Glossary of joinery terms | Westbury Windows and Joinery

A handy windows, doors, & joinery glossary

In the wonderful world of windows and doors, it might seem like there are plenty of industry terms and words for you to learn. Whether you’re an architect in need a quick reminder or you simply want to be clued up for your own home project – it can be beneficial to build an understanding of these commonly used words, and learn their meanings.

Here’s our handy windows, doors, and joinery glossary, featuring some of the key words that you are likely to come across in the industry. Feel free to bookmark the page so it’s to hand as you progress through your home renovation or new build project. Enjoy using your new vocabulary! 

4-16-4 toughened clear glazing – Glazing consisting of 4mm clear glass / 16 mm cavity / 4mm clear glass.

Accoya: A revolutionary modified timber that is made from fast growing, FSC certified radiata pine, making it highly sustainable. It has a class one durability status, it’s virtually rot proof and will not swell or twist, meaning products made from Accoya will last longer and need little maintenance. 

Argon: Gas that is used to fill the space between two panes of glass in a double glazed window. As it is denser than air, it results in an energy-efficient window. 

Bi-fold doors – Bi-fold doors, also known as folding sliding doors, or folding and stacking door, open outwards and consist of various door leaves that stack against one another to create an opening. 

Bottom hung sash – The lowermost window within a frame.

Bottom rail: The bottom horizontal rail of a window, with a drip bead that overhangs to ensure no water ingress. 

Bolection mould: A decorative moulding found on doors which projects beyond the face of a panel or frame. Used to enhance panel door designs and the bottom panels on French doors. 

Box sash window – A sash window which uses weights and pulleys on a piece of sash cord to counterbalance the sash and make it easier to move heavier windows.

Capping system – The application of aluminium or vinyl sheeting applied to the exterior of a window, door or roof lantern.

Cartwheel: A round, decorative window that looks like a cartwheel, usually used in top floors where the rooms have a steeper pitch and cannot accommodate for square or rectangular windows. 

Casement windows: Windows that are fixed to their frame with either a top or side hung hinge. 

Clerestory: Inspired by the upper parts of large churches, this is a series or row of windows that run along the top length of walls or roof lanterns, above eye level.

Common rafter: Part of a roof lantern. The common rafters run directly up and down from the central ridge to the bottom. The rafters create the main structure in which the glazing units sit in. 

Cornice – Decorative framework of a window.

Cupola: A rounded dome structure that is added to a roof or ceiling – usually works well with octagonal roof lantern designs. 

Decorative sash horns: A common external feature on sash windows, which are traditionally included for decorative purposes. 

Diamond-paned windows – Windows consisting of several small diamond-shaped panes of glass.

Door frame – A complete door frame is made up of one header, two jamb legs, and a threshold.

Dormer windows – A type of roof window which projects vertically from a pitched roof.

Double doors – Two adjacent doors within a single frame.

Double glazing – Window panes consisting of two layers of glass with a space between them designed to exclude noise and minimise heat loss. Triple glazing is also available and provides another layer of protection.

Double-hung sash windows – Two sash windows within one frame, both of which can move up and down independently of one another.

Draught gasket – An essential element that ensures the window is sound and draught-proof. They are only visible when the window is open. 

Espagnolette locking system – A flat metal locking system that is typically installed on the vertical frame of a French door or casement window. A handle is attached to a metal rod which is within the window frame and when the handle is turned the system will secure the opening.

Fanlight window – A semi-circular window often placed above a door, or another window with glazing bars separating the panes of glass to look like an open fan. Sometimes known as a transom light/window.

Fenestration – The arrangement of windows within a building.

FENSA: A government-authorised scheme that observes building regulations for replacement windows and doors. If a company is a FENSA certified installer, then they are assessed on a regular basis to ensure that their methods meet building regulations.

Finial: A decorative finish for the top of roof lanterns usually made from solid resin to ensure that it is resistant to the elements. Finials come in a range of shapes so that can be specified to suit homeowners personal tastes, however, they also play a functional role and protect the kingpin from weathering. 

Frame head: The top horizontal piece of a window frame. 

French casement windows: Similar to a standard casement window, these designs are made without the central mullion, creating a clear and unobstructed view when opened. They can be often used for second-floor dormer windows or for first-floor windows where the opening is not large enough to meet current escape regulations. 

French doors: Similar in appearance to a French window, these classic glazed doors come in pairs and traditionally open outwards to give access to balconies, patios, and other external areas. When open, they create one large opening with no disruptions to the view outside. 

FSC: A worldwide certification system that allows businesses and consumers to use sustainable timber products. It assesses a timber’s chain of custody and the way the forest it has been sourced from is managed. 

Gunstock style: A style of door panel, when the stiles change in size as they go down the door.

Glazing bar: Bars that run between adjacent panes of glass in a window, adding to the fenestration of the window and the design’s style.

Glazing bead: This ensures that water does not ingress between the timber joints in a window – we make ours from Accoya with a secret stainless steel fixing, which is then sealed with a silicone bead. 

Head drip – A highly durable external head drip is mounted on the top of a window casing to help protect the windows from rain and conceal the trickle vent.

Hip and rafter cappings – Powder-coated sections with aluminium cladding that hold the glass panes in place and ensure the building is watertight. 

Hip rafter – A key structural component that spans from the corner of the roof lantern to the ridge.

Ironmongery: A wider term for any kind of door or window hardware, also known as architectural hardware. This includes everything from handles and locks to window hinges. 

Jack rafter: Part of a roof lantern. As opposed to the common rafters, the jack rafters run at an angle in the corners of the roof lantern, giving additional support to the sides. The rafters create the main structure in which the glazing units sit. 

Jamb: The side post of a casement window. 

Kerb: An upstand that a roof lantern will sit on, which has the flat roof membrane tucked inside to ensure the whole structure is waterproof. 

Kingpin: A laminated timber section on a roof lantern, which connects the hip rafters to the ridge. It will usually have a finial fixed to the top. 

Knot-free – Knots in timber are scars left from removed branches. These scars appear as darker circles where the tree continues to grow them. Knot-free timber is free from these scars.

Laminated glass – Laminating glass is a process used to prevent the glass from shattering when broken.

Leaf/door leaves – A leaf is a panel that fills the doorway, becoming a door once hinged.

Low E coating – Low emissivity, meaning a surface that emits low levels of radiant heat.

Meeting rail: The middle horizontal rail where two sashes meet when a sash window is fully closed. Will include a draught excluder. 

Microporous paint – A paint that allows water vapour to pass through it, but acts as a barrier to liquids.

Mid rail – The middle horizontal structural member.

Moulding – Also known as casing, it is used to trim the perimeter of windows and doors for a decorative finish.

Mullion: A vertical element that forms a division between two casement windows. 

Muntin – Narrow bars dividing the panes of glass within windows.

Oriel windows – A style of bay window found on an upper-storey for a building.

Outer face boards – Creates the rebate for the outer sash.

Palladian window – A Venetian style window consisting of a central arched window, flanked by two rectangular, narrow windows.

Parapet: Where the sidewall extends slightly above the level of a flat roof, creating a low barrier.

Pitch – Measured in degrees, a roof’s pitch is the steepness of the roof. The pitch is calculated by dividing the vertical rise by the horizontal span of the roof.

Pre-tensioned heavy-duty balances – Heavy-duty weights used to hold a sash window in place once opened.

Quoin – Masonry blocks often used to surround windows, or on the corners of brick walls.

Rafter cappings: Protective ‘caps’ which are fitted over the rafters of a roof lantern to protect them from the external elements, giving them longevity. We make ours from aluminium, which is a particularly strong material. 

Raised and fielded panel: An attractive, decorative panel with grooved edges and a fielded or moulded element to its face – entrance doors will often have anything from two to six panels depending on their style. 

Redwood: This is a slow-growing timber, which gives an aesthetically pleasing, smooth grain. Its attractive grain makes it ideal for use on the interior of our products, resulting in an exceptional moulded and painted finish. 

Ridge cap: The central member which runs along the top length of a roof lantern, at its highest point. Where the two sides join in the middle.  

Roof lantern – A glass skylight that is built upon a flat roof to add height to a ceiling and allow natural light to enter the room below.

Roof light – A glass skylight that extends up from a flat roof. Also known as a roof lantern.

Round-arched window – A window that is fully arched at the top.

Sash windows: A window with two moveable sashes, with one sitting in front of the other. They bring a traditional, classic aesthetic to a home. They are available with the traditional cord and weights or can be spring balanced for a modern touch. 

Side hung friction hinge – Hinges that are fitted on the side of a window frame at the top and bottom, allowing the window to be pushed open from the side similar to a door.

Sidelights: Usually narrow-shaped windows fitted next to entrance doors. These windows do not typically open, but they do let in lots of natural light into entrance hallways. 

Sill – A ledge-like shelf at the bottom of a window, either inside or outside of the building, that creates an overhang and drip to protect your property from the weather.

Sill horn – The extension of the lip of a window sill that sits outside of the edge of the casing.

Slave leaf – A locking system applied to French doors and windows, to allow for the use of one handle. The slave left is the door that is often fixed in place and used less regularly. 

Spring sash window – A sash window that uses a spring-loaded device used for counterbalance.

Staff bead – The detailed trim that frames your sash box and keeps the sashes in place within the sash box. 

Teknos: An innovative paint brand that has developed a microporous paint system, which enhanced the longevity of timber products. Their paint is made from a blend of water and plastic, resulting in a highly durable and protective layer of paint once dry.

Thermoplastic Elastomer (TPE) – A mixture of polymers, typically plastic and rubber, that is used to create a weather seal on windows.

Threshold: A strip of wood that forms the bottom of a doorway, which is crossed when entering a house or room. Usually made from durable hardwood to withstand the footfall. 

Top hung friction hinge – Hinges fitted to the top or bottom of the frame, allowing the window to swing open from the top, or push open from the bottom.

Top hung sash – The uppermost window within a frame.

Top rail: The top horizontal rail of a window.

U-Values: A number used to measure the energy efficiency of a door or window. The lower the number, the better the insulation. It’s advised that you look out for the whole product U-Value, rather than just the central pane U-Value. 

Vent – A timber glazed sash window with an aluminium shroud that is installed in a roof lantern to help ventilate the roof. Typically they are powered by an electric motor and can include rain sensors and thermostats.

Venting: Cracking which is a problem that laminated glass is prone to if not polished properly. 

Warm edge spacer bar – Insulation applied to the edges of a window which keeps the panes of glass apart and reduces the amount of heat lost through a sealed unit.

Weatherstripping – The process of sealing openings such as doors and windows from the elements.

Weighted Sound Reduction Index (Rw) – A unit used to measure how effective a material is at insulating sound. The higher the Rw number the better the sound insulator will be.