The parlance of buildings can be confusing, there are technical terms and common terms that find their way into everyday use. Some meanings are nuanced and others plain and obvious. Below is a hopefully useful guide should you come across any words or annotations you are not sure about.

It is an active document, and written from experience rather than researched or referenced – please feel free to drop me a line about any omissions or amendments you see fit!


AAV – Air-Admittance valve, sometimes known as a ‘Durgo‘. A one way ‘flap’ valve made of rubber that permits air to be sucked into a drain system, but seals once pressure has normalised to prevent smells escaping. Normally looks like a flat top with small air gap to a plastic vertical pipe. Also refer to SVP, Drain and Sewer.

Abutment – The junction between a roof and a wall, can be a side or head/ridge abutment. See flashing and soaker.

Balustrade – The guarding of a handrail, either on stairs or next to a big drop. Traditionally made up of vertical timbers known as Balusters. In common modern language, Balustrade, bannisters, guarding, spindles and handrail can be said to be interchangeable. Building regulations require them to be non-climbable and not permit a 100mm sphere through any gaps for safety.

Bannister – The guarding of a handrail, either on stairs or next to a big drop. Traditionally made up of vertical timbers known as Balusters. In common modern language, Balustrade, bannisters, guarding, spindles and handrail can be said to be interchangeable. Building regulations require them to be non-climbable and not permit a 100mm sphere through any gaps for safety.

Batten – A small timber section usually in 25,38,50mm sizes square or rectangular. Used for packing out service voids behind plasterboard, as supporting sub-structure for timber cladding and to support roofing slates & tiles.

Beam UKB/UB – A structural member in a horizontal line, usually steel, sometimes concrete. Timber types not usually referred to as a beam unless made of Glulam. UKB refers to United Kingdom Beam, UB refers to Universal Beam, seen on engineers drawings. Numbers adjacent refer to sizes and weights of standard steel profiles, for example 178x102x19UKB refers to a UK Beam of nominal size of 178x102mm by 19KG/m weight grade. Sometimes referred to as RSJ or Rolled Steel Joist, or I-Beams

Breather membrane – A fabric sheet material that is specially woven to prevent liquid water penetrating, but allows water vapour to pass through over time. Used in modern construction to weatherproof timber frame structures and roofs as an underlay ‘felt‘. Can be thought of similar to a ‘Gore-Tex’ jacket. The industry uses the phrase ‘breathable’ to describe the passage of vapour or airborne moisture evaporating away.

Breathable construction – Traditional vapour-open construction would allow walls, floors and (to a lesser extent) roofs to absorb or pass air-borne water vapour, or humidity. Walls and floors are often said to ‘breathe’ in this scenario, as vapour is absorbed by, and released by them over time. Crucially soft constructions such as cob require breathable coatings to prevent trapping in water vapour that will then condense and form liquid water, leading to rot, damp, softening and possibly collapse. Modern paints, plastics, plasterboard, cement pointing, render and the like are all vapour-closed and will not ‘breathe’. See breather membrane, vapour control layer and cob.

Cantilever – Used to describe a structure that projects beyond a point of support, like a diving board. Beams, roofs and even parts of whole buildings can cantilever out over a drop. A cantilevered roof would have no line of support on its outermost edge.

Cavity Wall – A (usually) masonry brick or block wall built with an air gap down the middle. The two ‘leaves’ of the wall are tied together using wall ties. The cavity in the middle serves to prevent water penetrating to the inside, a bit like two skins of a tent. Modern construction also includes insulation on the outer face of the inner leaf. Generally most construction requires a min 50mm air-gap to ensure the system works effectively.

Cavity tray – A water impermeable material (usually plastic sheeting) draped downwards across the cavity in a cavity wall to direct any water that forms droplets in the cavity (on the back of the outer leaf for example) out again. Used above openings or where a cavity wall becomes inside below. Used in conjunction with weep holes to allow water to flow out again.

CHS Circular Hollow Section – A round steel tube, either as a vertical column, or in a beam / tie /brace scenario

Cob – a traditional earth built structure. Earth and straw are mixed and stacked up to form thick solid walls. Once dry they form solid and relatively warm walls. Must be kept dry to prevent instability. Usually protected by large eaves, a raised stone plinth and breathable finishes like limewash and lime or earth plasters and clay based paints.

Column UKC/UC – See Beam for detail, but is a structural member found in a vertical arrangement rather than horizontal. Column profiles tend to be evenly sized, rather than rectangular, and offer stiffer strengths so sometimes engineers use column profiles as beams to keep overall beam depth smaller.

Cornice – Usually a decorative strip or moulding at the junctions of a wall and ceiling in older properties. Can also be used to describe the moulding at the top of a historic column or porch-like structure.

Corbel – A series of bricks progressively stepped outwards on each course to cantilever out or overhang. Traditionally used to hold ends of roof structures at eaves or to support the end of a beam or truss. Can be a stone corbel, typically found in barns or churches for the same purpose.

Cover level – The height of the lid of an inspection chamber or manhole. CL.

Cranked Beam – Refer to Beam for detail. Cranked is usually a horizontal beam that has a welded joint to form an elbow in a steel to create a raked angle.

Dormer – A projecting extension to a roof slope, usually to permit a window (with vertical glass) to a space within. ‘Dormer loft conversions’ would normally refer to the large boxy types, spanning full-width across a house, but the smaller discreet traditional styled ‘dolls house’ roof windows are also called dormers.

Downstand – A beam or bit of ceiling that protrudes down below the general ceiling level. A boxed in beam after a wall is knocked down would be typical of this. Not always necessary in alteration work, but a cheaper way than hiding a beam above the ceiling.

Drain – A pipe used to discharge rainwater and or foul water (See Sewer). Functionally drains and sewers perform the same function, drains are usually privately owned, sewers, connect more than one property and are under the control of the local water authority. South West Water locally here.

DPC/DPM – Damp Proof Course / Damp Proof Membrane. A thin layer within a construction to prevent the passage of ground water up into the materials used for construction. Modern materials are usually a tough plastic sheet laid from a roll. They are overlapped or ‘lapped’ and taped together to form a continuous barrier to moisture. Traditionally layers of slate or engineering bricks were used to similar effect. Older properties may not have one, and provided moisture is not allowed to enter and is allowed to evaporate away and not become trapped, this is not an issue. Injecting in chemical DPCs into walls rarely works and should be undertaken only as a whole-house view, while tackling the cause and not a localised ‘magic pill’ solution.

Durgo – See AAV

Eaves – The lower edge of a sloped or pitched roof, where the rain would run-off it. Usually located with the guttering. Usually overhanging the wall below for protection from weather. Can be a significant overhang (needs to be if the walls are cob) or very tightly ‘clipped’ to just the depth of the gutter and a fascia board. Sometimes they can be a ‘rain-drop’ style without any guttering to catch the rainwater.

Elevation – A drawing of a (building’s) exterior, drawn flat (parallel with the wall surface) with no perspective. Used in technical drawings worldwide for many purposes, but the word can also be used to describe one particular wall or side of a building, eg “the south elevation of the house” Useful to explain a building in drawn format, but can be misleading as people experience buildings through perspective from multiple positions, and usually from ground level looking up.

EPS – Expanded polystyrene. Used in architecture as an insulating material. Made from beads of foamed polystyrene. Generally considered cheaper and less effective than EPX

EPX – Extruded Polystrene. Used in architecture as an insulating material. Made from extruded polystyrene forming a closed cell structure (think Aero bars or a Wispa compared to the balls of polystrene in EPS). Generally considered more effective than EPS


Fascia – A cover strip, usually covering the construction joints or ends. The fascia of a TV for example would be the plastic trim around the sides. In architectural terms it usually refers to the cover strip at the ends of the rafters or verge on a roof. Typically timber, PVC, or pressed metal.

Felt – The underlayer membrane of a roof to prevent any moisture that makes its way through the primary roof covering. Traditionally in a bitumen-coated felt product (like shed roofs) but now generally replaced by breathable or breather membranes. Draped over the rafters, below the roofing battens it allows water to drain away down to the eaves. Condensation can form on the underside if cold roof spaces are not properly ventilated.

FFL or FL – Finished Floor Level, or Floor Level. Refers to the height of a floor level, either referenced to a datum against other building features, such as the other floors, or back to an Ordnance Survey Datum such as sea level.  Also sometimes used are Above FFL, or Below FFL as AFFL and BFFL respectively.

Flashing – Waterproof flexible material to cover joints or corners in external construction to prevent water ingress. Typically lead, but can take the form of cement, fibreglass, aluminium/tin and plastics. Detail, sizes, overlaps and junctions have all been improved through years of development. Lead flashings are long lasting and can be beautifully finished if done well. The Lead Sheet Association publishes the industry standard for detail and how it should be folded, welded and lapped.

Gable – The term used to describe the flat triangular wall under a pitched roof. A Dutch Gable is a more ornamental curved design, often with a parapet front edge. A Gable wall is perpendicular to the eaves in most roof designs.

G or Gulley/Gully, an open grated drainage point connected to below ground drainage. Sometimes RWPs discharge onto gullies, sometimes they collect surface water. Usually with a concealed ‘U’ bend to form a water trap to prevent drainage gases escaping. Sometimes a Back Inlet Gulley (BIG) is used to discharge a rainwater pipe into the back of an open gulley below ground for a neater appearance.

Glulam – Short for Glue-laminated timber, it is a technique to join together smaller pieces of timber together to form a single bigger beam. Developed in modern construction with the advance of glueing techniques and adhesives, as well as an increasing shortage of big timber members. (There are simply less big trees around!) Can span surprisingly long distances, but bigger dimensions than the equivalent in steel, but offers a different aesthetic.

Hip – The term used to describe a ‘corner’ in a roof. Opposite to a valley. Usually where a sloped or pitched roof turns a 90˚ corner, as opposed to a gable end.

Invert Level, IL, The height of the bottom of the lowest pipe inside a manhole or inspection chamber. Usually referenced back to a datum such as FFL. Refer to Cover Level also.

IC – Inspection Chamber, a lidded hatch usually at ground level to access drainage pipework to permit clearing of blockages with drain rods

I-Beams – Refer to Beam

Joist – A structural timber member used in the horizontal plane to make up floors and flat roofs



Kerb – Commonly the raised step to an egde of a pavement, but also used to describe a similar step up on a roof, for example a raised kerb to sit a rooflight upon on a flat roof, so rainwater is kept away from the junction between roof covering and the rooflight itself.

Lean-to – A pitched roof ‘lent’ up against a flat wall

Lead rolls ‘ broomstick’ – A traditional method of joining lead roofing or cladding. Lead expands quite significantly during hot and cold cycles, and this is exacerbated as sheet sizes get bigger. Therefore smaller sheets are joined together by folding up adjoining ‘trays’ and then rolling over a rounded timber core or ‘broomstick handle’ so they stay together and dont let in water. Think of the ‘stripes’ you see on church roofs.

MH – Manhole, a lidded hatch usually at ground level to access drainage pipework or services conduits. Generally larger than inspection hatches or rodding eyes

Newel – The big timber at the end of a balustrade, usually called a newel post.

Nib – Any sticking out or pointy bit. Usually used to describe short projecting bits of wall, to support a beam, like the left over bits of wall either side of a new big opening. Structurally useful to avoid complex engineering or burying columns into party walls etc. Can form alcoves between.

Notch – A small cut out in a joist or rafter, usually to allow the installation of pipes or cables. Refer to this post.


Parapet – Where a wall projects up above the roofline, capped with a coping stone to protect, and lead flashings to join onto the roof. More exposed to the weather than a verge or overhanging eaves but offers a different architectural style. See Dutch Gable. Sometimes used to describe the projecting Party fire walls of Victorian terraces.

Perp ends – Short for perpendicular, the vertical joints in brickwork. ‘Open Perp joints’ would mean no mortar filled in some vertical joints to permit the passage of moisture – see weep holes.

PFC – Parallel Flange Channel, a description of a steel member, shaped like a squared ‘C’.

PIR – Can be Passive Infra-Red used for automatic light or alarm sensors, or short for Polyisocyanurate insulation (think of the typical rigid foil backed yellow insulation boards you see.) A flammable hydro-carbon based insulation, but generally offers the best available heat-retention properties for the thickness.

Plan – Used to describe drawings showing a flat cut-through of a building layout, sometimes ‘floorplan’. See Section and Elevation.

Posi-joist – A trade name for a system using a metal web zig-zag between two smaller pieces of timber to increase the overall depth and therefore ability to span further, by making a mini truss. Can also be a Posi-Rafter for roofing. Allows services and pipes to pass through side-to-side which offers considerable benefits. Can span further than traditional solid timbers.

Pointing – The mortar joints between brick and stone walls.

Purlin – A structural member (steel or timber) running at a perpendicular direction to rafters, to brace and split the span of the rafters. Seen in open roof spaces (as opposed to truss roofs)

Rafter – A structural timber member used in an angled slope (10˚-90˚) to make up a pitched roof. Sloped down following the fall line of a roof. (Battens, purlins and ridges are perpendicular to the fall line)

Resilient bars – Slightly flexible metal folded strips, usually about 25-50mm or thereabouts in size. Fixed to partitions and ceilings at regular centres to then fix plasterboard onto. This slightly ‘decouples’ the plasterboard wall surface from the underlying structure and helps prevent sound energy passing through. Invaluable in achieving proper acoustic separation.

RHS – Rectangular Hollow Section, to describe hollow steel pipes in a rectangular shape.

Rooflight – A specially designed window to be inserted into a roof. A typical trade name would be a ‘Velux’. Generally fall under two categories, a pitched roof or a flat roof. Hard to find products to suit the pitch range between 10˚ and 15˚ as this isn’t really either a flat roof, nor a pitched roof in terms of weather sealing.

RSJ – Refer to Beam

RWP – Rain water pipe, a vertical pipe to carry rainwater from the guttering, usually to stormwater drains or soakaway

SAP Calculation – A calculation to work out heat losses – see this separate description.

Section – A drawing showing a cut through the building in a vertical plane. See plan and elevation.

Sewer – A drain serving more than one property, therefore under control of the water authority. (See Drain)

Soffit – The underside of a projecting or overhead structure. Usually found at the Eaves of a building, or under an overhead floor. Normally only used on exterior features.

Soaker – A flashing of usually folded lead tucked under a roofing slate or tile, and folded up the wall a bit (usually 75mm). Laid in a series under every course of slates/tiles and then a cover flashing is folded down over the uprights to seal the junction between a wall and a roof.

Soakaway – An area where rainwater or surface water is discharged to, to drain away to natural groundwater. Often formed by an area of coarse stones below ground, or plastic ‘crates’ to give some air-gaps to the subsoil and improve drainage. May need testing to ensure that ground conditions allow sufficient speed of discharge. There are specific limits on how close Soakaways can be to boundaries, buildings and such. If one cannot be accommodated, then the drain and sewer network must take the rainwater too, but this is not ideal. Some areas such as Lyme Regis do not allow soakaways due to unstable bedrock conditions.

Stack & branch – terms used in drainage, a stack is usually a vertical pipe used to collect drainage from horizontal branches and take down to the below-ground system. See Drain, Sewer, SVP

Standing seam – Similar to Lead Rolls but a tighter fold without a timber core, offering a sharper more modern look. Typically used on zinc and steel metal roofs, but also on lead and copper.

Stringer – The diagonal strongest member of a stairs, on the outermost edges. Holds the treads and risers of the actual steps.

SVP – Soil Vent Pipe, a vertical pipe to carry foul water from the plumbing facilities. Usually venting to fresh air at the top to prevent back-pressures or vacuums that would lessen the performance of the drains. Traditionally the SVP is the head (highest point) of any drainage system, but the acronym can be used for stacks mid-way along a system too, where it might also be shortened to a SP if not vented. Refer to Drain, Sewer also.

Truss – A structural system of making up small sized timbers into a strong lattice (like the zig-zags on a tower crane) to form long-spanning roof system. Makes conversion of loft spaces difficult, but an economical way of building, and uses more sustainable fast-grown timbers.


u-value – An expression of heat loss through a part of a building (relevant to the temperature difference). Measured in W/m²K. The lower the number the less heat is lost. For example a typical window u-value might be 1.6W/m²K, whereas a wall in an extension might be 0.28W/m²K, with much more heat lost through the window. Workmanship, air-tightness and climatic factors make the whole thing more complex that these simple measurements, but they are a good basic measure of thermal performance of a build-up, or single component. See SAP Calculation.

Valley – The internal corner or ‘crease’ of a roof where two roofs of perpendicular directions meet. Usually 90˚ but not always. Can be a swept valley where the slates/tiles curve around the corner for a seamless look, or a lead valley where a channel lined in lead forms a little mini sloped gutter between the two roof planes. Opposite to a hip.

Vapour control layer / VCL / Vapour check – A physical barrier to water vapour percolating through parts of structure. Usually a polythene sheet taped together but can be achieved in wet applied plasters. Sometimes pre-built into plasterboards. Necessary in roof and timber framed buildings to prevent excess vapour finding its way into the structure and insulation that needs to remain dry. Not used in ‘breathable construction’

Verge – The raked edge of a roof, usually on a gable, perpendicular to the eaves

Wall Tie – A rigid connection between the inner and outer leaves of a cavity wall, to ensure that they act as one, structurally. Typically made from folded stainless steel heavy gauge wire with a drip detail ‘kink’ to ensure any tracking water drips off. Also used in conjunction with plastic clip washers to hold insulation in place. Remedial and timber frame wall ties are screwed into the other leaf for the same purpose.

Web – The middle bit of an I-Beam. The top and bottom two parallel bits are the flanges, and the single middle perpendicular bit is the web. Often a place for fixings, timber packers, insulation or bolt holes to be fitted to.

Weep Holes – Small holes, usually in the vertical perp joints of brickwork or stone to allow water out of a cavity construction. Best practice uses plastic/metal formers to ensure the weep holes remain open and not filled up with mortar squeezing in from above. See cavity tray and cavity wall.