A warehouse is a storage building usually characterized as a single storey steel structure with or without mezzanine floors. The designing of warehouse includes designing of the structural elements including principal rafter and roof truss, column and column base, purlins, sag rods, tie rods, gantry girder and bracings. Structurally metal roofing for a warehouse can be classified as structural and non structural or architectural. Whereas structural metal roofing is directly attached to purlins and eliminates the need of any solid support beneath, non structural metal roofing requires some solid support beneath in the form of plywood, a metal roof deck etc. Non structural systems would include tiles, corrugated metal roofing as well as panels. Metal roofs are available in sheets and sections. Sheets which come in standard sizes can be cut whereas sections are usually custom built. Custom made panels made of high quality metal roofing are fastened with screws and rubber washers and can simulate looks of slate, tiles etc. The most commonly used metal roofing materials for warehouses include steel, aluminum, copper, zinc, and titanium. Metal alloys in different colours, styles and textures are also available. Wig mentions that longer purlin spans, lower slopes and choice of colours determine the material.
The different types of metals that can be used for warehouse roofing are:
- Copper – the most expensive of all materials, the roofing is beautiful and is generally used in smaller places. Though prone to weathering to a green colour with time, it is resistant to corrosion and does not need to be painted. Care has to be taken to avoid direct contact with other metals. It has value in recycling.
- Aluminum- A very preferred choice – it is lightweight, durable, ductile, malleable and corrosion resistant. It has a light reflectivity of over 80% and is used to a great deal in industrial buildings. One needs to avoid direct contact with dissimilar metals, concrete or mortar; and moisture entrapment during storage to avoid staining.
- Zinc- Resistant to corrosion, it’s durable (a lot more than steel too) and can be recycled. The only downside is the very expensive initial cost.
- Steel – One of the most common, steel is an inexpensive material. Its strength and its prevention from rusting by using a zinc coating, a sealant or some paint are some of its advantages. Steel sheets for roofs can be manufactured from colour coated Galvalume / Galvanized, Soft Steel and High Tensile Steel.
- Titanium- It has high strength, is light weight and is immune to atmospheric corrosion
- Tin- Commonly seen in Indian residential metal roofing they are a great prevention from the winds.
- Stainless Steel – With a lustrous finish it doesn’t need any extra coatings, but its built-in anti corroding properties make it expensive. After installation, the surface needs to be cleaned to remove contaminations that can lead to surface corrosion. Space frames are popular in stainless steel.
- Galvanised Steel – It is steel coated with zinc which is often used in commercial and industrial applications. It is economical, can come in direct contact with concrete and masonry, is light weight and durable. It needs to be insulated with bituminous coating if it’s going to come in direct contact with copper.
- Alloys – Different metal alloys are good for metal roofing; the cost however depends on what the alloys consists of. Terne metal (zinc-tin alloy to cover steel) is light weight, durable and has low expansion though it cannot be nailed through metals and must be painted soon after installation. Also, it needs to be primed on both sides before installation. Then there are alloys like galvalume or zincalume (blend of zinc, aluminum and silicon-coated steel) which are more widely used as a base metal under factory coated colours.
Advantages of Metal Roofing in a Warehouse
The advantages for using metal roofing in a warehouse are:
- They have great fire resistance; hence one finds them in majority of industrial buildings.
- Low end metal roofing lasts 20 years while some sheet materials can last up to 100 years. Most steel roofing come with a 50-year guarantee. There are high quality paint systems/coatings used on metal roofing to resist chalking and fading of their colors. Simultaneously, these paint systems help in shedding off the dirt and resist the growth of fungal matter leading to low maintenance too.
- In severe wind conditions, it has been found that metal roofs outlast other roofing products. Also, metal roofing is ideal in northern or mountainous regions since painted metals roofs help in snow shedding.
- With coatings of various colors and patterns, and availability in a number of profiles, metal roofing offer extensive design flexibility to architects.
- There is no denying the strength of metal roofing and they are amongst the lowest weight roofing products – a benefit in seismic areas and in retrofitting projects. This also reduces the load on the structure.
- Most metals can be recycled without losing any valuable properties leading to eco-friendliness. Aluminium and copper roofing products are generally available with 95% recyclable content.
- Installation and construction is easy and quick and in many cases metal roofing can be installed over existing roofs also, saving costly tear off.
- Metal roofs reflect heat effectively, thus minimise the heat of absorption leading to lowering of air conditioning costs.
Disadvantages of Metal Roofing in a Warehouse
- The difference in temperature or the sound of falling rain sometimes can produce noise like in case of curved or corrugated metal roofing. This can however be avoided by proper installation and using roofing material that contains structural barriers to minimise the sound effect.
- Initial cost is expensive.
- Dissimilar and incompatible materials can cause unexpected and rapid corrosion.
- Transport of metal roof components is an energy intensive activity.
- Metal roofing renders the interiors warmer but Fabian mentions that with the use of an insulating layer (air-ventilation, insulation material such as Styrodur, Thermocol etc) one can easily control the radiation to the interior.
Different Systems for Metal Roofing for a Warehouse
There are different types of roofing systems that may be used in warehouse and industrial buildings. These tend to fall into some broad categories, which are described in the following sections.
- a) Single-skin trapezoidal sheeting
Single-skin sheeting is widely used in agricultural and industrial structures where no insulation is required. It can generally be used on roof slopes down to as low as 4° provided that the laps and sealants are as recommended by the manufacturers for shallow slopes. The sheeting is fixed directly to the purlins and side rails, and provides positive restraint (Figure 2). In some cases, insulation is suspended directly beneath the sheeting.
Generally steel sheeting is made of galvanised steel grades S280G, S320G or S275G to EN 10326. Due to the wide range of product forms, no standard dimensions for sheeting exist, although there are strong similarities between products and shapes. The steel sheets are usually between 0.50 and 1.50 mm thick (including galvanisation).
- b) Double skin system
Double skin or built-up roof systems usually use a steel liner tray that is fastened to the purlins, followed by a spacing system (plastic ferrule and spacer or rail and bracket spacer), insulation and an outer sheet. Because the connection between the outer and inner sheets may not be sufficiently stiff, the liner tray and fixings must be chosen so that they provide the level of restraint to the purlins. Alternative forms of construction using plastic ferrule and Zor rail and bracket spacers are shown in Figure 3 and Figure 4. As insulation depths have increased to provide greater insulation performance, there has been a move towards ‘rail and bracket’ solutions, as they provide greater stability. With adequate sealing of joints, the liner trays may be used to form an airtight boundary. Alternatively, an impermeable membrane on top of the liner tray may be provided.
- c) Standing seam sheeting
Standing seam sheeting has concealed fixings and can be fixed in lengths of up to 30 m. The advantages are that there are no penetrations directly through the sheeting that could lead to water leakage, and fixing is rapid. The fastenings are in the form of clips that hold the sheeting down but allow it to move longitudinally (Figure 5). The disadvantage is that significantly less restraint is provided to the purlins than with a conventionally fixed system. Nevertheless, a correctly fixed liner tray will provide adequate restraint to the purlins.
- d) Composite or sandwich panels
Composite or sandwich panels are formed by creating a foam insulation layer between the outer and inner layer of sheeting. Composite panels have good spanning capabilities due to composite action in bending. Both standing seam (Figure 6) and direct fixing systems are available. These will clearly provide widely different levels of restraint to the purlins. Sandwich elements for roofs generally have a width of 1000 mm with thicknesses between 70 and 110 mm, depending on the required insulation level and structural demands. Despite being relatively thick elements, the self-weights are comparatively low. Thus the elements are easy to handle and assemble. Component lengths of up to 20 m for roofs and walls permit constructions without or with few joints. The basic material for the outer layers is usually galvanised coated steel sheeting with thicknesses of 0.4 to 1.0 mm. The inner layers of sandwich panels are often lined or slotted; special designs are available with plane surfaces. Close-pitch flutings have also been established, which are fully profiled, yet appearing as a plane surface from a certain distance. Some patterns for external surfaces of sandwich panels are shown in Figure 7.
Standing Seam Metal Roofing for Warehouse
Standing Seam Metal Roofing (a commonly used sheet metal roof), is one of the most important technological advances in building construction in warehouses. This state-of-the art system, which is used on virtually all metal buildings, provides a weathertight seal and accommodates expansion and contraction. Utilizing formed, interlocking seams, the standing seam roof can literally cover an entire building with minimal penetration by structural fasteners. As a result, the standing seam metal roof fulfills the building owner’s for a warehousing need for durable, long-term protection against virtually any type of weather, while working in tandem with the forces of nature. It imparts an appealing finished look to the roof by concealing the washers and screws by using clips. The standing seam system is an ideal option for both new roofs and reroofing projects for a warehouse.
Durable and Light weight
The standing seam roof is both durable and lightweight. Each metal roof panel weighs only 1-1/2 lbs. per square foot. The standing seam system provides outstanding drainage from rain and snow, effectively solving ponded water problems, leaks and other related troubles associated with flat roof systems. Designed as a water barrier, the raised seam assists drainage. The panels are joined together by a weathertight seam raised above the roof’s drainage plane. Special sealants are factory-applied inside the seams during roll forming of the panel. Automatic field seaming machines produce weathertight connections between the metal roof panels. The system’s concealed clip assembly is roll formed or crimped into the panel seams without penetrating the steel weathering membrane. These clips perform two functions -they secure the panels to the structural system, reducing the need for structural fasteners, and they enable the roof to expand and contract as the outside temperature changes.
Standing seam metal roofs can be designed with a low slope or be highly visible with a steep slope -adding distinctive aesthetic appeal to a warehouse. The roof panel can also be used for mansards, fasciae and similar applications. The structural standing seam roof is the industry’s alternative to flat built-up and single-ply systems. It spans from structural support to structural support in new and retrofit construction with slopes of 1/4: 12 or more. Non-structural (architectural) standing seam roofs are used for visually exposed roofs, mansards, fasciae and similar applications. These systems require slopes of 3: 12 or greater to shed water.
Fiberglass blanket insulation is the most common material used in new standing seam metal roofs and reroofing projects for a warehouse. The insulation blankets are installed directly under the panels and stretched over the supporting structural members. In most retrofit projects for a warehouse, the standing seam roof can be installed over an existing built-up roof, eliminating costly and time-consuming tear-offs. Leaving the old roof in place also avoids the release of potentially hazardous materials into the environment and eliminates the disposal cost of the existing roof. The 66 percent recycling rate of steel products is the highest for any building product.
Metal roofing is used in long span, column free structures such as stadiums, airport hangers, etc and especially in industrial buildings and warehouses. The applicability of metal or steel roofing in warehouses has increased since the insulation and ventilation solutions have also paired up. Low cost, light weight and low maintenance were and still are the popularity pushers in a warehouse. Modern technology has added precision, safety, insulation, lifetime looks and textures to up the style quotient. There is also considerable reduction in both labor effort and material used. Life-cycle cost studies confirm the fact that the metal roofs are among the most cost-effective roofing systems available for a warehouse.