Reliable renovation of standing seam metal roofs

When a private investor relies on a contractor with insufficient expertise, the financial investment in roof renovation can result in serious long-term consequences. The aim of this article is to outline the key principles of proper design for standing seam metal roofs and to draw attention to the most common mistakes made during their design and installation.

Roof renovations in practice

A large number of roof refurbishments are currently being carried out using both private and public funding. Older roof coverings, such as bitumen shingles or fibre cement sheets, are often at the end of their physical and functional lifespan.

Repairs to such roofs are frequently undertaken without removing the original covering. A new roof layer is installed directly over the existing one, fixed through a separation layer. This approach can be advantageous, saving both time and cost, especially when the removal and disposal of hazardous materials would be expensive.

For this reason, it is important for property owners to familiarise themselves with construction pricing in advance. Understanding the cost differences between renovation methods allows for informed decisions and realistic budgeting.

Importance of the substructure and load-bearing layers

The longevity of a new roof depends not only on the roof covering itself but also on the condition of the layers beneath it. The state of roof decking, sheathing and load-bearing elements is often an integral part of the renovation process.

In many cases, additional masonry or structural works are required to create a solid and stable base for the metal roof. A reliable substructure prevents moisture penetration, reduces heat loss and forms the foundation for correct installation, helping to avoid recurring defects.

Inspecting the roof structure before installation

Before installing a new metal roof, the roof structure, battens and all load-bearing components must be thoroughly inspected. Timber elements must be in good condition to ensure maximum stability and long-term reliability.

Poorly prepared structural components can lead to roof instability and damage to the metal covering. Inspection may also include adjusting the height of counter battens, which has a direct impact on effective roof ventilation.

Principles of reliable standing seam metal roof design

Standing seam metal roofs have been used for centuries. In the past, roof spaces were mainly used for storage. Today, most roof voids are converted into living space, which significantly affects the design and composition of the entire roof build-up.

A durable and fault-free standing seam roof can only be expected if sufficient attention is given to the following key aspects:

• effective ventilation of the roof build-up
  
• proper allowance for thermal expansion of metal sheets
  
• suitable roof pitch
  
• correct selection and execution of metalwork details and junctions
  
• installation in accordance with approved and agreed technical solutions
  

Ventilation of the roof build-up

Condensation forming on the underside of metal roofing is a natural phenomenon. Some metals are particularly sensitive to condensation and may react with corrosion if moisture is not properly managed.

Roof ventilation addresses not only the removal of condensed moisture but also the drying of moisture trapped within the roof structure, for example from freshly installed timber.

Proper ventilation is a critical requirement for long roof life, especially in buildings with habitable loft spaces.

Requirements for roof build-up layers

For most standing seam metal roofs, the following roof build-up principles apply:

• a ventilated double-skin roof construction with a correctly sized air gap
  
• reliable design of air intake and exhaust openings
  
• a properly installed vapour control layer or vapour barrier
  
• use of a vapour-permeable membrane above the thermal insulation
  

Ventilated roof constructions are well proven and widely used. In practice, counter battens are installed on the main roof structure, such as rafters, to create the necessary ventilation cavity. Their height is largely determined by the roof pitch and rafter length.

As mentioned above, condensation forming on the underside of the metal sheet has to be dealt with. The solution is a suitable separation layer between the substrate and the roof covering itself. Metals differ in how well they resist condensate combined with the heat generated by sunlight. Depending on the colour of the metal roofing, its surface temperature ranges from 60 to 90 °C. In the past, roofing felt, that is a bitumen membrane, was used as the separation layer under metal roofs. This solution is unsuitable for metals that are sensitive to hot water corrosion (for example titanium zinc or galvanised steel). In such cases a structured mat has to be used as the separation layer. A structured mat is a separation layer laid directly under the metal sheet on the roof decking, and it prevents the underside of the sheet from coming into contact with any moisture. This layer consists of a polyamide mat 7 to 8 mm thick and a backing of vapour-permeable membrane. The structured mat also has sound-insulating properties and helps bridge minor unevenness in the timber decking. It is always used when large-format chipboard, OSB, MDF or CETRIS boards are used instead of timber boarding.

Recurring defects in roof ventilation

If the design does not specify the height of the air gap, builders tend to go for the cheapest option, which is whatever the timber merchant has in stock. The most commonly sold roof batten today measures 60/40 mm. For structural reasons it is fixed to the substrate on its wider side, so the air gap ends up only 40 mm high. This cross-section is not suitable for metal roofs at all. With an air gap of less than 50 mm, the roof build-up cannot be considered effectively ventilated. Other common defects include:

• air intake openings blocked by poorly installed soffit cladding
• the roof covering finished at the ridge without functional ridge ventilation (see detail no. 1)
• ventilation blocked by incorrectly placed thermal insulation
• a roofing underlay membrane that does not do its job

For example, longitudinal membrane joints are only overlapped even on pitches of 20° or less, the membrane is finished incorrectly at the eaves, and it is joined to roof penetrations (chimney, roof window, soil vent pipe and so on) without being carefully taped with system components. It is also a misconception that a structured mat under the metal sheet can ventilate the whole roof build-up.

Another frequent mistake is building standing seam metal roofs as single-skin, unventilated roofs.

The decking under the roof covering is often made from poor quality boards with a moisture content above 20 % and the wrong dimensions (the optimum is 80 to 160 mm wide by 24 mm thick). So-called open boarding, where every other board is left out, is also wrong.

Thermal expansion of roofing panels

In the past, standing seam roofs were made almost exclusively from 2 x 1 m sheets, so problems caused by thermal expansion hardly ever occurred. With the arrival of machines for forming standing seams, coil sheet metal became standard in modern roofing practice. This excellent innovation has its pitfalls, though. Architects design buildings with very attractive but demanding shapes. Roofing panels are often longer than 10 m. Occasionally, mainly on sports buildings, they can even exceed 30 m.

A double standing seam roof made from coil sheet has to allow for the thermal expansion of the material both across and along the panel. Across the panel, expansion is handled by setting the correct seam profile on the seaming machine (see fig. 1). The correct sheet thickness in relation to its developed width also needs attention.

Fig. 1: Seam profile geometry of a double standing seam roof

Thermal expansion along the panel needs attention once panels are longer than 3.0 m. The roof covering is fixed to the substrate with a system of sliding (expansion) clips and fixed clips (see fig. 2).

Fig. 2: Fixed and sliding clips for fixing roofing panels to the substrate

For ordinary panel lengths up to 10 m, standard sliding clips are used. For the thermal expansion of the roof to work as a whole, the connection of the panels at the eaves and at the ridge also has to be designed properly. An expansion gap of 8 to 10 mm must be kept at the eaves flashing (see fig. 3) and at the ridge.

Fig. 3: Expansion joint between the roofing panels and the eaves edge

Where unavoidable, panels up to 15 m long can be used. In that case the expansion gap must be increased to 15 mm and extended sliding clips must be used.

Roofs with extremely long panels need careful attention, and the whole design has to be assessed in context. On roofs with panels longer than 15 m, transverse expansion joints must be made within the roof area. The detail of the transverse expansion joint has to match the specific roof pitch. The position of roof penetrations such as roof windows, chimneys and rooflights must also be taken into account. These penetrations should be placed within the zone of fixed clips.

Thermal expansion of metals must be allowed for in all linear flashings, such as valleys, cornices, parapets and every type of gutter. It is especially important for parapet gutters and valley gutters between roofs.

Table 2: Coefficients of thermal expansion of metals and selected building materials

Roof pitch

Getting the roof pitch right deserves real attention. It is governed not only by roofing standards and guidelines but also by height limits set in local zoning plans.

Experience shows that the minimum pitch of 3° for metal roofs, allowed by standards in the past, is not always sufficient for seamed roofs. Based on current knowledge, a standing seam roof can be expected to perform from a pitch of 7°. A double standing seam is rainproof, not watertight, as many people wrongly assume. If a double standing seam ends up under water, for example because of water pressure from melting snow at the cold edges of the roof or above larger roof penetrations, moisture gets under the roof covering.

Under certain conditions, double standing seam roofs can be built at a pitch of 3°. In neighbouring Austria, for example, a double standing seam roof may be laid at 3° provided there are no penetrations, no valleys and no transverse joints in the roof area, meaning the panels must be made in one piece without joints.

In practice, roofs with a borderline pitch between 3° and 7° need extra care, with solutions that reduce the risk:

1. seam sealant used along the full length of the seams
2. a more watertight underlay, creating a waterproof secondary roof
3. measures that keep condensation away from the roof covering and the substrate beneath it (a suitable separation layer)
4. and above all, adequate ventilation of the roof build-up (an air gap of 80 to 100 mm)

Unfortunately, even professionals are often unsure how to specify roof pitch. Some use degrees, others percentages. For completeness, here is a pitch conversion table.

Metalwork details and junctions

The choice and construction of carpentry and metalwork must suit the roof pitch set by the designer. A key functional element of every roof is properly designed roof drainage and its details, including the metalwork finish of the roof covering at the eaves. Just as every story has a beginning, every roof installation starts at the eaves. On borderline pitches of 3° to 7°, remember to lower the substrate so that no back fall forms where the roof covering discharges into the gutter.

This can be done in at least two ways:

• lower the counter batten by 2 to 4 mm across the width of the first eaves board (about 150 mm)
• reduce the thickness of the eaves board by 2 to 3 mm.

The first option takes more work, but it is structurally safer and more reliable because it does not reduce the required embedment depth for the fixings.

Flashings around roof penetrations also have to match the roof pitch. Metalwork details on double standing seam roofs should be made as seamed joints without direct fixing to the substrate (no perforation of the material) and without relying on sealants or silicone. The cheapest profiled metal roofing systems use a self-drilling roofing screw as their standard component for fixing the covering to the substrate. This fixing has spread through the whole roofing trade like a virus. Sadly, even experienced tradespeople often catch it, and instead of indirect fixing with a stiffening clip strip, they use roofing screws and drill through valuable material, perforating it and greatly shortening its life.

Roofing screws have no place in traditional metal roofing craftsmanship.

In terms of detailing and junctions, a metal roof must work as a whole, including every detail. The most common cause of metal roof failure (up to 90 % of cases) is the wrong choice of metalwork details and their unreliable execution. This deserves close attention during design and, of course, during installation. Thorough preparation can only be provided by a knowledgeable specialist, a designer with enough expertise and experience in the field.

Carrying out the metal roofing work

An essential part of preparing a metal roof renovation is choosing a reliable and experienced contractor for sheet metal work who has mastered the chosen technology.

Before starting work, the contractor must take over the site and the substrate and make a detailed written record of their condition in the site diary. When checking the carpentry, hidden defects should be uncovered wherever possible, as they are common in roof renovations. Examples include ineffective roof ventilation caused by insulation that is too thick for the rafter depth, an unsuitable substrate under the covering, or a poorly installed underlay and its details.

Reliable work also requires a careful study of all design documents, drawings and details. A personal consultation with the project designer or the site supervisor is recommended. During installation, all the designed solutions must be applied, using the most modern machine and hand tools available for reliable metal roofing work. If these principles are followed, you can expect a reliable result and a roof that works.

Finally, a brief note on the state of the metal roofing trade. Sadly, the trade is currently at something of a crossroads. There is a shortage of qualified staff, a shortage of young people entering the trade and a lack of proper recognition for those who create value from sheet metal with their own hands. At the same time, modern, industrial and above all cheap roofing systems are making their way into the traditional craft, and their installation methods are eroding the professional pride and expertise of tradespeople. I believe that with the joint effort of people who care about honest metal roofing craftsmanship, this unflattering trend can be reversed.

Author: Ing. Gabriel Boros, PROJECT CONSULTING
The author is a specialist in metal roofing from Rheinzink and Prefa Aluminiumprodukte with 30 years of experience in the field.

References: Company documentation, PREFA ALUMINIUMPRODUKTE GMBH
Company documentation, RHEINZINK GmbH & Co. KG