Corrugated galvanised iron (CGI) sheets

General information

Corrugated Galvanized Iron or steel (CGI) sheets are used for general roofing purposes.

Selection:

Table: example of sizes

Thickness is an important factor for strength and durability.

Minimum recommended thickness:

• 0.551 mm (26 gauge) for permanent shelters and shelters subject to snow loads.

• 0.475 mm (28 gauge) can be used for shelters when the wind speed is lower than 250 km/h, the distance between laths/battens/purlins is 0.50 m maximum and the area is not prone to snow.

• 0.4 mm (30 gauge) can be used for shelters when the wind speed is lower than 200 km/h, the distance between laths/battens/purlins is 0.60 m maximum and the area is not prone to snow.

In exceptional circumstances such as major difficulties of accessibility (mountainous and remote areas), it is possible to use exceptional solutions à choose thinner CGI sheets to reduce the weight of the transportation, if CGI sheets have to be carried by humans or animals. In this case, the roof frame must be modified à the spacing of the CGI sheet support (lath/batten/purlin) must be reduced to keep its strength. In such cases it is highly recommended to seek the advice of a shelter expert.

For a more accurate result, you can verify the appropriate CGI sheet thickness for your shelter design using the Roof Estimate Form (Excel spreadsheet) provided with the CGI sheet roof covering manual.

It is highly recommended to use measurement in millimetre instead of gauge when ordering or purchasing CGI sheets, as the conversion of galvanized steel sheets can be confusing.

The gauge conversion depends on the material and its shape. Below is the conversion for galvanized steel sheet (including CGI sheet, ridge cap and fascia cap).

Table: Conversion table for galvanized steel sheet (only)

Zinc coating thickness and method of galvanization are the most important factors for durability. CGI sheets are protected from corrosion by a zinc coating applied to the steel sheet.

The zinc coating should be thick enough to ensure the durability of the CGI sheet by preventing corrosion. A 20 µm coat of zinc should be applied per side of the CGI sheet (total of 40µm for both sides of the CGI sheet equivalent to 275g/m²). This coating can be exceptionally lowered to 240g/m² when higher grade is not available.

The galvanization method used must be hot-dip galvanization as it better ensures the durability of the CGI sheets. Other methods such as electro-galvanization should be avoided because the coating thickness is thinner than hot-dip galvanization and the durability of the CGI sheet will be reduced, as a result. (Example: Electro-galvanized has a great finish but the zinc coating thickness can be 10 times lower than that of hot-dip galvanized, which means that in less than one year the CGI sheets lose their protection and corrosion starts).

It is possible to increase the zinc coating thickness in order to increase the service life (durability) of the CGI sheets.

In a marine environment (within 1 km of the coast), CGI sheets can be painted with zinc-rich paint to increase service life. More details are provided in the CGI sheet roof covering manual (link to be included).

Other metal and alloys such as aluminium-zinc and aluminium can also be used to protect the steel sheet from corrosion. Aluminium coating can be an alternative in very high corrosive environments (marine environment).

Quality control:

When purchasing CGI sheets, it is recommended to go to the suppliers with a caliper gauge and a coating thickness gauge in your hand, in order to show that you can verify the quality of the CGI sheets on the spot. This way you may avoid disappointments.

• The thickness of the CGI sheet can be verified using a caliper gauge (EMEAMETACG16).

• The zinc coating thickness can be verified using a coating thickness gauge or magnetometer (EMEAMEASGC01), which measures the thickness of the zinc coating applied to the CGI sheet.

Both verifications are crucial to ensure the durability and strength of the CGI sheets.

Safety and storage:

Always use gloves and hard shoes while manipulating CGI sheets.

Store in a dry and ventilated area. Avoid condensation. Place the CGI sheets on a wooden pallet and ensure that there is a slight slope. These measures will help in avoiding white rust.

Basic indications regarding the quantity of CGI sheets per roof:

Always add 0.05 to 0.075 m to the length of the roof slope to be covered with CGI sheets to allow for overlapping at the eaves. This extra length of CGI sheet allows for the rainwater to fall in the gutter or to the ground without wetting the roof frame. Example: if the roof slope measures 2.32 m, then the length of the CGI sheet should be 2.37 m (2.32 + 0.05 m).

Total quantity of CGI sheets to order must include the overlapping of each sheet with the next one which can be 5 to 26% of the surface to cover:

• Depends on the wind, the roof pitch, and if the length of the roof slope takes one or several CGI sheets (see the tables below).

• The overlapping of the CGI sheet sides (side overlap) is already deduced in the usable width (see in the first table). The suggested side overlap is 2 corrugations. The side overlap can be 1 corrugation only if the wind speed is lower than 100km/h and roof pitch higher than 15° (26%).

Percentage of overlapping if one CGI sheet for the entire roof slope:

Percentage of overlapping if several CGI sheets for the entire roof slope:

To have a good approximation of the quantity of CGI sheets needed to cover a gable roof (pitch = 20°, wind speed > 150 km/h), you need to:

• measure the length of the roof and the length of the roof slope (example: length of roof = 6.6 m, length of roof slope = 1.87 m)

• add 5 cm to the length of the roof slope (total length of roof slope = 1.87 + 0.05 = 1.92 m)

• multiply the two values to get the surface of one roof slope to cover (example: surface of one roof slope S = 6.6 x 1.92 = 12.67 m2)

•  Example: the percentage of overlapping in our case is 13 % as one CGI sheet is enough for the entire roof slope, and the surface of the CGI sheet is 0.92 x 2 m à N = 12.67 / (0.92 x 2.00 x (1 – 0.13)) = 7.9 CGI sheets

• round up this value to get the quantity of CGI sheets to cover one roof slope (example: 7.9 à 8 CGI sheets / side)

• multiply this value by 2 as there are 2 roof slopes in a gable roof (example: 8 x 2 = 16 CGI sheets to cover the entire roof)

It is recommended to include a 10% loss to ensure that you will have enough CGI sheets to cover the roof. (Example: 16 x 1.10 = 17.6 à 18 CGI sheets to be ordered for the entire roof)

Usage:

When purchasing CGI sheets to repair existing roofs, it is important to ensure that the pattern of corrugations is compatible, otherwise leakage may appear. You need to check the corrugation pitch and depth of the existing CGI sheets. In most cases, corrugation pitch is 76 mm and corrugation depth is 18 mm.

See drawing in item sheet header

Assemble the CGI sheets on wooden frames with roofing nails (see item EHDWNAILRS01). The quantity of roofing nails depends on the wind speed, location of the shelter and thickness of the laths/battens/purlins used. The values mentioned in the table below are approximate. They were calculated for a shelter with a 15-30° gable roof pitch built in a rural area with low vegetation, using a 38mm thick lath/batten/purlin. If the laths are thinner, then you need to multiply the quantity of roofing nails by 2.

• The roofing nails must be placed at the top of the CGI sheets corrugations, never at the bottom, in order to avoid leakage.

• When positioning the roofing nails, you need to place more roofing nails all around the edges of the roof: at eaves overhang, ridge, verges and hip angles (areas submitted to increased wind pressure).

• When the shelter needs to withstand very high wind speed, using roofing screws (see item EHDWSCRSR046) with sealing washers may be more effective than roofing nails.

For a more accurate quantity of CGI sheets and quantity of fasteners/fixings needed to securely attach the CGI sheets to the roof, it is highly recommended to use the Roof Estimate Form (Excel spreadsheet) provided with the CGI sheet roof covering manual (link to be included).

Suggested materials to accompany the CGI sheets:

• Caliper gauge (EMEAMETACG16)

• Coating thickness gauge or magnetometer (EMEAMEASGC01)

• Ridge cap (EBUIBSHEGR20)

• Fascia cap 5 EBUIBSHEFC20)

• Roofing nails with sealing washers (EHDWNAILRS01)

• Roofing screws with sealing washers (EHDWSCRSR046)

• Gutters (EBUIBSHEGR20)

Some specifications may vary from national standards and local market conditions. For locally sourced items, check the relevant national standards or specifications. Any important variation from the EIC standard needs to be approved by a specialist.

Specifications

Remark:

Grade: It is possible to use a structural steel (low quality such as S220GD), but it is not recommended as it is usually more expensive than mild steel for cold forming.