Knowledge Base
Planning, regulations and construction, explained.
Topic 14
Structural engineering
The structural engineer’s role, from calculations for beams and openings to foundations, cracks and movement.
22 articles
The engineer's role
01Do I need a structural engineer?You should involve a structural engineer when proposed work changes how your building is supported, or when you need a diagnosis of possible structural movement. A straightforward inspection, an alteration design and ongoing construction advice are different services, so agree which you need before commissioning the work.02What are structural calculations, and when are they needed?Structural calculations explain why the proposed beams, walls, floors, foundations and connections can safely carry the relevant loads. They are needed where the design must demonstrate structural adequacy, particularly for alterations that cannot be justified using suitable standard details alone.03Designer vs structural engineer: who does what?The architectural designer develops the layout and building details, while the structural engineer designs or assesses the parts that carry loads and keep the building stable. Their work overlaps, so the important question is who is responsible for each element and for coordinating the design as a whole.04What your structural engineer needs: surveys, trial pits and existing drawingsThe engineer needs reliable information about the existing building, the proposed changes and any known defects or previous alterations. Measured drawings are a starting point, but concealed supports or uncertain foundations may require targeted opening-up, trial pits or other investigations before a design can be completed.05How long calculations take, and what affects the costThe time and cost of structural calculations depend on the scope, the information available and the amount of investigation and coordination required. A fee for one clearly defined element is not directly comparable with a commission covering a whole alteration, revisions and construction visits.06Will the engineer inspect or sign off the finished work?An engineer's inspection and report must be agreed as part of the appointment, rather than assumed to follow a calculations-only service. This is separate from building-control approval and its completion documentation.
Beams and openings
07Steel beams: sizing, fire protection and installationA steel beam must suit your opening's loads, span and supports; width alone is not a design. Its bearings, connections and fire protection must be specified alongside the steel itself.08What is a padstone?A padstone is a load-spreading support beneath a beam end, used where the concentrated load needs to be distributed over the masonry below. Whether one is needed, and its size and specification, should follow the structural design rather than a standard detail copied from another house.09Goalpost frames for wide openingsA goalpost frame is an arrangement of a horizontal beam supported by two columns, which can provide a structural solution around a wide opening. Whether it is suitable depends on the loads, connections, foundations and the building's sideways stability, not just the width of the proposed doors.10Lintels for new doors and windowsA lintel supports construction above a door or window opening and transfers its load to the supports at each side. A new or enlarged opening needs a suitable designed support; replacing the window itself does not establish that the existing lintel is adequate.11Temporary support during the works: props and needlesProps and needles are temporary supports used to keep a structure stable while permanent alterations are carried out. Their arrangement must suit the actual loads and construction sequence; knowing the final beam size does not tell a contractor how to support the building safely during demolition.
Lofts and chimneys
12Loft floor joists, steels, purlins and ridge beamsA loft conversion normally needs a structural assessment of both the new floor and the altered roof, including how their loads reach the walls and foundations. Existing ceiling joists and roof supports should not be assumed suitable for a habitable room or removed to create space without a replacement design.13Chimney breast removal: gallows brackets or a steel beam?Removing a chimney breast requires a designed way to support any chimney masonry left above; a steel beam or, in some circumstances, gallows brackets may be considered. Gallows brackets are not a universally accepted shortcut, and the condition of the wall and the neighbour's chimney arrangement can rule them out.
Foundations
14Foundation types: strip, trench fill, raft and pilesStrip, trench-fill, raft and piled foundations transfer building loads to the ground in different ways. The suitable type depends on the soil, loading, expected movement and construction constraints; none is automatically the right or cheapest choice for every London extension.15How deep do foundations need to be?There is no single foundation depth that is correct for every extension. The design must respond to the ground, trees, nearby structures and drains, and the loads being supported; a blanket “one metre” allowance is not a site assessment.16Trees and clay soil: shrinkage and heaveClay can shrink as it loses moisture and swell as it becomes wetter, moving the ground that supports a building. Trees can influence that moisture balance, so foundations near trees on shrinkable clay need a site-specific assessment of both subsidence and heave.17Trial pits and soil investigationsTrial pits expose shallow ground or existing foundations so that an engineer can check conditions rather than design entirely from assumptions. They are one investigation method, not a substitute for every soil test or a guarantee that the whole site has been characterised.18Underpinning: when it's needed and how it's doneUnderpinning strengthens or extends an existing foundation where an engineering assessment shows it is necessary, for example to address an inadequate footing or support an altered loading arrangement. It is not the automatic remedy for every crack or every subsidence claim, and it requires building-regulations approval.19Can my existing extension take another storey?An existing extension may be capable of supporting another storey, but that must be demonstrated from its structure, foundations and the proposed additional loads. Its age, apparent solidity or original approval does not establish spare capacity for a different building.
Cracks and movement
20Which cracks are structural?A crack's significance depends on its pattern, location and accompanying movement, not width alone. Some cracks result from drying or temperature changes, while new or worsening cracks can need structural investigation; a photograph cannot establish the cause.21Subsidence: signs, causes and insuranceSubsidence occurs when sinking ground beneath a building affects its foundations. Suspected subsidence needs professional assessment and early contact with the buildings insurer; a crack alone does not establish the cause or cover.22Settlement cracks after a new extensionCracks after an extension can result from drying materials, temperature changes or movement as the new construction settles, but not every new crack is harmless settlement. The appropriate response depends on the pattern, whether it is changing and whether other parts of the building are moving.