Structural engineering glossary

Glossary terms from A to Z

Acrow propadjustable steel prop

An adjustable steel prop used to hold up floors, walls and roofs temporarily while structural work takes place. The name comes from an early manufacturer but is now used for any telescopic prop. Props must sit on a firm base and be spaced to suit the load, which is why larger openings need a designed propping arrangement.

Basement impact assessmentBIA

A report many London councils require with a planning application for a new or deeper basement. It looks at ground conditions, groundwater, surface water and the likely ground movement and damage to neighbouring buildings, then sets out how the risks will be controlled. Requirements differ between boroughs, so the council's own policy sets the scope of a Basement Impact Assessment.

Beam and base underpinning

An underpinning method where reinforced concrete bases are cast at depth at intervals, with a reinforced concrete beam formed under or through the wall to span between them. It suits walls where continuous mass concrete would need to go very deep, and it limits excavation. The layout and sequence come from the engineer's underpinning design.

Build over agreement

Permission from the sewer authority (in London, Thames Water) to build over or close to a public sewer or lateral drain. Extensions at the back of terraced houses often sit over shared drains. The authority normally wants details of the pipe, its condition and how the foundations will protect it. Our guide on building over a public sewer explains the process.

Building Controlbuilding inspector

The service that checks building work complies with the Building Regulations. It can be your local council or a registered private inspector. For structural work they review the engineer's calculations and drawings, inspect key stages on site and issue a completion certificate at the end. Read more in Building Control and structural work.

Building Regulations Part AApproved Document A

The part of the Building Regulations that deals with structure. It requires buildings to carry their loads safely, resist ground movement and avoid disproportionate collapse. Approved Document A gives guidance on meeting these requirements, and most structural calculations reference it alongside the Eurocodes. A Part A compliance statement sets out how a design satisfies it.

Cavity wall

An external wall built as two leaves of masonry with a gap between them, usually tied together with metal wall ties. Most London houses built from the 1920s onwards have cavity walls, while older Victorian and Edwardian homes typically have solid walls. The cavity resists rain penetration and often now holds insulation. Ties can corrode over time, so see signs of wall tie failure.

CCTV drain surveydrainage survey

A survey where a small camera is pushed through the drains to record their route, size, depth and condition. It shows cracks, root ingress, displaced joints and blockages. It is useful before building near drains, when investigating subsidence and when a sewer authority asks for evidence of condition. We arrange a CCTV drainage survey and report.

Chartered engineerCEng, MICE, MIStructE

An engineer registered as CEng after meeting the competence standards of a professional institution. MICE means Member of the Institution of Civil Engineers and MIStructE means Member of the Institution of Structural Engineers. Both indicate professional review and a commitment to ongoing development. All our structural inspection reports are signed off by a chartered engineer who is MICE. See engineer or surveyor.

Chimney breast

The part of a chimney that projects into a room, containing the flues. Removing one, especially on a lower floor, leaves the remaining breast or stack above with nothing beneath it, so it needs support from steel, gallows brackets or a beam. Any removal also affects your neighbour where the stack is shared. Our chimney breast removal guide covers the options.

Completion certificatefinal certificate

The certificate Building Control issues once work has been inspected and found to comply with the Building Regulations. Solicitors ask for it when you sell, so missing certificates can delay a sale. It usually depends on receiving structural calculations and any site inspection records. If one was never obtained, consider retrospective approval.

Consequence class

A classification that reflects how serious the consequences of a structural failure would be, based mainly on a building's use, size and number of storeys. Houses and small blocks of flats fall into the lower classes. The class decides what level of robustness the design must provide to prevent disproportionate collapse, which a disproportionate collapse assessment addresses.

Crack monitoring

Measuring cracks repeatedly over a period, typically several months, to see whether they are still moving, getting wider or opening and closing with the seasons. The pattern helps an engineer distinguish ongoing subsidence from historic movement. Readings are taken with tell-tales or precise studs and a calliper. It usually starts with a crack monitoring baseline report.

Crack tell-taletell-tale

A small plastic gauge fixed across a crack, with two overlapping plates marked with a grid and a crosshair. As the crack moves, the crosshair shifts across the grid, showing movement in millimetres. Tell-tales are cheap, simple and readable by a homeowner, but need regular, dated readings to be useful. They often form part of a structural movement investigation.

Cracking categoriesdamage categories

A widely used scale that grades visible cracking in masonry from 0 (negligible) to 5 (very severe), based mainly on crack width and ease of repair. Categories 0 to 2 are usually cosmetic, while 3 and above suggest structural movement worth investigating. Crack width alone is not the full story, as our guide are cracks in my house serious explains.

Dead loadpermanent load

The weight of the building itself and anything permanently fixed to it: walls, floors, roof coverings, finishes, plaster and fixed equipment. Dead load is calculated from material weights and does not change over time. It is often the largest load on beams in older London houses, where heavy masonry sits above. Our guide to structural calculations explains how loads are added up.

Deflection

How much a beam, joist or floor bends under load. A beam can be strong enough not to break yet still deflect enough to crack plaster, jam doors or feel bouncy. Engineers check deflection against limits set as a fraction of the span, and stiffness often decides beam size rather than strength. See steel beams and padstones explained.

Disproportionate collapse

A failure where damage to one element, such as a wall or column, triggers collapse far greater than the original damage. Part A requires buildings to be robust against this, with the measures depending on the consequence class. It matters most when adding storeys or converting houses into several flats. We provide a disproportionate collapse assessment where Building Control asks for one.

Dormerdormer loft

A box-shaped structure built out from a sloping roof to create headroom and floor area in a loft. Rear dormers are common on London terraces and semis. Structurally, the new floor, the dormer walls and the altered roof need beams and connections designed to carry the loads back to the walls. Read our loft conversion structural guide.

Eurocodesdesign codes

The European structural design standards now used in the UK, published as British Standards with UK national annexes. They cover loads, concrete, steel, timber, masonry and geotechnical design, among others. They replaced the older British Standards for most new design. Calculations submitted to Building Control will usually reference them, as described in what are structural calculations.

Flitch beamflitch plate beam

A beam made by bolting a steel plate between two or more timber joists or sections. It is stronger than timber alone but lighter and easier to handle than a full steel beam, and it fixes easily to existing timber floors. Flitch beams are often used in loft conversions and to trim around new openings. See loft conversion structural package.

Gallows bracket

A steel bracket bolted to a wall to support a chimney breast left above after the lower part has been removed. Building Control accepts them only in limited situations, often only where the brickwork is sound and the remaining breast is modest, and many require a steel beam instead. Your engineer will check which is suitable in a chimney breast alteration design.

General Structural InspectionGSI

A structural inspection report covering the whole property, rather than one specific defect. The engineer looks at walls, roof, floors, foundations as far as visible and signs of movement, then reports on overall condition and any concerns. It suits buyers and owners wanting a complete structural picture. Compare options in which report do I need.

Glulamglued laminated timber

Engineered timber made by gluing layers of timber together with their grain running lengthwise. It produces large, consistent, strong beams that can span further than ordinary timber and can be curved. Glulam is popular where an exposed timber beam is wanted in an extension or vaulted room, and it has a lower embodied carbon than steel for many uses. See embodied carbon assessment.

Heaveclay heave

Upward movement of the ground as clay soil absorbs water and swells. In London it often follows the removal of a large tree, as the dried clay slowly rehydrates over years. Heave can lift and crack foundations and floor slabs, so new foundations near felled or removed trees may need heave protection. Our guide on London Clay, trees and foundations explains more.

Hip to gablehip to gable loft

A loft conversion where a sloping hipped end of a roof is replaced by a vertical gable wall, enlarging the loft. It is common on semi-detached and end-of-terrace houses and is often combined with a rear dormer. The new gable wall and altered roof need checking for loads and stability. See the loft conversion structural package.

HMOhouse in multiple occupation

A property let to several people from more than one household who share facilities such as a kitchen or bathroom. Larger HMOs need a licence from the council. Converting a house to an HMO or flats can mean extra loads, new fire separation and Building Regulations work. Our guide converting a house into flats or an HMO covers the structural side.

Imposed loadlive load

The load from people, furniture, stored items and other things that can move or change, as opposed to the fixed weight of the building. Standard values for different uses are set in the Eurocodes, with higher values for areas such as balconies and storage. A change of use or new plant can increase imposed load, which a floor loading assessment checks.

Joistfloor joist

One of the parallel beams, usually timber in London houses, that support a floor or flat roof. Joists span between walls or beams and are sized by their span, spacing and loading. Notching or cutting them for pipes and cables, or adding heavy loads such as a bath, can overstress them. A floor strengthening design puts this right.

Lateral restraintrestraint straps

Connections that tie walls to floors and roofs so the walls cannot bow or lean outwards. Galvanised steel restraint straps fixed across joists and built into the wall are the most common form. Restraint is often lost when floors are altered or walls rebuilt, and it must be reinstated. It is a frequent cause of bulging walls in older houses; see remedial works design.

Lintel

A beam spanning over a door, window or other opening in a wall to carry the masonry above. Lintels can be steel, concrete or timber. Standard manufactured lintels suit ordinary openings with normal loads, while wider openings or heavy point loads need a designed beam. Rotting timber lintels are common in Victorian bays, as our guide on bay window movement explains.

Load-bearing wallstructural wall

A wall that carries load from floors, roofs or walls above, as well as its own weight, down to the foundations. Removing or opening one needs a beam, padstones and temporary support, with calculations approved by Building Control. Whether a wall is load-bearing is not always obvious from below. Read do I need an engineer to remove a wall.

London Clayshrink-swell clay

The stiff clay underlying much of London. It shrinks when it dries out and swells when it gets wet, a behaviour called shrink-swell. Trees draw water from it, so foundations near trees can move seasonally or long term, causing cracks. Foundation depth guidance increases with clay shrinkability and tree proximity. See London Clay, trees and foundations.

Mansardmansard roof

A roof with a steep lower slope, close to vertical, and a shallow upper slope, creating an almost full extra storey. Mansards are common on London terraces and in conservation areas where planning allows. They add significant load to the existing walls and foundations, which must be checked. See our mansard roof extension guide.

Method statement

A document setting out how a piece of work will be done safely and in what order, including temporary support, sequence, equipment and checks. For structural work it might cover removing a wall, underpinning in bays or excavating a basement. Building Control, a party wall surveyor or the council may ask for one. We prepare a construction method statement.

Mini-piled underpinning

An underpinning method where small diameter piles are installed next to or through a wall, then linked with a reinforced concrete beam or caps that pick up the wall. It suits poor or deep ground, nearby trees and sites with limited access, as the rigs are compact. It is usually more expensive than mass concrete. See what is underpinning.

Moment framegoalpost frame, portal frame

A steel frame of beams and columns with rigid connections, so it resists sideways forces by bending rather than relying on walls for bracing. They are used when a large rear or side opening removes the walls that kept the house stable. Connections are heavier than ordinary beam seats. See the rear extension structural guide.

Needleneedling

A short beam passed through a hole in a wall and supported on props either side, so the wall above is held up while the section below is removed. Needles are a standard part of temporary works for installing beams and underpinning. Their spacing, size and the props beneath them should be checked. Read temporary works and propping.

Padstone

A solid block, usually concrete or engineering brick, placed under the end of a beam to spread its load into the wall below. Without one, the concentrated load can crush the masonry. Padstone size depends on the beam reaction and wall strength, and is shown in the calculations. See steel beams and padstones explained.

Party wall

A wall shared between two properties, such as the dividing wall in a terrace or semi. Under the Party Wall etc. Act 1996, work such as cutting in beams, raising the wall or digging foundations nearby requires notice to your neighbours. Notice is two months for work to the party wall itself and one month for new walls on the boundary or nearby excavation. See the Party Wall Act.

Party wall awardparty wall agreement

A legal document prepared by party wall surveyors when a neighbour dissents to a notice. It sets out what work may be done, how and when, access rights, and usually includes a schedule of condition of the neighbour's property. Awards often ask for structural drawings and calculations to be reviewed. We provide a party wall structural review.

Permitted developmentPD

Rights that allow some extensions, loft conversions and alterations without a full planning application, within set size and design limits. They do not apply everywhere, for example in some conservation areas or on flats. Permitted development removes the need for planning, not Building Regulations, so structural approval is still needed. See the rear extension guide.

Pilingpiled foundation

Foundations formed by driving or boring columns of concrete or steel deep into the ground to reach firmer strata or to carry loads through friction. Piles suit poor ground, deep clay affected by trees and heavy loads. In London, small rigs install mini-piles where access is tight. Your engineer will advise in a foundation design.

Purlin

A horizontal beam running along the length of a roof slope, partway up, supporting the rafters. In many London lofts the purlins are propped by struts down to internal walls. A loft conversion often removes those struts, so the purlins need new support or replacement with steel. See the loft conversion structural guide.

RAACreinforced autoclaved aerated concrete

A lightweight concrete used in roof, floor and wall panels, mostly between the 1950s and 1990s. It is weaker than ordinary concrete and can deteriorate, particularly if water gets in, with some panels failing with little warning. It is rare in houses but appears in some flats and mixed-use buildings. See RAAC: what property owners need to know.

Raft foundationraft

A reinforced concrete slab spread under the whole building or extension, rather than strips under the walls. Rafts spread loads over a large area and help bridge soft spots, so they suit weak or variable ground and some sites affected by trees. They need careful design of thickness and reinforcement. See a foundation design.

Retaining wall

A wall that holds back soil or water at a change of ground level, such as a garden terrace, a lightwell or a basement wall. It must resist the soil pushing sideways as well as the vertical loads. Leaning, cracking or bulging retaining walls need prompt assessment. We provide retaining wall design.

Retrospective approvalregularisation

A way to get Building Control approval for work already carried out without it. You apply to the council for a regularisation certificate, and the work may need to be opened up so it can be inspected, with calculations to show compliance. It is common when selling a home. See retrospective Building Regulations approval.

Ridge beam

The beam at the apex of a roof where the rafters meet. In a traditional cut roof the ridge board only locates the rafters, but in loft conversions a structural steel or timber ridge beam is often added so the rafters can hang off it, stopping the roof pushing the walls apart. See the loft conversion structural package.

RSJuniversal beam, steel beam

Rolled steel joist, a common name for a steel I-shaped beam. Modern sections are called universal beams (UB) and are specified by size and weight per metre. They carry loads over openings where walls are removed. The right section depends on span and loads and is shown in calculations. See load-bearing wall opening design.

Schedule of condition

A dated record of the condition of a property, with photographs and notes of existing cracks and defects, taken before nearby building work starts. If damage is later alleged, it shows what was there beforehand. It is common under party wall procedures and basement projects. We prepare schedules of condition.

Settlement

Downward movement of a building as the ground compresses under its weight, mostly soon after construction. Most old houses settled long ago and are stable, though the cracks or sloping floors may remain. Settlement differs from subsidence, which is caused by ground movement after the building has settled. See subsidence signs.

Soakaway

An underground pit or crate system that collects surface water from roofs or paving and lets it soak into the ground. It only works where the soil drains well, which London Clay often does not, so infiltration testing is needed before relying on one. Soakaways also need to be set a safe distance from buildings. See soakaway and infiltration testing.

Specific Structural InspectionSSI

A structural inspection report focused on one defined issue, such as a crack, a bowing wall or a sagging ceiling. The engineer inspects, explains the likely cause and recommends what to do. It is quicker and more targeted than a full inspection, and is often requested by mortgage lenders. See the Specific Structural Inspection report.

Steel calculationsbeam calculations

Structural calculations that size a steel beam or column and its supports. They set out the loads, beam size, deflection, bearing and padstone design, and connections. Building Control needs them before approving the work. We send calculations and drawings to Building Control and liaise with them. See what are structural calculations.

Strip foundation

A continuous strip of concrete beneath a wall, wider than the wall, to spread its load into the ground. Traditional strip foundations are fairly shallow, with brick footings in older houses. For new work in clay they often become trench fill foundations instead. See our foundation design service.

Structural engineer

An engineer who designs and assesses the parts of a building that carry loads, such as beams, walls, floors, roofs and foundations. Homeowners need one to remove walls, convert lofts, build extensions, investigate cracks and satisfy Building Control. Fees vary with the job; see structural engineer cost in London.

Subsidence

Downward movement of the ground beneath a building, not caused by the building's own weight. In London the most common cause is tree roots drying out clay soil, followed by leaking drains washing out the ground. Signs include tapering diagonal cracks and sticking doors and windows. See subsidence signs and what to do.

SuDSsustainable drainage systems

Ways of managing rainwater close to where it falls, slowing it down before it enters sewers. Examples include soakaways, permeable paving, green roofs, rain gardens and attenuation tanks. Councils often require a SuDS strategy for new homes and larger extensions as a planning condition. We prepare a sustainable drainage strategy.

Temporary workspropping

Any work that holds up the structure or ground temporarily during construction, such as props, needles, shoring and excavation support. Poor temporary works are a common cause of collapses on small sites. Larger openings, basements and underpinning need a designed scheme. See temporary works design and propping scheme.

Trench fill foundation

A foundation where a narrow trench is dug and filled almost to ground level with concrete. It is quick to build and suits the deeper foundations often needed in London Clay, especially near trees. Depth is set by soil type and tree influence, and Building Control will usually inspect the trench. See tree influence assessment.

Trimmertrimmer joist

A joist that runs across the ends of cut joists around an opening such as a stairwell or loft hatch. It transfers the load from the cut joists to the full-length joists either side, called trimming joists, which are often doubled. New staircases in loft conversions need trimming designed. See the loft conversion guide.

Underpinningmass concrete underpinning

Deepening or strengthening a foundation so it reaches firmer, stable ground. The traditional method, mass concrete underpinning, digs out and fills short sections under the wall in a set sequence. Alternatives include beam and base and mini-piled underpinning. It is used to treat subsidence and for basements. See what is underpinning.

Wall tiescavity wall ties

Metal ties that connect the two leaves of a cavity wall. Older steel ties can corrode and expand, causing horizontal cracks along the mortar joints, while too few ties can let walls bulge. Replacement ties can be fitted from outside. See our wall tie condition survey.

Wind load

The force wind exerts on walls and roofs, pushing or pulling depending on direction. It depends on location, height, exposure and the shape of the building. Wind load matters most for gable walls, tall walls, roof structures and buildings losing internal walls. Calculations follow the Eurocodes; see structural calculations.