The answer to the design defect vs. construction defect question depends on where the building first departs from the governing requirements. A forensic structural engineer finds that point with three checks in order. The design is checked against the code of record, then the as-built condition against the design, then what changed after completion.
Design defect: a condition in which the construction documents, built exactly as drawn, would not meet the code, referenced standards or project requirements that governed them. Construction defect: a condition that originates in how the work was built rather than in what the documents call for. Whether a particular field condition falls in this category is determined from the documentary record, field observations and any testing the question requires, not assumed.
Why the distinction matters in a dispute
A design question is tested against the code; a construction question, against the approved documents and tolerances.
What are the types of construction defects?
Legal references commonly sort the types of construction defects into four groups: design, material, construction (workmanship), and subsurface deficiencies. Structural work adds a timing question: whether the condition existed at completion or developed later.
Before any check: establish the code of record
The engineer confirms which code edition applied to the project from the permit records and the jurisdiction’s adoption and transition rules. That step comes before any calculation, because loads and detailing rules differ between editions.
Pennsylvania’s UCC regulations adopting the 2021 International Codes took effect January 1, 2026. Philadelphia’s Bulletin B-2501 says applications received on or after July 1, 2026 “must comply with the 2021 editions.” New Jersey’s adoption of the 2024 model codes took effect August 17, 2026, with a six-month grace period.
Check one: the design as drawn against the code
The engineer models the structure exactly as the drawings and specifications show it, taking member sizes and reinforcing from the documents. IBC Chapter 16 sets structural design requirements. The 2021 IBC requires construction documents to provide the information specified in Section 1603.
If a beam on the drawings lacks the capacity the code requires, the shortfall exists on paper. It would be present even if the beam were installed perfectly.
Check two: the as-built condition against the design
The engineer then documents what was built. The engineer records which methods were used, what each one can and cannot show, and whether testing or further diagnosis is needed before a cause can be stated.
Recorded dimensions, reinforcing, and connections are compared with the drawings and with the referenced standards for the material. Examples are ACI 117 tolerances for concrete construction and TMS 402/602 for masonry.
Shop drawings, RFIs, plan revisions and special inspection records are reviewed as part of the documentary record of what was approved and when. IBC Chapter 17 governs special inspections and tests, with required inspections set out in Section 1705.
Check three: what changed since completion
Added rooftop equipment or heavier finishes in a commercial building, or a converted attic in a home, can raise demand. The engineer compares current loads with the design loads stated on the drawings, or derived from the code of record where none are stated.
Deterioration can also reduce capacity. Reinforcing corrosion and steel section loss are measured and analyzed as a potential contributor.
When more than one cause contributes
A single condition can have more than one contributor. When it does, the engineer considers each potential contributor and how they interact.
Table: How a forensic structural engineer classifies a defect
| Category | What it is | What the engineer compares | Typical evidence |
|---|---|---|---|
| Design defect | Documents that would not meet the code of record or project requirements if built as drawn | Design as drawn vs code and referenced standards | Drawings, specifications, calculations, code edition at permit |
| Construction defect | A condition that originates in the work as built | As-built vs drawings, specifications and tolerances | Field observations and measurements, testing where needed, compared with the approved drawings, shop drawings and RFIs |
| Later alteration or deterioration | Changes in load or condition after completion | Current loads and condition vs original design basis | Permit history, alteration drawings, photos, corrosion and section-loss measurements |
| Combination | Two or more of the above acting together | Each potential contributor and how they interact | All of the above, as applicable to the question |
For attorneys: records a construction defect expert witness needs
Records that commonly help a construction defect expert witness separate design from construction include the following, where they exist:
- Permit set and any revised construction documents, with specifications
- Structural calculations and geotechnical report
- Shop drawings, submittals, and RFI log with responses
- Special inspection and testing reports, and the final inspection report
- Permit records for later alterations, and maintenance records
- Photos from construction and from the date distress was first observed
Early steps preserve evidence: photograph conditions before repairs and keep removed materials. See our construction litigation support and testimony article.
FAQ: design defect vs construction defect
What are the four types of construction defects?
Legal references commonly list four types of construction defects: design deficiencies, material deficiencies, construction or workmanship deficiencies, and subsurface or geotechnical deficiencies. A forensic structural engineer also asks whether a condition existed at completion or developed later through alterations, added loads or deterioration. One problem can involve several categories.
What is considered a design defect?
A design defect is a condition in the construction documents that would not meet the governing code or referenced standard even if built exactly as drawn. Examples include an undersized beam or inadequate reinforcing shown on the drawings. The engineer confirms it by analyzing the design as drawn against the code edition that applied to the permit.
How does an engineer evaluate a possible construction defect?
The engineer documents what was built, notes what each method used can and cannot show, and compares the findings with the approved drawings, specifications, and published tolerances. Shop drawings, RFIs, revisions, and inspection records are reviewed as part of the record of what was approved and when. Each documented departure is then evaluated against that record and the applicable tolerances.
Which code applies to an older building?
The edition that applied, called the code of record, is identified from the permit records and the jurisdiction’s adoption and transition rules.. For an older building, that may be a municipal code that predates the current statewide code. Pennsylvania, Philadelphia, and New Jersey all moved to newer editions in 2026, so recent projects need the same check.
Talk to a forensic structural engineer
Gregory S. Baker, P.E., founded Penn Valley Engineering in 2005. He is licensed in PA, NJ, DE, MD, and FL and has served as an engineering expert in the New Jersey Superior Court and Pennsylvania courts. See our structural investigative and forensic services for residential and commercial buildings.
Contact Penn Valley Engineering to discuss a construction defect assessment. Call South Jersey (Cinnaminson) at (856) 872-3537, Philadelphia at (215) 821-7686, or Central Florida (Brevard County) at (321) 394-8026. Email info@pennvalleyengineering.com.
This article is general information, not legal advice, and not an opinion on any specific project or matter.
Sources
- Pennsylvania Department of Labor & Industry: Uniform Construction Code (Triennial Update effective January 1, 2026)
- Jensen Hughes: Pennsylvania Adopts 2021 I-Codes into the UCC
- Philadelphia L&I Bulletin B-2501: Implementation of 2021 International Codes
- New Jersey DCA: Adoption of 2024 Model Codes
- ICC: 2021 IBC Chapter 16, Structural Design
- Montana DLI: Designing to the 2021 IBC (Section 1603 reference)
- STRUCTURE magazine: Special Inspections (2021 IBC Chapter 17)
- American Concrete Institute: ACI 117-10, Specification for Tolerances for Concrete Construction and Materials and Commentary (preview)
- The Masonry Society: TMS 402/602
- FindLaw: Types of Construction Defects
By Gregory S. Baker, P.E., Penn Valley Engineering | As of October 2026
Penn Valley Engineering is a structural engineering consulting firm dedicated to Total Client Satisfaction.