What are structural calculations and when do you need them?
Building Control asks for them, builders work from them, and most homeowners never see inside them. Here is what structural calculations are and why they matter.
The short answer
Structural calculations are a structural engineer's written proof that each new or altered part of a building, such as a beam, joist, lintel or foundation, can safely carry the loads on it without failing or bending too much. They are prepared to meet Building Regulations Part A, normally using the Eurocodes, and you need them whenever building work changes how loads travel through the structure.
In a typical London home that means removing a load-bearing wall, building an extension, converting a loft, taking out a chimney breast, adding a basement or strengthening a floor. Building Control will not approve these works without them.
What is inside a calculation package
A domestic package might run from a few pages for a single beam to dozens for a loft or a basement. The format varies between engineers, but a good package follows a logical order that a Building Control officer can check line by line.
- Design basis: the standards used, the materials and any assumptions about the existing building
- Loads: the weight of the structure itself, the people and furniture it supports, and snow and wind where relevant
- Load take-down: how those loads travel from roof to foundation
- Member design: checks on each beam, joist or column for bending, shear and deflection
- Bearings and padstones: checks that the masonry under each beam end can take the concentrated load
- Connections and foundations: bolted joints, post bases and footings where they apply
- Stability: how the building resists sideways forces, especially when walls are removed
- A summary schedule of member sizes, with drawings or marked-up plans showing where each one goes
How the loads are worked out
Engineers split loads into two main types. Permanent loads are the weight of the building itself: brickwork, timber, roof tiles, plaster and screeds. Variable loads are things that come and go, such as people, furniture, snow on the roof and wind pressure on the walls. For domestic floors, the Eurocodes set a standard figure for the imposed load per square metre, which covers normal household use.
Those loads are then multiplied by safety factors before the members are checked, so a beam is designed to carry noticeably more than it will see in normal life. The engineer also checks deflection, meaning how much a beam sags under load. A beam can be strong enough and still bend enough to crack the plaster below, so deflection often governs the size in house alterations.
Sideways loads are easy to overlook. Wind pushes on the walls, and the house resists it partly through its internal walls acting as bracing. When several walls come out to create an open-plan ground floor, or the whole rear wall is replaced with glazing, the engineer has to show the house is still stable. That is why some schemes need a rigid steel frame rather than a simple beam on two posts.
Who uses the calculations
Building Control reviews them to confirm the design meets Part A, then inspects on site against them. Your builder uses them to order the right steel, timber and concrete, and to follow the propping and installation notes. A steel fabricator will take the member sizes and connection details from them. If the work involves a party wall, the adjoining owner's surveyor may ask to see them too.
They keep their value long after the work is finished. When you sell, the calculations and the Building Control completion certificate together show a buyer's solicitor that the alteration was designed and approved properly. We send the calculations and drawings to Building Control on your behalf and deal with any questions they raise.
What makes good calculations
Good calculations are based on the actual building, not a generic assumption. The engineer needs to know the wall thickness, what sits above, which way the joists run and what the existing foundations are likely to be. Online beam calculators size a member in isolation, but they say nothing about whether the brickwork under the bearings can take the load or whether the floor above is supported elsewhere.
They should also be buildable. A beam that is technically adequate but cannot be lifted into a terraced house, or a connection detail no fabricator can make, causes delays and variations on site. Every design and calculation we produce goes through a three-step quality control process, with final sign-off by our Head of Engineering, partly to catch exactly these practical problems.
Finally, assumptions should be stated openly. If the engineer has assumed the existing foundation is a certain depth, or that a wall is solid brick rather than two leaves with a cavity, the calculations should say so and the builder should confirm it on site. A clear assumption that turns out to be wrong is easy to deal with. A hidden one is not.
When the design changes
Schemes change. The architect moves an opening, the client wants a wider span or the builder suggests a different beam that is easier to get. Any of these can invalidate the original calculations, and Building Control will expect the revised design before the work is inspected.
Site discoveries matter too. Opening up often reveals rotten timber, a missing lintel or a wall in poorer condition than expected. Tell the engineer straight away rather than improvising. A structural design revision is far cheaper than putting right something that was built without one.
Getting calculations for your project
For a straightforward job, such as a single wall opening, the engineer may only need photographs, a few measurements and a site visit. For larger work, the architect's drawings are the starting point and the engineer designs the structure around them. Our extension structural package and the calculations behind a loft conversion follow the same principles at a larger scale.
Every job starts with a fixed fee confirmed in writing before any work starts, and our typical turnaround is 10 working days.
Frequently asked questions
Do I need structural calculations for a non-load-bearing wall?
Usually not, because removing a wall that carries nothing does not change how loads travel through the building. The difficulty is being sure it really carries nothing. In older London houses, timber partitions often support the floor above. If there is any doubt, an engineer's site visit to confirm the wall's role is cheap compared with repairing sagging floors later.
Are structural calculations the same as structural drawings?
No, but they go together. Calculations prove that each element is adequate. Drawings show the builder where each element goes, how it connects and what size it is. For a simple beam, a marked-up plan may be enough. For an extension or loft, Building Control and the builder will usually want proper structural drawings alongside the calculations.
Can I reuse calculations from my neighbour's identical house?
It is not a good idea. Houses that look identical often differ in wall thickness, joist direction, previous alterations and ground conditions. Building Control will expect calculations prepared for your property and signed by the engineer who designed it. An engineer may be able to work faster if they have designed a similar scheme nearby, but the design still needs checking against your building.
Does Building Control check the calculations?
Yes. On a full plans application, the Building Control officer or a checking engineer reviews the calculations and may ask questions before approving. On site, the inspector checks the work against the approved design. Building Control does not take on design responsibility, though. That stays with the engineer who prepared the calculations.