Specifying corrugated roofing for an industrial building. How to assess substitutions

Operating conditions are design inputs for the envelope. A label such as “industrial use” leaves important questions unanswered. A precise description of the duty allows the team to judge whether a cheaper, similar-looking sheet can perform the same task on this roof.

Specify the profile, sheet sizes and effective cover dimensions used in the design. The sheets must match laps, supports and moulded accessories. If another profile is permissible, identify the parts of the design that must be checked again. Include a sheet layout and detail drawings to show whether a change affects quantities, cutting waste or joint positions. Identical overall width does not guarantee the same effective cover width or fixing pattern.

State the full reaction-to-fire designation or other required product property for the particular variant. Separate sheet performance from whole-roof requirements. Natural and coated sheets may have different declarations. Identify the necessary documents and the intended use they must cover. Check the unit and conditions behind every catalogue value. Mass per sheet, mass per nominal square metre of material and load per square metre of finished roof are different quantities. A useful specification lets bidders compare like with like.

Support spacing, fixings, laps and seals must work together under the design loads. Use accessories belonging to the chosen system and match fasteners to the base material and exposure. Cite the edition of the installation instructions used in the design. Check any new variant against that edition, including preparation of holes and corners. Mixing recommendations from different editions can alter how a joint works. A sheet substitution may require several system details and drawings to change along with the cost estimate.

Ask the contractor proposing a change for supporting documents and a comparison of geometry, performance, support requirements and accessories. Record the effects on the frame, cost and programme. “At least as good” gives no basis for approval. Equivalence must be established for this application. Agree who can approve a change and by when. Approval before delivery allows drawings to be corrected. Once the sheets are in place, assessing them is harder and remedial work more expensive.

Define batch identification, delivery documents and how to report shortages or defects. Inspect sheets before installation, particularly around corners and holes. A different factory-prepared variant may need a different laying sequence. Identify details that must be inspected before they are concealed. Record the actual fixings and seals at handover so the installed assembly can be checked against the approved detail. An invoice listing only a generic material name cannot identify the roof system.

At handover, provide a plan of materials, photographs of significant connections and the maintenance instructions. Retaining the profile designation and fixing detail makes future repairs easier. Identify safe inspection routes and the locations needing attention. State that subsequent equipment installations require assessment against the covering and the structure. These records remain useful beyond final payment and reduce the chance of an incompatible component being introduced during a minor repair.

Cover building use, geometry, performance, installation and handover in the specification. Assess substitutions before delivery using the documented differences and their effect on the complete design.