whereby untreated metal is exposed to the elements, causing deterioration through atmospheric oxidation. Loose fixings and fasteners, missing profile filler pieces, and poor detailing to roof projections, must be assessed during a roof survey. Profiled roofs may also contain fragile materials, and no attempt should be undertaken to inspect a roof by walking along lines of fixings. All faults identified in the survey need to be rectified prior to the application of the liquid coating specification. Cut end corrosion The degree of cut end corrosion can vary enormously and if severely damaged, may require a complete replacement. However, during a refurbishment, all rust should be removed by a wire brush or grinding wheel to obtain bright metal where possible, and rust inhibitors should be applied prior to the application of a metal primer. A flexible reinforcement is then generally used to account for expansion and contraction between the top and bottom profiled sheet. The roof coating specification is then applied, taking care to adhere to the manufacturer’s priming requirements and coverage rates. In the last 20 years, some cut edge corrosion systems have been routinely used on PVC plastisols, including many silicone-based technologies. However, if the repair system used is not breathable, condensation can occur on the underside of the roof sheets causing reverse side corrosion. In these cases, the roof area should not be approached as it may not be structurally safe. The only options for treating this is either re-sheeting or removal, and replacement of the bottom edge of the profile metal sheeting. When approaching this type of work, it is important to clarify if the client wants a protective treatment for the cut end corrosion – if so, use a system that doesn’t seal the joints. Most leaking metal 064 MARCH 2018 RCIMAG.COM Liquid waterproofing manufacturer’s training should be undertaken prior to application Metal profiled roof – treating the boltheads To access the full LRWA Guidance Note 10 - Waterproofing of Metal Profiled Sheet Roofing and Cladding, visit the ‘Downloads’ section at www.lrwa.org.uk profiled sheets will show in the gutter area of the roof – even in a twin skin structure – so it’s good practice to waterproof the gutter prior to carrying out the waterproofing to the sheets. Gutters will generally need to be cleared of all accumulated dirt, and a rust inhibitor should be applied prior to the application of a suitable metal primer. Sometimes a flexible reinforcement is required to allow for expansion and movement between the different gutter sections. The waterproofing specification should then be applied to the correct coverage rates recommended by the manufacturer. The gutter should be treated independently from the main roof with the coating material terminating under the profile sheet or under the gutter cover flashing. Other key considerations There are several other key factors to consider when dealing with waterproofing metal profile roof sheets. For example, it is important to identify the existing protective coating, which may need different preparation and priming products, depending on the specification. Installing a seamless waterproofing skin over the existing structure may exacerbate condensation problems. Depending on the roof structure and the building use, ventilation may have to be introduced. Fixings are one of the main reasons for water ingress in a metal roof. Over the years, sealing washers will have deteriorated and perished, meaning some will be loose and affected by rust, thus proving difficult to tighten up. All the fixings should be wire brushed and given individual treatment in accordance with the manufacturer’s specification – this may involve application of a reinforcement prior to the full waterproofing system. Contractors should also be aware of lap joints. A properly constructed pitched metal profile roof should have 150mm minimum end laps sealed with two butyl sealant strips as detailed in the ‘Metal Cladding & Roofing Manufacturers’ Association Technical Paper No. 6 Profiled Metal Roofing Design Guide’. Loose and missing profile fillers may also cause issues on-site at ridge and eaves details. They may have perished, become loose or have been removed by birds. This allows wind-driven rain to blow under the capping and into the structure, creating water ingress. For this type of problem there are two solutions: either fix a new, larger profile ridge capping over the existing flashing, or allow for complete encapsulation of the ridge detail with the coating system specification. This may also require gap filling to provide a suitable backing for the liquid coating. Areas where plants have been fitted after the completion of the original roof need to be addressed prior to the application of a liquid coating system. The best method is to insert an apron cover sheet on top of the metal profiled sheets, extending under the ridge capping down past the penetration, which will prevent ponding behind the penetration. Applying the waterproof coating It’s always advisable to wire brush or mechanically abrade the areas to remove rust and scale, and return the metal to a clean, bright metal wherever possible. Tighten bolts as necessary and cut back to approximately 5mm above the nut. An adhesion test will determine if the roof area and associated details require priming – the test area should be a minimum of 300mm2, and you should ensure that all areas of the roof are covered. Apply the liquid coating as specified, which is usually by airless spray. If more than one coat is required, it is recommended that contrasting colours are used to ensure correct coverage. For contractors working on any project, liquid waterproofing manufacturer’s training should be undertaken prior to application. It’s also important to remember that roofs which are structurally unsound due to total corrosion, particularly around fixings, must be replaced.
RCI March 2018
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