Almost any project that removes a wall, forms a new opening, or alters how a building carries its loads needs a structural engineer at some point. What varies is when in the process they're brought in, and how thoroughly their design gets followed once it reaches site — both of which affect the outcome more than most homeowners expect.
Understanding the sequence helps you check it's happening properly on your own project, rather than assuming a beam has been 'sorted' because someone mentioned a size in conversation.
Identifying which elements are actually load-bearing
Before any calculation happens, the engineer or an experienced surveyor needs to establish which walls, floors or roof elements genuinely carry load, since this isn't always obvious from appearance alone. A wall that looks structural can be a later stud partition, and a wall that looks decorative can be carrying the floor above — assumptions here get corrected properly with a site inspection, sometimes involving opening up a small section to check.
This step also identifies where loads travel once a wall is removed — down through remaining walls, onto padstones, and ultimately into the foundations — because the whole path matters, not just the opening itself.
Commissioning calculations and details
Once the load path is understood, the engineer produces calculations sizing the new beam or structural element, along with drawings showing bearing lengths, padstone sizes, and connection details to existing structure. These documents are what gets submitted to building control, and what the builder should be constructing to precisely, not approximating from a general steel size mentioned on the phone.
On more complex projects, this can include temporary works design — how the structure is safely propped while the new beam goes in — which is a separate and equally important piece of engineering that shouldn't be left to guesswork on site.
Confirming bearing, padstones and connections
A beam is only as good as what it sits on. The engineer specifies padstone sizes and bearing lengths to spread the load into the surrounding wall without crushing it or overloading a weak spot, and connection details for how the new steel ties into existing masonry or timber. These details are frequently where site conditions differ from what was assumed on paper, and any discrepancy found during construction should go back to the engineer rather than resolved on site by judgement alone.
Where an existing wall or foundation is found to be in worse condition than assumed once it's exposed, the engineer may need to revise the design, and building control will expect evidence this revision happened rather than assume it did.
Site inspections at key stages
For structurally significant work, the engineer typically inspects at key points — once an opening is formed and before the beam goes in, and again once it's installed — to confirm the site matches the design assumptions. This isn't duplication of the building control inspection; it's a separate check that the engineering, specifically, has been followed.
Some engineers issue a certificate of structural adequacy once satisfied, which is a useful document to keep alongside the building regulations completion certificate for any future sale.
Keeping one current drawing on site
It's common on longer projects for a detail to be revised after an initial site visit uncovers something unexpected. The risk this creates is a builder working from an outdated drawing while a revised one sits in someone's inbox unopened. Making sure only the current revision is on site, and superseded versions are removed or clearly marked, avoids a structural element being built to specifications that were already known to be wrong.
This is a coordination task that falls to whoever is managing the project day to day, and it's worth confirming explicitly who that responsibility sits with before work starts.
Common questions
Do I always need a structural engineer for a loft conversion?+
Almost always, since a loft conversion typically involves new floor joists, strengthened structure to carry the additional load, and sometimes steel beams at the roof — all of which need calculating rather than estimated by eye.
How much does the engineer's involvement cost relative to the job?+
It varies with complexity, but it's a small proportion of typical project costs relative to the risk of an under-designed structural element. It's not an area worth cutting to save money.
What if the builder wants to use a different beam size than specified?+
Any change from the engineer's specified size or detail should go back to the engineer for approval before it's built, since a substitution — even one that looks equivalent — can affect deflection, bearing and building control sign-off.
Get an engineer involved in your structural project
We work with structural engineers on every job that needs one, and can arrange calculations and site inspections as part of the build.