The jargon decoder

Renovation glossary

107 terms you’ll meet in quotes, drawings and on site, defined in plain English by the engineers who use them. If a builder says it and you nod without knowing why, it should be on this page.

A

Air brick #: A perforated brick that ventilates the void under a suspended floor or a cavity. Blocking them is a classic cause of damp and rot. Repairs & Problems guides →
Approved Document #: The official guidance documents (Parts A–S) explaining how to comply with the Building Regulations in England, Part L covers energy, Part B fire, Part P electrics. Planning & Permissions guides →
Architrave #: The decorative moulding framing a door or window opening, covering the joint between frame and plaster. Decorating & Paint guides →
Article 4 direction #: A council order removing permitted development rights in a specific area, meaning you need planning permission for work that normally wouldn't require it. Planning & Permissions guides →

B

Balustrade #: The guarding along stairs, landings or balconies, spindles, handrail and newel posts together. Building regs set minimum heights and gap sizes. Structural guides →
Battens #: Thin timber strips fixed to a wall or roof as a base for tiles, cladding or plasterboard. Roofing Materials guides →
Bearer #: A horizontal timber that carries and spreads load, for example under a water tank or decking joists. Structural guides →
Blockwork #: Walls built from concrete blocks, denser 7.3N blocks for load-bearing work, lightweight aircrete for insulation. Brick, Block & Mortar guides →
Breathable membrane #: A roofing/wall underlay that lets water vapour escape while keeping liquid water out, reducing condensation risk. Insulation & Membranes guides →
Building control #: The inspection regime (council or approved inspector) that checks work complies with the Building Regulations and issues the completion certificate you'll need when selling. Planning & Permissions guides →
Building Regulations #: The legal minimum standards for structure, fire, energy, ventilation, electrics and more. Separate from planning permission, most renovation work needs building regs sign-off even when no planning is required. Planning & Permissions guides →

C

Cavity tray #: A damp-proof barrier built into a cavity wall above openings or where a roof meets a wall, directing water back outside through weep holes. Brick, Block & Mortar guides →
Cavity wall #: A wall of two leaves (usually brick outer, block inner) with a gap between, now normally filled or part-filled with insulation. Brick, Block & Mortar guides →
CDM Regulations #: Construction (Design and Management) Regulations 2015, health-and-safety law that applies to ALL building work, giving homeowners duties they usually discharge by appointing competent contractors. Managing the Project guides →
Chartered surveyor #: A surveyor qualified through RICS, the people behind valuations, homebuyer reports and party wall awards. Trades & Professionals guides →
Chasing #: Cutting channels into walls to bury cables and pipes before plastering. Heating, Plumbing & Electrics guides →
Cold bridge #: See thermal bridge. Insulation & Membranes guides →
Completion certificate #: The document building control issues when work passes final inspection. Solicitors ask for it when you sell, chase it the moment work finishes. Planning & Permissions guides →
Condensation #: Moisture from indoor air hitting cold surfaces, the most common 'damp' in UK homes, fixed by ventilation, heating and insulation rather than tanking. Repairs & Problems guides →
Conservation area #: A designated area of special character where permitted development rights are restricted and demolition needs consent. Planning & Permissions guides →
Contingency #: Money held back for the unknowns, engineers typically carry 10–15% on renovation budgets, more on older buildings. Costs & Budgeting guides →
Curtilage #: The land immediately around a house, permitted development rules limit how much of it outbuildings can cover. Planning & Permissions guides →

D

Damp-proof course (DPC) #: The horizontal barrier near the base of walls stopping ground moisture rising. Bridging it (raised ground, render) is a frequent cause of damp. Brick, Block & Mortar guides →
Damp-proof membrane (DPM) #: The plastic sheet under a concrete floor slab that stops ground moisture coming up through the floor. Floors & Screeds guides →
Day rate #: What a trade charges per day rather than per job. Useful for small or open-ended work; get fixed quotes for defined packages. Costs & Budgeting guides →
Dead load #: The permanent weight a structure carries, the building itself and fixed finishes, as opposed to live loads like people and furniture. Structural guides →
Defects liability period #: The window after completion (often 6–12 months) in which the builder must return and fix defects, usually tied to the retention payment. Managing the Project guides →
Dormer #: A boxed extension through a roof slope creating headroom and a vertical window, the workhorse of loft conversions. Loft Conversions guides →
Dot and dab #: Fixing plasterboard to walls with dabs of adhesive instead of a stud frame, fast and common, but hollow-sounding and poor for hanging heavy loads. Materials – Interior guides →
Dry rot #: A fungus that destroys structural timber in damp, unventilated spaces and can spread through masonry. Needs professional treatment and, above all, the moisture source fixed. Repairs & Problems guides →

E

Easement #: A legal right over someone else's land, like a drainage run or right of way, that can constrain where you build. Planning & Permissions guides →
Eaves #: Where the roof meets the top of the walls. Loft conversions live or die by eaves height; extensions near boundaries are height-limited at the eaves. Loft Conversions guides →
Engineered timber #: Manufactured structural wood, glulam, LVL, I-joists, stronger and straighter than sawn timber for long spans. Steel & Structural Timber guides →
EPC #: Energy Performance Certificate, the A–G energy rating every home needs when sold or let. Renovations are the moment to climb the bands. Heating, Energy & Eco guides →
Extension over drains #: Building over or near public sewers needs a build-over agreement from the water company, a routine but easily forgotten approval. Extensions guides →

F

Fascia #: The vertical board along the eaves carrying the gutter, usually paired with a soffit underneath. Roofing Materials guides →
Fink truss #: The standard W-braced roof truss in post-1960s homes. The webs make lofts hard to convert without structural redesign. Loft Conversions guides →
First fix #: Everything installed before plastering, structural carpentry, pipe runs, cable runs, ducting. Second fix is what you see afterwards. Managing the Project guides →
Flashing #: The lead or composite weatherproofing where a roof meets walls, chimneys or windows. Failed flashing is a top cause of leaks. Roofing Materials guides →
Flitch beam #: A composite beam of steel plate bolted between timbers, stiffer than timber alone, lighter to handle than pure steel. Structural guides →
Footings #: See foundations, the concrete underground that spreads the building's load into the soil. Foundations & Groundworks guides →
Foundations #: The below-ground structure carrying the building. Depth depends on soil, trees and drains, why extension quotes carry a 'subject to ground conditions' line. Foundations & Groundworks guides →

G

Gable #: The triangular upper section of an end wall between roof slopes. A hip-to-gable conversion builds the sloping end up into a full gable for loft space. Loft Conversions guides →
Gang #: A crew of trades working together, 'a 2+1 gang' means two tradespeople and a labourer. Gang size × day rate × duration is how labour is priced. Costs & Budgeting guides →

H

Header tank #: The small cold-water tank (usually in the loft) feeding an open-vented heating system, often removed when switching to a sealed system or combi. Heating, Plumbing & Electrics guides →
Herringbone strutting #: Diagonal timber or steel bracing between floor joists that stops them twisting and shares load between them. Structural guides →
Hip #: The sloping external corner where two roof planes meet. Hipped roofs steal loft space, hence hip-to-gable conversions. Loft Conversions guides →
Hipped roof #: A roof that slopes on all four sides rather than ending in vertical gables. Roofing Materials guides →

I

I-joist #: An engineered joist shaped like an I, light, straight and deep, spanning further than sawn timber floors. Steel & Structural Timber guides →

J

Joist #: The horizontal timbers (or steel/engineered sections) carrying floors and ceilings. Structural guides →

K

K-value #: See U-value; the thermal conductivity of a single material (lambda), where lower conducts less heat. Insulation & Membranes guides →

L

Lath and plaster #: Pre-1940s wall/ceiling construction: plaster pressed onto thin timber laths. Fragile, dusty to remove, and a sign wiring and pipework may be equally dated. Renovation Essentials guides →
Lawful Development Certificate #: Council confirmation that work is lawful (usually as permitted development). Not compulsory, but gold when you sell, get one for any significant PD project. Planning & Permissions guides →
Lead time #: How long between ordering something and it arriving. Windows, steels, kitchens and roof windows have the lead times that wreck programmes. Managing the Project guides →
Lintel #: The beam over a door or window opening carrying the wall above. Replacing windows can trigger lintel checks; new openings always need one. Structural guides →
Live load #: The variable loads on a structure, people, furniture, snow, that design must allow for on top of dead load. Structural guides →
Load-bearing wall #: A wall carrying floors, roof or other walls above. Removing one needs an engineer's design and building regs approval, never assume a stud wall isn't load-bearing. Structural guides →

M

Making good #: Repairing surfaces disturbed by the works, patching plaster, filling, redecorating. Whether it's included is a classic quote gap. Costs & Budgeting guides →
Mansard #: A roof/extension form with a near-vertical slope (typically 70°) and flat top, giving maximum loft volume, common on London terraces, usually needing planning permission. Loft Conversions guides →
Mastic #: Flexible sealant used at joints, around windows, worktops, baths. 'Mastic man' is a real trade at the end of a job. Decorating & Paint guides →
MVHR #: Mechanical ventilation with heat recovery, whole-house ducted ventilation that recovers heat from stale air; standard in airtight new builds and deep retrofits. Heating, Energy & Eco guides →

N

Newel post #: The main vertical post anchoring a staircase handrail at bottom, turns and top. Materials – Interior guides →
Noggin #: A short timber block fixed between studs or joists for stiffness and fixings. Structural guides →

O

OSB #: Oriented strand board, the engineered sheet used for roof decks, walls and hoardings. OSB3 is the moisture-resistant structural grade. Steel & Structural Timber guides →
Overheads and profit (OHP) #: The builder's margin on top of labour and materials, typically 15–25% on domestic work. Cheap quotes usually just hide it in the rates. Costs & Budgeting guides →

P

Padstone #: A dense concrete or engineering-brick block spreading a steel beam's point load into the wall below. Structural guides →
Party Wall Act #: The 1996 Act governing work on shared walls, boundary walls and excavation near neighbours. You serve notice; neighbours consent or surveyors produce an award. Planning & Permissions guides →
Party wall award #: The legal document surveyors produce when a neighbour dissents to party wall notice, recording condition, working hours and protections. Budget £1,000–£2,000+ per dissenting neighbour. Planning & Permissions guides →
PC sum #: Prime cost sum, an allowance in a quote for items not yet chosen (say £3,000 for a kitchen). If your choice costs more, the price rises. Interrogate every PC sum before signing. Costs & Budgeting guides →
Permitted development (PD) #: Work you can do without planning permission under national rules, many lofts, single-storey extensions and outbuildings qualify, within strict size limits. Flats and designated areas are largely excluded. Planning & Permissions guides →
PIR insulation #: Rigid polyisocyanurate foam boards, the workhorse insulation for roofs and floors, roughly twice as effective per centimetre as mineral wool. Insulation & Membranes guides →
Planning permission #: Council consent for development that isn't permitted development. Decisions target 8 weeks; factor it into any programme. Planning & Permissions guides →
Prelims #: Preliminaries, the running costs of a job that aren't labour or materials: scaffold, skips, welfare, supervision, insurance. Typically 8–15% of a project. Costs & Budgeting guides →
Principal contractor #: Under CDM, the contractor in overall charge of a project with more than one trade on site. Managing the Project guides →
Provisional sum #: A quote allowance for work that can't be priced yet (e.g. 'underpinning if required'). The final cost replaces the allowance, up or down. Costs & Budgeting guides →
Purlin #: A horizontal beam supporting roof rafters mid-span, often the timber you see running across an unconverted loft. Structural guides →

R

Rafter #: The sloping timbers carrying the roof covering. Structural guides →
Render #: The external cement or silicone coating on walls. Modern thin-coat silicone systems flex and breathe better than hard sand-cement. Cladding & Render guides →
Retention #: A percentage of the contract price (commonly 2.5–5%) held back until defects are fixed at the end of the liability period. Costs & Budgeting guides →
Ridge #: The horizontal top line of a roof. Ridge height limits are a key permitted-development constraint for lofts. Loft Conversions guides →
Rising damp #: Ground moisture rising up walls where the DPC has failed or been bridged. Rarer than sold, most 'rising damp' is condensation or a bridged DPC. Repairs & Problems guides →
RSJ #: Rolled steel joist, the colloquial name for the steel beams (technically UBs/UCs) that carry openings when walls come out. Structural guides →

S

Sarking #: Boarding or membrane fixed over rafters beneath the roof covering. Roofing Materials guides →
Screed #: The levelling layer (traditionally sand-cement, increasingly flowing anhydrite) laid over a slab or insulation to take floor finishes. Floors & Screeds guides →
Second fix #: The visible installations after plastering, sockets and switches, radiators, sanitaryware, doors, skirting. Managing the Project guides →
Section 106 / CIL #: Planning obligations and the Community Infrastructure Levy, payments larger schemes make to councils. Most home extensions are exempt, but check CIL on big rebuilds. Planning & Permissions guides →
Skim #: The thin finish coat of plaster (2–3mm) that gives walls their smooth final surface. Materials – Interior guides →
Snagging #: The end-of-job inspection listing defects, chips, sticking doors, missed sealant, for the builder to fix before final payment. Managing the Project guides →
Soakaway #: An underground pit or crate system dispersing rainwater into the ground, required where surface water can't join a drain. Foundations & Groundworks guides →
Soffit #: The horizontal board under the eaves closing the gap between fascia and wall, often ventilated. Roofing Materials guides →
Soil vent pipe (SVP) #: The vertical pipe taking waste to the drain and venting sewer gases above the roof. Moving bathrooms means thinking about the SVP first. Heating, Plumbing & Electrics guides →
Steels #: Site shorthand for structural steel beams and posts. Fabrication lead times (2–4 weeks) make steels an early ordering priority. Structural guides →
Stud wall #: A non-masonry wall of timber or metal studs lined with plasterboard. Quick to build, but sometimes still load-bearing. Materials – Interior guides →
Subsidence #: Downward movement of the ground under foundations, from clay shrinkage, trees or leaking drains, causing stepped cracking. An insurer and engineer matter, not a filler matter. Repairs & Problems guides →

T

Tanking #: Waterproof coatings or membranes holding water out of basements and wet rooms. Brick, Block & Mortar guides →
Thermal bridge #: A path of poor insulation, a steel, a junction, a solid lintel, where heat escapes and condensation or mould can form. Detailing kills bridges, not just thickness. Insulation & Membranes guides →
Trickle vent #: The small controllable vent in a window frame providing background ventilation, required in most new and replacement windows since 2022. Windows, Doors & Glazing guides →
Trussed rafter #: Factory-made triangulated roof frames (see Fink truss). Never cut one without an engineer's design. Structural guides →

U

U-value #: How fast heat passes through a building element (W/m²K), lower is better. Building regs set maximum U-values for new walls, roofs, floors and windows. Insulation & Membranes guides →
UB / UC #: Universal Beam and Universal Column, the standard steel sections engineers specify, e.g. '203×102 UB'. Structural guides →
Underpinning #: Strengthening or deepening existing foundations, for subsidence, basement digs, or when new loads exceed what old footings can carry. Foundations & Groundworks guides →

V

Vapour control layer (VCL) #: The membrane on the warm side of insulation stopping indoor moisture reaching cold surfaces inside the build-up. Insulation & Membranes guides →

W

Wall plate #: The timber bedded on top of a wall that roof rafters or joists bear on and fix to. Structural guides →
Warm roof #: A roof insulated above or between rafters so the structure stays warm, the standard for flat roofs and loft conversions, avoiding condensation traps of 'cold roof' build-ups. Insulation & Membranes guides →
Weep hole #: Small openings in mortar joints letting water drain out of a cavity above trays and lintels. Brick, Block & Mortar guides →
Wet rot #: Timber decay from persistent moisture, less aggressive than dry rot, fixed by curing the leak and replacing affected wood. Repairs & Problems guides →
Wet underfloor heating #: Heating pipes cast into screed running at low temperatures, pairs naturally with heat pumps; needs floor build-up height that retrofits must find. Heating, Energy & Eco guides →