The email at 11:14 AM
A general contractor sends a message to the engineer of record mid-morning. The drawings assumed a 40 lb/ft² mechanical load on the penthouse slab. The owner now wants a 1,200 lb rooftop air handler sitting on four point bases inside a 3 by 5 foot footprint. Can the slab take it. By when can the engineer confirm.
The answer lands in the contractor's inbox before end of day. Not as drawings. Not as a calc package. As a one-and-a-half page sealed letter on firm letterhead, addressed to the contractor by name, stating what was reviewed, what basis was used, what assumptions apply, and what the professional engineer's opinion is. PDF. Signed. Sealed.
That document is a sealed opinion letter. It is also the doc Ontario's construction industry quietly runs on, the one a principal signs two or three times a week, and the one almost nobody writes about.
What the category actually is
A sealed opinion letter is a short-form sealed engineering document, usually 1 to 3 pages, written to a named recipient on firm letterhead, carrying the professional seal of a licensed engineer. It responds to a specific question the recipient asked. The engineer does not visit a site to write one. The engineer reads the drawings, considers the proposed change, and produces a letter stating a professional opinion supported by a basis.
The difference from a field review report is structural, not stylistic. A field review report is generated by a site visit. The observation comes first, the narrative follows. Its structure is enumerated (observation, finding, recommendation), and the length follows the complexity of what the engineer saw.
A sealed opinion letter is the inverse. The question comes first. The observation is desk-based. The letter's structure follows correspondence convention: date block, recipient address, project reference, narrative body under all-caps or sentence-case headings (DOCUMENTS REVIEWED, ASSUMPTIONS, ASSESSMENT, CONCLUSIONS, LIMITATIONS), a closing paragraph, and the seal block. The engineer is not reporting what happened on site. The engineer is answering a question.
That shift, observation-forward to question-forward, changes what the document has to do. A field review report establishes a record. A PE sealed letter establishes an opinion. Both carry the same professional liability. The mechanism for exercising that liability differs.
The shapes the industry asks for
Within the category, a handful of shapes recur often enough to deserve names. These are the five the construction industry most frequently asks a structural engineer to produce.
Load review letter. Called when a proposed load exceeds what the original design assumed, whether an RTU, a chiller, a generator, an MRI, a safe, a forklift in a warehouse, or heavy snow during a retrofit. The engineer reviews the governing drawings, compares the new demand against the existing structure's capacity, and states whether the existing structure can accept the load, whether reinforcement is required, or whether the proposal must change. 1 to 3 pages. Often accompanied by a single appended calc if the capacity margin is tight.
Shoring requirement letter. Issued to the contractor before demolition or partial renovation, confirming the shoring scope the engineer of record requires for the proposed work. A shoring letter names the elements being removed, identifies the loads that need to be carried during the work, and either endorses the contractor's proposed shoring arrangement or defines the arrangement the engineer requires. Because the letter is contractor-facing and becomes part of the trade's planning record, its scope statements and limitations carry disproportionate weight. A vague shoring letter is the document behind a partial collapse.
Coring review letter. Tabular, location-by-location. The contractor submits a coring layout through an existing slab and the engineer reviews each hole against the slab's reinforcement, embedded conduit, and post-tensioning. Most coring review letters are mostly tables: hole ID, location, slab type, acceptability, and hole-specific constraints. The narrative framing is short. The judgment is dense.
Feasibility letter. Typically owner-facing, often landlord-facing. Can this wall be removed. Can this mezzanine carry an office fitout. Can this roof support a rooftop terrace. The engineer's opinion is binary at the top, conditional in the middle, and bounded by a limitations paragraph at the bottom.
Hydro vault review letter. A specialty subset. The local distribution company (LDC) requires sealed engineering review of on-site transformer vault designs as a condition of connection, with the LDC's own engineering specification governing structural adequacy, ventilation, and access provisions. The Electrical Safety Authority enforces the Ontario Electrical Safety Code on the electrical side in parallel. The letter is the document the LDC reads before it will energize the transformer.
These are not exhaustive. Many firms maintain additional letter shapes for wall-opening reviews, tenant-improvement load reviews, preliminary and detailed existing-building assessments, and post-event walk-throughs. The common property is format: short-form sealed correspondence answering a specific question on short notice.
Why brevity is what makes the category defensibility-critical
On an observation-forward document, defensibility erodes through transcription errors. The engineer saw the right thing. The written record introduced the inconsistency.
On a question-forward document, defensibility erodes through compression errors. The engineer knew what they meant. The letter did not say it.
A sealed opinion letter has to compress a professional judgment into a page and a half. That compression is under time pressure. Opinion letters frequently carry 24 to 72 hour turnarounds because a trade is waiting to proceed. What the engineer cuts to make the page is where defensibility leaks.
The scope statement. A letter that says "we have reviewed the proposed loading" without naming which drawings, which revision, which dates, and which structural elements were considered leaves the defensible boundary of the opinion unclear. In a dispute, the scope statement is the evidentiary answer to the question "what did you actually look at."
The basis. A load review letter that states "the existing structure can accept the proposed load" without citing the drawing the engineer measured against, or the applicable code clause, or the material strength value used, is an opinion without a footing. The basis is what moves the opinion from assertion to engineering judgment.
The assumptions. Short letters routinely depend on assumptions the engineer cannot verify without a site visit: that the existing structure was built per the drawings, that no unauthorized modifications have occurred since construction, that the loads identified by the recipient are accurate. Naming those assumptions is how the engineer properly scopes the liability. Omitting them is how the liability silently broadens.
The limitations. Every sealed opinion letter ends with a limitations paragraph by convention. "This opinion applies to the conditions described above and the drawings referenced. Any change to these conditions may invalidate the opinion." A letter without a limitations paragraph has no outer boundary.
These four elements are cheap to include and expensive to omit. On a 15 page field review report, a missing limitations paragraph is a minor finding. On a 1.5 page sealed letter, it is half the document's structural integrity.
The regulatory landscape
Sealed opinion letters sit under the same professional framework as every other sealed engineering document in Ontario. The practitioner's obligation to sign and seal is defined by PEO Regulation 941 section 53, rewritten by O. Reg. 837/21 and in force in its current form since 2021. The practice obligation to exercise reasonable care is defined by the common-law duty of care and the firm's own standard of practice.
Specific regulatory hooks vary by letter shape.
- Load review letters generally reference OBC Part 4 for structural design loads and the project's governing material standards (CSA A23.3 for concrete, CSA S16 for steel, CSA O86 for wood, CSA S304 for masonry).
- Shoring requirement letters reference OBC Part 4 for the structural evaluation and the shoring-specific provisions in the contract documents. Some cite CSA S269 for formwork and falsework. A contractor opinion letter on shoring may additionally reference occupational-health-and-safety obligations.
- Hydro vault review letters reference O. Reg. 22/04 (Electricity Distribution Licences), the Electrical Safety Authority's standards, and the specific local distribution utility's engineering specification.
- Coring review letters reference the governing structural drawings and the element-specific provisions of the original design. For post-tensioned slabs, the underlying tendon layout drawings carry as much weight as the general structural drawings.
- Feasibility letters reference the OBC part relevant to the proposed modification and any applicable specialty regulation. For renovation of existing buildings, OBC Part 11 is often the relevant framework.
PEO has published practice advisories that touch the letter category without naming it explicitly. The advisories on the "designer" role, on retention of engineering documents, and on the use of seals all apply. A sealed opinion letter is an engineering document. The practice obligations that attach to any engineering document attach to it.
Why the trade press does not cover the category
Search engineering publications for "field review report" and results are everywhere. Search for "engineer opinion letter Ontario" or "sealed opinion letter" and the trade press is near silent. This asymmetry is not accidental.
Field review reports are externally visible. They are cited in permit conditions, referenced in municipal audits, and occasionally surface as exhibits in construction disputes. A journalist or a trade association has reasons to write about them.
Sealed opinion letters are private correspondence. They travel from engineer to contractor, or engineer to owner, or engineer to utility. They are not filed with a municipality. They do not appear in any external repository. The reader of the letter is the recipient named in the salutation. No third party indexes them. No association tracks them. No publication writes about a document it cannot read.
This invisibility has a practical consequence. The body of accumulated best-practice writing about sealed opinion letters is thin, even though the category itself is routine. Younger engineers learn to write them by copying their principal's prior letters, absorbing conventions that were never written down. Firms develop house formats in isolation from each other. The craft is mature. The literature is not.
That gap is the reason a firm's letter conventions vary so widely. It is also why a firm's most load-bearing defensive habits, things like scope statements, explicit assumptions, and named limitations, are often inherited verbally from a senior engineer's voice rather than codified on a template.
Where this leaves the category
A sealed opinion letter is the daily habit of a working Ontario engineering practice. A structural firm signs more of these than any other sealed document type in a given year. They are brief, they are defensible only when they are explicit, and they are the least-studied category of a profession that otherwise writes a lot about its own documentation.
For engineers drafting them, the operating question is whether the firm's house letter carries all four defensibility elements, scope, basis, assumptions, and limitations, in every letter that goes out, consistently, under the time pressure the category imposes. For firms writing them, the operating question is whether the craft is codified anywhere outside the principal's head.
A free audit tool for the category lives at /tools/letter-audit. Paste a draft letter and get an independent read on scope clarity, assumption documentation, named basis, and limitations. For firms interested in how Fermito drafts sealed opinion letters end-to-end from a PE's verbal brief, the demo is at /demo.
