How to Install Galvanised Steel Roofing Sheets? [PDF]

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The roofs made of galvanized steel sheets have been trending these days with the rise in the construction of large warehouses and godowns. The roof made out of galvanized steel sheet is a low-budget easy-to-install option and serves as long as any other roofing material.

In this article, we discuss the material specification and installation guide of the sheets, ridges, flashing, and gutter of the galvanized steel sheet roofing.

Material Specification

1. Galvanised Steel Sheet

1. The sheets shall be of 750 grade of coating unless otherwise specified.
2. The sheets shall be free from cracks, split edges, twists, surface flaws, etc.
3. They shall be clean, bright, and smooth.
4. The galvanizing shall be non-injured and in perfect condition. 
5. The sheets shall not show signs of rust or white powdery deposits on the surface.
6. In corrugated sheets, the corrugations shall be uniform in depth and pitch and parallel with the side.
7. Purlins of the specified material or M.S rolled sections of requisite size shall be fixed over the principal rafters.
8. The galvanized coating on J or L hooks, and bolts shall be continuous and free from defects such as blisters, flux stains, drops, excessive projections, or other imperfections that would impair serviceability.

2. Ridges and Hips

1. The ridges and hips of galvanized sheets roof shall be covered with ridge and hip sections of plain G.S. sheet with a minimum lap of 20 cm on either side over the G.S. sheets.
2. The end laps of ridge and hips and in between the ridges and hips shall also be not less than 20 cm.
3. The ridges and hips shall be of 60 cm overall width plain G. S. sheet, 0.6 mm or 0.8 mm thick as given in the description of the item and shall be appropriately bent in shape.

3. Valley and Flashing

1. The valley shall be 90 cm wide overall plain G.S. sheet 1.6 mm thick or another size as parallel in the item bent to shape and fixed.
2. They shall lap with the C.G.S. sheets not less than 25 cm, which on the other side. 
3. The end laps of the valley shall also be not less than 25 cm.
4. The flashing shall be of plain G. S. sheet of 40 cm overall width 1.25 mm thick or 1 mm thick as specified in the item bend to shape and fixed. 
5. They shall lap not less than 15 cm over the roofing sheets. 
6. The end laps between flashing pieces shall not be less than 25 cm.

4. Gutter

1. The gutter shall be fabricated from plain G.S. Sheets 1.25 mm thick or other sizes as required.
2. The overall width of the sheet referred to therein shall mean the peripheral width of the gutter including the rounded edges, the longitudinal edges shall be turned back to the extent of 12 mm and beaten to form a rounded edge. 
3. The ends of the sheets at junctions of pieces shall be hooked into each other and beaten flush to avoid leakage.

Installation Procedure of Galvanised Sheet Roofing

1. Galvanised Steel Sheet

1. The G.S roof shall not be pitched at a flatter slope than one vertical to 5 horizontal.  The normal pitch adopted shall usually be one vertical to 3 horizontal.
2. The G.S sheets shall be laid with a minimum lap of 15 cm at the ends and two ridges or corrugations at each side. 
3. The above minimum end lap of 15 cm shall apply to slopes of 1 vertical to 2 horizontal and steeper slopes. 
4. For flatter slopes, the minimum permissible end lap shall be 20 cm. 
5. The minimum lap of sheets with ridge, hip, and valley shall be 20 cm measured at right angles to the line of the ridge, hip, and valley, respectively.
6. Lapping in G.S. sheets shall be painted with a coat of approved steel primer and two coats of paint with approved paint suitable for the G.S. sheet before the sheets are fixed in place.
7. The G.S sheets shall be fixed to the purlins or other roof members such as hip or valley rafters etc. with galvanized J or L hook bolts and nuts, 8 mm diameter, with bitumen and G.I. limpet washers or with a limpet washer filled with white lead.
8. Three number of minimum hook bolts shall be placed at the ridges of corrugations in each sheet on every purlin, and their spacing shall not exceed 30 cm.
9. The spacing of these seam bolts shall not exceed 60 cm along each of the staggered rows. 
10. The holes for all bolts shall be drilled and not punched in the ridges of the corrugations from the underside, while the sheets are on the ground.

2. Ridges and Hips

1. The ridges shall be fixed to the purlins with the same 8 mm dia G. I. hook bolts, nuts, bitumen, and G. I. limpet washers.
2. Similarly, hips shall be fixed to the purlins with the same 8 mm dia G.I bolts.
3. On either side, at least one of the fixing bolts shall pass through the end laps of ridges and hips.
4. The end laps of hips and ridges shall be joined together by galvanized iron seam bolts 25 x 6 mm size,
5. The surface of G. I. sheets of ridge and hip sections and the roofing sheets shall be painted with a coat of primer and two coats of paint suitable for painting G.S sheets before they are fixed in place.

3. Valley and Flashing

1. The flashing and valley sheets shall be fixed to the purlins and valley rafters with the 8 mm dia G.I hook bolts and its accessories.
2. At least one of the bolts shall pass through the end laps of the valley pieces on the other side.
3. The free end of flashing shall be fixed at least 5 cm inside masonry with the mortar of mix 1:3 (1 cement: 3 coarse sand).
4. The surface of G. S. sheets under overlaps shall be painted with a coat of approved primer and two coats of approved paint suitable for painting G. S. sheets.

4. Gutter

1. The gutter shall be supported on and fixed to M.S. flat iron brackets bent to shape and fixed to the requisite slope. 
2. The spacing of brackets shall not exceed 1.20 meters.
3. At the places where these brackets are to be fixed to the sides of rafters, they shall be of 40 x 3 mm section bend to shape and fixed rigidly to the sides of rafters with 3 Nos. 10 mm dia bolts, nuts, and washers.
4. The brackets shall overlap the rafter not less than 30 cm, and the connecting bolts shall be at 12 cm centers.
5. The overhang of the vertical portion of the bracket from the face of the purlin shall not exceed 22.5 cm with this arrangement. 
6. For connection to down take pipes, a proper drop end or funnel-shaped connecting piece shall be made out of G.S. sheet of the same thickness as the gutter and reverted to the gutter, the other end tailing into the socket of the rain-water pipe.

FAQs on Galvanised Steel Sheet Roofing

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