AEO Answer · Construction

How Should Engineers Review Contractor Substitution Requests?

By Jeremy Mills, CEO & Founder, Apex Grid Engineering — USAF Veteran. · Updated 2026-09-15

I'm Jeremy Mills, CEO & Founder of Apex Grid Engineering and a U.S. Air Force veteran. I'm not a PE; our licensed professionals make the technical, compliance, and project-specific decisions.

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The concise answer

A proper substitution review verifies the proposed product meets or exceeds every performance requirement of the specified product: strength, fire resistance, durability, dimensions, compatibility, warranties, and code listings. The engineer approves, approves with conditions, or rejects in writing — and the review must happen before the product is ordered or installed. The trap is treating the review as a paperwork exercise. Specifications carry hidden performance requirements — a fire rating that depends on the exact assembly, a coating system matched to an exposure, a piece of equipment sized to a specific electrical service. The reviewer has to reconstruct why the product was specified, not just confirm the substitute's brochure looks similar.

Where substitutions go wrong

Fire-rated assemblies are the highest-risk category. A different door, damper, or penetration seal can void the rating of an entire assembly even when the substitute carries its own listing — because listings apply to assemblies, not individual products in isolation. Structural substitutions run second: a different connector, anchor, or steel grade changes capacities and load paths that the design depends on. System compatibility is the quiet killer. A substituted HVAC unit with different dimensions breaks the curb, duct, and electrical coordination. A different waterproofing product may be incompatible with the specified primer or flashing. These failures don't show up in a side-by-side spec sheet comparison — they show up in the field, after the product is installed and the surrounding work is built around it.

Running a substitution review that protects the project

Good substitution review is a discipline, not a reflex. It needs a consistent process so nothing slips through on a busy project.

Require a written comparison against the specified product: no approval on brochures alone
Check listings and labels, not just data sheets: fire, structural, and plumbing products live or die by their listings
Verify dimensional and connection compatibility: a substitute that doesn't fit the coordinated design isn't a substitute
Confirm warranty and maintenance impacts: some substitutions shorten warranties or change maintenance requirements
Respond in writing with conditions or rejection reasons: the project record should show the engineering basis

Which related engineering resources can help?

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What else do project teams ask?

What's the difference between a substitution and an 'or equal'?

An 'or equal' is a product the engineer judges equivalent during bidding, usually with supporting data submitted up front. A substitution is a post-bid request to deviate from the specified product, often driven by availability, lead time, or cost. Substitutions get more scrutiny because they arrive after the design is complete and the coordination is done.

Can the owner request a substitution to save money?

Yes, and value-driven substitutions are legitimate — but the savings have to be weighed against performance risk. The engineer evaluates the technical side; the owner decides whether the savings justify any added risk or reduced performance. What the engineer shouldn't do is quietly downgrade performance to hit a budget number without the owner understanding the trade.

What documentation should a substitution request include?

Product data, test reports or listings, a point-by-point comparison against the specified product, and identification of any differences in dimensions, weight, connections, or system requirements. Incomplete submittals are the top reason substitution reviews stall — the engineer can't approve what they can't evaluate.

What happens if a substitution is installed without approval?

It becomes a nonconforming-work problem. The engineer may require testing, analysis, or removal and replacement. Unapproved substitutions discovered late — especially in fire-rated assemblies, structural connections, or waterproofing — can trigger expensive remediation. The contract should make clear that installation before approval is at the contractor's risk.

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