Glossary
Prefab & offsite construction, term by term
Plain, objective definitions of the components, methods and machines used in industrialised house building. Where relevant, it also covers how each step is automated in a production line.
Building elements
- Batten A batten is a thin strip of timber fixed across a roof or wall to carry tiles, cladding or a ventilation gap.
- Building envelope The building envelope is the physical barrier between inside and outside: the external walls, roof, lowest floor, windows and doors.
- Centre bearing wall A centre bearing wall is a load-bearing interior wall, usually along the middle of a house, that supports floors or roof between the external walls.
- Cladding Cladding is the outer skin of a building, such as timber boards, panels, render or brick slips, which protects the wall from the weather and gives the facade its look.
- Closed panel A closed panel is a prefabricated wall or roof element that leaves the factory boarded on both faces, with insulation and often windows, services and cladding fitted.
- Floor cassette A floor cassette is a prefabricated floor section: joists with sheeting and often insulation and services. It is built off site and craned into place as a unit.
- Floor joist A floor joist is one of the parallel horizontal beams that carry a floor and span between walls or beams.
- Gable end A gable end is the triangular upper part of an end wall, between the two slopes of a pitched roof.
- Header A header is the horizontal member that spans the top of a door or window opening in a framed wall and carries the load from above around the opening.
- Infill wall An infill wall is a non-load-bearing external wall fitted between the floor slabs or frame of a building, closing it in without carrying its structure.
- Lintel A lintel is a horizontal beam that spans the top of a window or door opening and carries the load of the wall, floor or roof above it.
- Multi-stud A multi-stud is two or more studs fixed together into one member, used at corners, wall junctions, openings and point loads.
- Nail plate A nail plate is a toothed steel plate pressed into timber across a joint to connect the members of a truss or frame.
- Nogging A nogging is a short horizontal piece of timber fixed between studs or joists to stiffen the frame and give a fixing point for boards and fittings.
- Notch A notch is a recess cut into a piece of timber so that another member can sit in or over it.
- Open panel An open panel is a prefabricated wall frame that is sheathed on one side only, or not at all, leaving the cavities open for insulation and services on site.
- Prefabricated house A prefabricated house is a house whose main parts (walls, floors, roof) are manufactured in a factory and assembled on site.
- Rafter A rafter is a sloping roof beam that runs from the ridge down to the wall plate and carries the roof covering.
- Roof element A roof element is a prefabricated section of roof or ceiling, built as a flat panel of joists or rafters with boarding and often insulation, and craned into place.
- Roof truss A roof truss is a prefabricated triangulated timber framework that supports a roof and spans the space between external walls.
- Rough opening A rough opening is the framed hole in a wall that a window or door is later fitted into, sized slightly larger than the unit itself.
- Service cavity A service cavity is a zone on the inside of a wall or ceiling, in front of the vapour barrier, where cables and pipes run without piercing it.
- Storey-and-a-half house A storey-and-a-half house has a full ground floor and a second floor built into the roof, with sloping ceilings and knee walls.
- Stud A stud is one of the vertical members that make up the frame of a wall panel.
- Thermal bridge A thermal bridge is a local area of the building envelope where heat escapes more easily than through the surrounding construction.
- Top and bottom plate The top and bottom plates are the horizontal timbers at the head and foot of a wall frame that the studs are fixed between.
- Truss types Roof trusses come in many shapes, such as fink, mono, attic, scissor and girder trusses, each suited to a different roof form, span or use of the roof space.
- Volumetric module A volumetric module is a complete three-dimensional section of a building, such as a room or apartment, built in a factory and transported to site as a unit.
- Wall panel A wall panel is a prefabricated section of wall: a timber or steel frame, often already fitted with sheathing, insulation, windows and services. It is built in a factory and assembled on site.
- Wind barrier A wind barrier is the layer on the outside of the insulation that stops wind blowing through it while letting moisture escape.
Methods
- Design for manufacture and assembly (DfMA) DfMA is a design approach that shapes a building from the start so its parts are easy to manufacture in a factory and quick to assemble on site.
- House factory A house factory is a production plant where the walls, floors, roofs or complete modules of houses are manufactured before delivery to site.
- Lean production Lean production is a way of organising manufacturing around a continuous flow of value to the customer, removing every step that does not add to it.
- Light gauge steel (LGS) Light gauge steel is thin, cold-formed steel sections used as an alternative to timber for the frames of walls, floors and roofs.
- Mass customisation Mass customisation is producing individually designed products with the efficiency of series production.
- Modern methods of construction (MMC) Modern methods of construction (MMC) is the UK umbrella term for building methods that move work from the site into a factory or use new processes on site.
- Offsite construction Offsite construction is building in a factory rather than on the building plot, then delivering the finished elements or modules to site.
- Panelized construction Panelized construction builds a house from flat prefabricated wall, floor and roof panels that are assembled on site.
- Precast concrete Precast concrete is concrete cast into elements such as walls, slabs, beams and stairs in a factory, then transported and assembled on site.
- Prefabrication Prefabrication is the practice of manufacturing building components in a factory and assembling them on site.
- Takt time Takt time is the pace at which a product must be completed to meet demand: the available production time divided by the number of units required.
- Timber frame Timber frame construction builds walls, floors and roofs from a framework of timber studs, joists and rafters or trusses, closed with boards.
Machines & lines
- Angled cutting Angled cutting is crosscutting timber at an angle across its width, with the saw blade swivelled but still upright, so the cut face stays vertical.
- Buffer station A buffer station is a position in a production line where elements can wait between two processes, so a short stop in one does not stop the other.
- Butterfly table A butterfly table is a hinged work table that turns a wall, floor or gable element over so both faces can be worked on and the element can be stood up.
- Compound cutting Compound cutting is a cut where the saw blade is both swivelled and tilted, so the cut face is angled across the timber and inclined at the same time.
- Crosscut saw A crosscut saw cuts timber across the grain to length and angle, producing the individual members for trusses, walls and floors.
- Element gluing Element gluing is applying adhesive between the frame and the boards of a wall, floor or roof element, so they act together as a stiffer unit.
- End-of-line stacking End-of-line stacking is the last step of a production line, where finished elements are collected and stacked in the order they will be loaded and erected.
- Framing station A framing station is the machine that assembles studs, plates and noggins into a wall frame and nails them together.
- Jib crane A jib crane is a crane with a horizontal arm (the jib) that swings around a column or wall bracket and carries a hoist or tool.
- Jig A jig is a fixture that holds parts in exactly the right position while they are assembled, so every copy comes out the same.
- Laser projection Laser projection shows the outline of each part as lines of laser light on the work table, so operators can place parts without measuring or marking.
- Machine vision Machine vision uses cameras and image processing to let a machine see and measure the parts in front of it, and act on what it finds.
- Multifunction bridge A multifunction bridge is a gantry that travels over a wall or floor element and carries several tools, fastening, sawing, milling and marking the element in one pass.
- Nailing bridge A nailing bridge is a gantry that travels over a wall panel and drives the nails fixing sheathing boards to the frame automatically.
- Notching saw A notching saw is a dedicated saw for cutting notches and recesses in timber, typically with one horizontal and one vertical blade working together.
- Robotic cell A robotic cell is an enclosed workstation where one or more industrial robots carry out a defined task automatically.
- Truss press A truss press is the hydraulic machine that presses nail plates into the joints of a roof truss.
- Wall production line A wall production line is a sequence of linked stations that takes a wall from framing through to a finished panel.
Data & software
- BIM (building information modelling) BIM is the practice of designing and managing a building through a shared digital model that holds both its geometry and information about every part.
- Bottleneck A bottleneck is the step in a production process with the least capacity, which limits the output of the whole line.
- CAD/CAM CAD/CAM links computer-aided design with computer-aided manufacturing, so machines are driven directly by the design data rather than by manual measurement.
- CDT file A CDT file is the data format Randek systems use to describe a building element precisely enough for machines to produce it.
- Production planning Production planning decides what a factory produces, in what order and on which line, to meet delivery dates with the capacity available.
- Traceability Traceability is being able to follow each produced element back through its production: when, where, from what material and with what result it was made.
Materials
- Blown-in insulation Blown-in insulation is loose insulation material (cellulose, glass or stone wool) blown into a cavity until it fills it to a specified density.
- Cross-laminated timber (CLT) Cross-laminated timber (CLT) is a solid wood panel made from layers of boards glued crosswise, each layer at right angles to the one below.
- Glulam Glulam (glued laminated timber) is a structural timber product made by gluing layers of timber together with the grain running in the same direction.
- Gypsum board Gypsum board (plasterboard, drywall) is a sheet of gypsum plaster between paper or fibre facings, used for interior linings and fire protection.
- LVL (laminated veneer lumber) LVL (laminated veneer lumber) is an engineered timber made from thin veneers glued together with the grain in the same direction.
- Moisture content (timber) Moisture content is the weight of water in a piece of wood expressed as a percentage of its dry weight.
- OSB (oriented strand board) OSB (oriented strand board) is a structural board made from layers of wood strands glued and pressed together, with the outer layers aligned for strength.
- Plywood Plywood is a board made from thin wood veneers glued together with the grain of each layer at right angles to the next, giving strength in both directions.
- Sheathing board Sheathing boards are the large sheets (OSB, plywood, particleboard, gypsum) fixed to a timber or steel frame to close it, brace it and form a surface.
- Structural timber Structural timber is sawn wood that has been strength graded so it can be used in load-bearing construction with known properties.
- Tongue and groove Tongue and groove is an edge profile where a ridge on one board fits into a slot on the next, locking boards together into a continuous surface.
- Vapour barrier A vapour barrier is a membrane on the warm side of the insulation that limits how much moisture from indoor air can migrate into the wall or roof.
Quality & sustainability
- Construction waste Construction waste is the surplus material produced when building, from timber offcuts and board trimmings to packaging and damaged goods.
- Dimensional tolerance Dimensional tolerance is the permitted deviation from a specified size, such as how much a wall panel may differ from its drawn length, height or squareness.
- Downtime Downtime is the time a machine or production line is not producing, whether because of breakdowns, changeovers, waiting for material or planned maintenance.
- Embodied carbon Embodied carbon is the greenhouse gas emitted in producing, transporting and assembling a building’s materials, as opposed to the emissions from running it.
- Ergonomics in production Ergonomics in production is designing work so people can do it without strain or injury: lifts, reaches, postures and repetition within safe limits.
Roof trusses
- Apex (ridge) The apex is the highest point of a pitched truss, where the two top chords meet under the ridge of the roof.
- Attic truss An attic truss leaves an open rectangular space in the middle of the roof, so the loft can be used as a room.
- Bottom chord The bottom chord is the lower member of a truss that ties the two ends together and usually carries the ceiling.
- Bowstring truss A bowstring truss has a curved top chord and a straight bottom chord, like a bow and its string, and spans large open halls.
- Cambered truss A cambered truss has a bottom chord that is slightly raised in the middle, giving a little extra ceiling height and a gently sloped ceiling.
- Cathedral truss A cathedral truss creates a high, vaulted ceiling, with the bottom chord following the roof slope for all or part of its length.
- Coffered truss A coffered (tray) truss has a bottom chord that slopes up at the sides and runs flat in the middle, creating a raised tray ceiling.
- Collar beam A collar beam is a horizontal member that ties a pair of rafters or top chords together part-way up the roof.
- Double fink truss A double fink truss doubles the W web pattern of a fink truss, adding webs so the truss can span further or carry heavier loads.
- Double Howe truss A double Howe truss extends the Howe pattern with extra verticals and diagonals, for long spans and heavy loads.
- Dual pitch truss A dual pitch truss has top chords at different pitches on each side, giving an asymmetric roof with an off-centre ridge.
- Duo-pitch roof A duo-pitch (gable) roof has two slopes meeting at a central ridge, with a gable wall at each end.
- Fan truss A fan truss has webs that fan out from points on the bottom chord to the top chord, giving the top chord extra support.
- Fink truss A fink truss is the standard pitched roof truss, with internal webs arranged in a W; it is the most widely used truss type for houses.
- Flat truss (parallel chord) A flat truss has parallel top and bottom chords, used for flat or low-slope roofs and sometimes for floors.
- Floor truss A floor truss is a parallel chord truss used as a floor joist, with an open web that services can pass through.
- Frame truss (A-truss) A frame truss (A-truss) is a roof truss built as an open frame of rafters, tie, collar beam and side struts, leaving the roof space free for a room.
- Gable end truss A gable end truss is the truss placed on top of a gable wall, with vertical studs instead of diagonal webs so cladding can be fixed to it.
- Gambrel truss A gambrel truss gives a roof with two slopes on each side, a steep lower slope and a shallow upper one, as on a traditional barn.
- Girder truss A girder truss is a strengthened, often multi-ply truss that carries the load of other trusses or of a point load.
- Heel (truss heel) The heel is the joint at each end of a truss where the top chord meets the bottom chord, usually directly over the supporting wall.
- Hip truss Hip trusses are a set of trusses of decreasing height that together form the sloping end of a hipped roof.
- Hipped roof A hipped roof slopes down on all sides, including the ends, so the building has no vertical gable walls.
- Howe truss A Howe truss has vertical webs and diagonals that slope towards the centre, a pattern used for longer spans and heavier loads.
- Inverted truss An inverted truss has its bottom chord sloping upwards away from the support, like a monopitch truss turned upside down.
- King post truss A king post truss is the simplest roof truss: two top chords, a bottom chord and one vertical post from the apex down to the middle of the tie.
- Knee wall A knee wall is a short vertical wall or strut under the slope of the roof, forming the low side wall of a room in the roof.
- Modified queen post truss A modified queen post truss adds diagonal webs to the queen post layout, so it spans further than a plain queen post truss.
- Monopitch truss A monopitch (mono) truss has a single sloping top chord, forming a roof that falls in one direction only.
- Overhang (eaves) The overhang is the part of the top chord that extends beyond the outside of the wall, forming the eaves of the roof.
- Piggyback truss A piggyback truss is a tall truss made in two parts: a base truss and a separate cap truss fixed on top of it on site.
- Queen post truss A queen post truss has two vertical posts instead of one, which lets it span further than a king post truss.
- Raised heel truss A raised heel truss has a taller heel, so the full thickness of ceiling insulation can continue out over the wall.
- Raised tie truss A raised tie truss has its bottom chord set higher than the wall plates, giving a higher ceiling or more headroom in the room below.
- Ridge beam roof A ridge beam roof is carried by a structural beam along the ridge, so the ceiling can be open all the way up to the roof.
- Roof pitch Roof pitch is the angle of the roof slope, given in degrees in most of Europe or as rise over run (e.g. 6/12) in North America.
- Scissor truss A scissor truss has a sloping bottom chord, so the ceiling underneath can follow the roof and form a vaulted room.
- Sloping flat truss A sloping flat truss is a parallel chord truss set at a slight slope, for low-pitched roofs where water must drain in one direction.
- Span (truss span) The span of a truss is the distance it covers between its supports, normally measured from the outside of one bearing wall to the outside of the other.
- Stub truss A stub truss is a truss with one end cut short and closed by a vertical member, so it stops before its full shape would.
- Studio truss A studio truss gives a ceiling that is vaulted on one side and flat on the other, creating a partly open, loft-like room.
- Top chord The top chord is the upper, usually sloping member of a truss that forms the roof slope and carries the roof covering.
- Web (truss web) Webs are the internal members of a truss that connect the top and bottom chords and divide the truss into triangles.
Walls
- Airtightness Airtightness describes how little uncontrolled air leaks through a building envelope, measured with a blower door test.
- Element joint An element joint is the connection between two prefabricated elements on site, where structure, insulation, air barrier and cladding must be made continuous.
- External wall An external wall separates the inside of a building from the outside, carrying loads and keeping out weather, cold and noise.
- Fire resistance Fire resistance is how long a building element keeps its load-bearing capacity, integrity and insulation in a standard fire test, expressed in Europe as classes such as REI 60.
- Lifting points Lifting points are the prepared places on a prefabricated element where it is attached to the crane, so it can be lifted safely without damage.
- Mineral wool Mineral wool is insulation made from glass or stone fibres, sold as batts, rolls or loose fill and used to fill the cavities between studs and joists.
- Partition wall A partition wall divides the space inside a building; it may be load-bearing or not, and is usually a stud frame boarded on both sides.
- Racking resistance Racking resistance is a wall’s ability to resist sideways forces, such as wind, without distorting from a rectangle into a parallelogram.
- Sound insulation Sound insulation is a building element’s ability to reduce the transfer of noise from one space to another, through the air or through the structure.
- Stud spacing Stud spacing is the distance from the centre of one stud to the next, typically 600 mm or 450 mm in Europe and 16 or 24 inches in North America.
- U-value The U-value measures how much heat passes through a building element, in watts per square metre per kelvin (W/m²K); the lower the value, the better it insulates.
- Ventilated cavity A ventilated cavity is the air gap behind the cladding of a wall, created by battens, that lets moisture drain and dry out.
- Wood fibre insulation Wood fibre insulation is made from softwood fibres, as flexible batts, rigid boards or loose fill, and is used in walls and roofs as a bio-based alternative to mineral wool.