From Architrave to Undercut: 18 Internal Door Terms Decoded

Buying an internal door ought to be simple: pick a style, measure the hole, pay, hang. In practice, the trade has its own vocabulary, and a single misunderstood word can mean a door that will not close, a handle that will not fit or a fire door that fails an inspection. Here are the 18 terms that come up most often, with a note on why each one matters when you are standing in a doorway with a tape measure.

The frame and the finish

1. Lining The timber frame fixed into the opening, behind the plaster. It is what the door hinges from and latches into. If your opening is slightly out of square - common in older UK homes - the lining is usually the culprit. Ask whether the fitter intends to replace it or work with what is there.

2. Casing Strictly, the whole frame assembly: lining plus the facing that sits against the wall. In everyday use, people often say casing when they mean the visible frame. Clarify whether a quote covers the casing, the lining or both.

3. Architrave The decorative moulding around the opening that hides the join between frame and wall. It comes in profiles such as torpe-do, ogee and chamfered, and is sold by the metre. Matching existing architrave in a period home can add cost and time, so check before you order.

4. Leaf (or slab) The door itself, without frame or ironmongery. In some quotes, slab means a flat, unglazed door, but in most it simply distinguishes the door from the bits around it. When comparing prices, check whether you are being quoted for a leaf only.

5. Stile The vertical edge pieces of a panelled door. Wide stiles look traditional and suit period properties; narrow stiles feel more contemporary. They also affect how much the door can be trimmed.

6. Rail The horizontal pieces of a panelled door. The lock rail is the one at handle height, and its position determines where the latch and handle sit.

7. Rebate The stepped cut in the frame that the door closes against. A damaged or shallow rebate can stop a new door sitting flush. Rebated pairs are two doors that overlap in the middle, often used for wardrobe or cupboard openings.

8. Door stop The thin strip fixed to the frame that prevents the door swinging through. It can be planted on afterwards or formed as part of the frame. Fire doors need an effective stop, and the seals often sit against it.

The door itself

9. Clearance The gap between the door and the frame, and between the door and the floor. Sides and top are usually around 2-3mm. Too tight and the door rubs; too wide and it looks untidy, lets sound through and can compromise a fire door. Floor clearance depends on flooring and ventilation.

10. Undercut The amount trimmed from the bottom of a door, often to clear a carpet or to allow airflow. Fire doors have strict limits, so never let anyone take a saw to one without checking the certification.

11. Handing Which side the hinges are on and which way the door opens. Get it wrong and you will have a door that swings into a wardrobe or blocks a light switch. Specify left-hand or right-hand as viewed from the side you push.

12. Lipping A strip added to the edge of a door to give it strength and allow trimming. Solid lipping lets a joiner plane the door to fit an awkward opening. Cheap doors with thin lipping cannot be trimmed much, which matters in older houses where nothing is square.

13. Flush door A flat, smooth door, usually hollow-core or solid-core. It suits modern interiors and is common for fire doors. Hollow-core versions are light and inexpensive; solid-core versions feel more substantial and block more sound.

14. Panelled door A traditional door with recessed panels, available in period-appropriate styles for Victorian and Edwardian homes. Panelled doors can be more forgiving visually in older properties, but they collect dust in the mouldings.

The bits that make it work

15. FD30 A fire door rating that gives 30 minutes of resistance. You will see it specified between a house and an integral garage, in flats and HMOs, and often for loft conversions. An FD30 door is not the same as a standard door with a fire label stuck on it.

16. Intumescent strip A strip that expands when hot to seal the gap between door and frame. Fire doors need them, either fitted into the door edge or the frame. Do not paint over them, and do not assume a fire door is compliant without them.

17. Backset The distance from the edge of the door to the centre of the handle spindle or latch. It determines whether your existing handles will fit and whether the latch clears the frame. Most internal doors use a tubular latch, but backset sizes vary.

18. Hinge types Butt hinges are the standard choice. Concealed hinges disappear into the door and frame, which suits flush doors. Parliament hinges project so a door can open flat against the wall. Rising butt hinges lift the door as it opens, useful over thick carpet. Fire doors need specified hinges, usually three of them.

Questions to ask before you pay

• What is the exact leaf size, including any trimming allowance?
• Is the existing lining sound, or will it need repair or replacement?
• What clearance will the door have at the sides and the floor?
• Is this door required to be FD30, and are the seals and hinges included?
• What ironmongery comes with it, and will my existing handles fit the backset?
• Who is finishing the door - painting, staining or varnishing - and when?

None of this is complicated once the words are clear. The trick is to ask before the door arrives, not after the fitter has packed up the van.