Disproportionate collapse assessment
An assessment of a building's robustness against disproportionate collapse, as required by Building Regulations Part A3.
About this service
Disproportionate collapse is about consequences. If a gas explosion, vehicle impact or local failure removes one wall or column, the rest of the building should not fall down in a way out of all proportion to that cause. Requirement A3 of the Building Regulations sets this out, and the extent of checking depends on the building's consequence class, which is driven by its use, number of storeys and floor area.
Houses of up to four storeys usually fall in the lowest class, where normal good practice is enough. Blocks of flats, buildings with communal or public areas, and five-storey buildings typically sit in higher classes that need explicit ties, notional removal checks or key element design. We carry out these assessments for new-build flats, conversions, mixed-use buildings and additional storey projects up to 15 m.
The assessment states the consequence class, sets out a robustness strategy, checks horizontal and vertical ties, and where needed designs key elements for accidental load. Building Control often requests it on conversions where the original building was never assessed. Calculations go through our three-step quality control process with final sign-off by our Head of Engineering. Fees start from £1,500 + VAT.
When you need it
- You're designing a building of three or more storeys or with communal use
- Building Control has asked for a disproportionate collapse assessment
- You're adding storeys to an existing building
What's included
- Consequence class assessment
- Robustness strategy
- Tying and key element checks
- Calculations and details
How it works
- Classify the buildingWe establish the consequence class from use, storey count, floor areas and occupancy, as set out in Approved Document A, and agree it with Building Control where it is borderline.
- Choose a robustness strategyDepending on the class, we adopt horizontal ties, vertical ties, notional removal of load-bearing elements or key element design, often in combination, choosing what is most practical for the building.
- Check the structureWe check that floors, walls and frames are properly tied together and that, if any single element is notionally removed, the resulting damage stays within the limits set by the guidance.
- Design upgrades and detailsWhere the existing or proposed structure falls short, we design ties, straps, connections or strengthened elements and detail them clearly so the contractor can install them correctly on site.
- Issue and liaiseThe assessment and calculations pass our quality control checks, are signed off and sent to Building Control, and we respond to their queries until they are satisfied.
What you receive
Why existing buildings are the hardest to make robust
On a new building, ties and continuity can be designed in from the start at little extra cost. Existing Victorian and Edwardian buildings are another matter. Timber floors often sit loosely in pockets in masonry walls with little positive connection. When such a building is converted into flats or gains a storey, it may move into a higher consequence class, and the existing structure must be shown to be robust or upgraded.
Practical upgrades include steel straps tying floors to walls, connections between new and old structure, and treating certain walls or beams as key elements. The aim is a sensible, buildable strategy rather than rebuilding the structure. Our guides to adding a storey and converting a house into flats explain where this fits in the wider project.
Frequently asked questions
What is disproportionate collapse?
It is a collapse far larger than the event that caused it, for example a whole block of flats failing after one wall is damaged in a gas explosion. Building Regulations Part A3 requires certain buildings to be robust enough to prevent this. The level of protection required depends on the building's consequence class.
Which buildings need a disproportionate collapse assessment?
Most houses of up to four storeys need only normal good practice. Flats, buildings with communal or public areas, and five-storey buildings generally fall into higher consequence classes needing explicit checks. Approved Document A sets out the classes, and Building Control will confirm what they expect for your building.
What is a key element in structural design?
A key element is a structural member whose loss would cause unacceptable collapse, so it is designed to withstand an accidental load instead. Guidance commonly uses a pressure of 34 kN per square metre applied from any direction. Key element design is used where ties or notional removal checks cannot practically be satisfied.
Does converting a house into flats trigger disproportionate collapse checks?
It can. A change of use to flats may move a building into a higher consequence class, and Building Control may ask for evidence that the structure is robust. Older houses often need tying upgrades between floors and walls. We assess the building and design proportionate improvements rather than wholesale rebuilding.