Code of practice for laying and fixing sloped roof coverings using plain and corrugated galvanized steel sheets
1987 Edition

Overview of Scope and Essential Specifications

Scope Details (Clause 3.1)

For effective design and assembly of sloped roofing using galvanized plain and corrugated steel sheets, the following must be established:

• Total surface area coverage
• Types and layout of supporting frameworks
• Drainage slope considerations
• Fixing provisions for ridge components and additional services
• Installation height or roof level

Important Design Notes (Clause 2.3)

• Screws for plain steel sheets should comply with IS 6739-1972
• Screws must be galvanized to resist corrosion

Rounding Rules (Clause 0.3)

• Test and design values should be rounded in accordance with IS 2-1960
• Maintain the specified number of significant figures

Quick Reference: Screws for Plain Steel Sheets

flowchart TD
    A[Begin Roofing Design] --> B[Calculate Surface Area]
    B --> C[Select Support Elements]
    C --> D[Determine Roof Slope]
    D --> E[Plan Ridge and Services Fixing]
    E --> F[Establish Installation Level]
    F --> G[Choose Screws (IS 6739, Galvanized)]
    G --> H[Complete Roof Design and Construction]

This section ensures a safe and durable design approach for galvanized steel sheet roofs.

Material Specifications According to IS 12093

• Galvanized Steel Sheets (Clause 2.1):

  • Must comply with IS 277:1985 covering plain and corrugated sheets.
  • Zinc coating grades available: 600 g/m² and 450 g/m².

• Fixing Screws (Clause 2.3):

  • Screws for plain sheets should meet IS 6739:1972 standards.
  • Screws must be galvanized to prevent corrosion.

• Value Rounding (Clause 0.3):

  • Round off test and calculated values as per IS 2:1960, keeping consistent significant digits.

Steel Sheet Characteristics from IS 277 (Typical Values):

Design Input Parameters (Clause 3.1):

• Surface area coverage
• Type and configuration of supports
• Slope for proper drainage
• Ridge fixing details
• Installation level

flowchart TD
    A[Materials] --> B[Galvanized Steel Sheets]
    A --> C[Fixing Screws]
    B --> D[Comply with IS 277:1985]
    C --> E[Comply with IS 6739:1972]
    D --> F[Zinc Coating: 450 or 600 g/m²]
    E --> G[Galvanized Screws]

This ensures materials meet corrosion resistance and performance standards.

Key Information for Designing Sloped Roofs with Steel Sheets

Clause 3.1: Required Data

• Total surface area to be covered
• Types and layout constraints of supporting members
• Slope requirements for drainage
• Fixing details for ridges and related services
• Elevation level for roof placement

Clause 4.1: Purlin Spacing Guidelines

• Minimum one intermediate purlin on simple rafters to prevent deflection.
• For fabricated trusses, spacing based on maximum allowable design distances.

Additional Notes:

• Screws for plain sheets must adhere to IS 6739-1972 and be galvanized.
• Round off values in accordance with IS 2-1960.

flowchart TD
    A[Initiate Roofing Design] --> B[Collect Required Information]
    B --> C{Information Needed}
    C -->|Surface Area| D[Measure Area]
    C -->|Support Elements| E[Determine Type and Limits]
    C -->|Slope| F[Define Drainage Slope]
    C -->|Fixing Ridge| G[Plan Ridge Fixings]
    C -->|Roof Level| H[Determine Installation Height]
    D & E & F & G & H --> I[Calculate Purlin Spacing]
    I --> J{Sheet Thickness}
    J -->|1.00 mm| K[Max Spacing: 2.0 m]
    J -->|0.80 mm| L[Max Spacing: 1.8 m]
    J -->|0.63 mm| M[Max Spacing: 1.6 m]
    K & L & M --> N[Finalize Design]

This section outlines essential parameters and spacing requirements.

Design Principles for Sloped Roofing with Galvanized Steel Sheets

Purlin Spacing (Clause 4.1)

• Maximum center-to-center purlin spacing varies with sheet thickness:

• A minimum of one purlin at mid-span on simple rafters to avoid sagging.
• End purlins are required for each sheet layer.
• For fabricated trusses, spacing based on maximum allowable distances.

Additional Design Considerations (Clause 3.1)

• Define surface area
• Identify support types and constraints
• Provide adequate slope for drainage
• Fix ridge pieces and related services
• Specify roof installation level

Other Specifications

• Plain sheet screws must comply with IS 6739-1972 and be galvanized.
• Round off values as per IS 2-1960.

flowchart TD
    A[Begin Design] --> B[Calculate Surface Area]
    B --> C[Choose Sheet Thickness]
    C --> D[Determine Purlin Spacing]
    D --> E{Simple Rafter?}
    E -->|Yes| F[Place End and Mid-span Purlins]
    E -->|No| G[Follow Truss Spacing Limits]
    F --> H[Ensure Drainage Slope]
    G --> H
    H --> I[Plan Fixing of Ridge and Services]
    I --> J[Set Roof Level]
    J --> K[Use Galvanized Screws (IS 6739)]
    K --> L[Apply Rounding Rules (IS 2)]
    L --> M[Complete Design]

This concise guide supports efficient roofing design.

Key Guidelines for Installing Corrugated Steel Sheets

Lap Requirements (Clause 4.3)

• Minimum end lap length: 150 mm
• Minimum side lap: overlap of 2 corrugations
• Laps may be extended to avoid cutting and to maintain waterproofing and structural strength

Fixing Methods

• Use hook bolts complying with IS 12093 for securing corrugated sheets
• Use slotted round head wood screws for attaching sheets to timber purlins

Installation Procedure

• Lay sheets starting from the eaves progressing upwards
• Ensure overlaps prevent water penetration
• Fix sheets at crest points of corrugations to reduce leakage risks

Lap Dimensions Summary

flowchart TD
    A[Start at Eaves] --> B[Position Sheet]
    B --> C{Check Overlaps}
    C -- End Lap < 150 mm --> D[Increase End Lap]
    C -- Side Lap < 2 Corrugations --> D
    C -- Adequate --> E[Fix at Corrugation Crests]
    E --> F[Install Next Sheet]

Following these steps ensures watertight and stable roofing.

Specifications for Ridges and Hips Installation

Overlaps and Laps (Clause 6.1)

• Minimum lap on each side over roofing sheets: 150 mm
• End laps between ridges and hips must be at least 150 mm
• Ridges and hips should be properly shaped by bending and securely fixed

Fixing Details (Clause 6.2)

• Utilize J or two M6/M8 hook bolts for attachment to roof members
• Ensure at least one fixing bolt passes through end laps on both sides; provide additional bolts if needed
• End laps to be secured with at least two M6 hook bolts per lap
• Set laps using red lead for sealing
• Edges must be straight; surfaces must be flat and parallel to the roof plane
• Ridges and hips must be fitted squarely onto the sheets

Hole Preparation (Clause 5.6)

• Drill holes for bolts/rivets in the crowns of corrugations; punching is not permitted
• Washer holes must match hook bolt diameter exactly
• Nuts to be tightened from above to maintain waterproof integrity

Summary Table

flowchart LR
    A[Ridges & Hips Installation] --> B{Lap Dimensions}
    B -->|Side Lap| C[≥ 150 mm]
    B -->|End Lap| D[≥ 150 mm]
    A --> E[Fixing with Hook Bolts]
    E --> F[J or 2 M6/M8 hook bolts]
    E --> G[Minimum one bolt through end laps]
    E --> H[Two M6 bolts per end lap]
    A --> I[Hole Making]
    I --> J[Drilled holes in corrugation crowns]
    I --> K[Exact bolt diameter matching]
    A --> L[Sealing with Red Lead]

These guidelines ensure secure and waterproof ridge and hip installations.

Standards for Valleys and Flashings – Dimensions and Fixing

Dimensions and Fixing Requirements (Clause 7.1)

• Width and thickness as determined by design or manufacturer’s specifications
• Overlap with corrugated sheets: minimum 150 mm on each side
• End laps must be at least 150 mm and coated with red lead for corrosion resistance
• Flashings and valleys must be bent to the required profile and firmly fixed

Fixing Instructions (Clause 5.6 & 6.2)

• Drill bolt/rivet holes in the crown of corrugations (do not punch)
• Washer holes must exactly fit the hook bolt diameters
• Tighten nuts from above to ensure a watertight seal
• Use J or M6/M8 hook bolts for fastening
• End laps should be joined with at least two M6 hook bolts and sealed with red lead
• Edges of valleys and flashings must be straight and level when installed end-to-end

Lapping and Protection

Additional Recommendations

• Do not embed sheets into gables or parapets
• Edges near walls should be bent upward and sealed with flashing or bitumen mastic gola

Diagram of Lap and Fixing:

flowchart LR
    A[Corrugated Sheet] -->|150 mm Lap| B[Valley/Flashing]
    B -->|150 mm End Lap| C[Valley/Flashing End]
    C -->|≥ 2 M6 Hook Bolts| D[Fixed Joint]
    D -->|Red Lead Coating| E[Corrosion Protection]

Follow these instructions to achieve durable, waterproof valleys and flashings as per the code.

Fastening Procedures for Corrugated Steel Sheets

Riveting (Clause 5.5)

• Use 6 mm diameter galvanized wrought iron rivets
• Attach two galvanized plain round washers per rivet
• Rivet spacing:
  • Side laps: maximum 200 mm intervals
  • End laps: rivets placed on every corrugation in staggered formation
• Rivets must be firmly drawn without causing indentations

Lap Requirements (Clause 4.3)

• Minimum end lap: 150 mm
• Minimum side lap: 2 corrugations
• Laps may be extended to avoid cutting sheets

Screws (Clause 2.3)

• Must comply with IS 6739-1972
• Must be galvanized

Hook Bolts (Clause 5.3)

• Types: J or L hook bolts, size M6 or M8
• Projection above nut: minimum 12 mm
• Grip length on purlin side: minimum 25 mm
• Minimum of 3 hook bolts per sheet per purlin aligned along ridge corrugations
• Maximum spacing between hook bolts: 300 mm

Summary Table

flowchart TD
    A[Steel Sheet] --> B[Side Lap: 2 Corrugations]
    A --> C[End Lap: 150 mm]
    B --> D[Riveting with 6 mm rivets]
    C --> E[Hook Bolt Fixing]
    E --> F[Minimum 3 bolts per purlin]
    F --> G[Spacing ≤ 300 mm]

These methods guarantee secure and corrosion-resistant attachments.

Quality Control and Inspection Guidelines

• Rounding of Measurements (Clause 0.3):

  • Round all test or analysis results as per IS 2:1960.
  • Maintain the same number of significant figures as specified.
  • Follow standard rounding rules (e.g., values ≥ 0.5 round up).

• Quality Checks:

  • Visual inspections for defects and dimensional accuracy
  • Verification against dimensional tolerances specified in IS codes
  • Material testing following accepted methods (tensile strength, hardness)
  • Use of statistical sampling plans per IS 2500 or IS 4905

• General Recommendations:

  • Confirm compliance with mechanical property requirements
  • Retain traceability through test certificates
  • Adhere to IS-approved testing and calibration procedures

Example of Rounding per IS 2:1960

For detailed inspection protocols, consult IS 2500 for sampling and IS 1608 for tensile testing.