Door Lining Kits: Sizes, Fitting, and How to Get the Width Right

Your carpenter fits door linings across six openings. The plasterer finishes to every one. The painter does two coats on all of them. Three weeks later you hang the doors and five won't close properly. One catches the frame at the top. Two have a visible gap down one side. The sixth fits but the architrave won't sit flat because the lining is 4mm proud of the plaster. Every one of those problems traces back to the same thing: the lining was the wrong width, not plumb, or not square. A door lining kit is one of the cheapest components in any build. Ripping it out and starting again costs a day's labour, replastering, and redecoration. Get it right the first time.

What it is and what it's for

A door lining kit is a three-piece timber frame that lines the inside of a door opening. Two vertical pieces (the jambs or legs) and one horizontal piece (the head) slot together using pre-cut housing joints. The lining serves two purposes: it gives the plaster a clean edge to finish against, and it provides the surface you screw hinges into when hanging the door.

Don't confuse a door lining with a door frame. A frame is heavier, rebated (has a step machined into it for the door to close against), and used for external doors where weather sealing and security matter. A lining is thinner, lighter, and used internally. The door stop (the strip the door closes against) is planted on separately after the door is hung, which gives you the ability to adjust its position for a perfect fit.

The lining goes in during first-fix carpentry, before the plasterer arrives. The plasterer uses the lining edge as a datum (a reference surface) and finishes the plaster flush with the lining face. After plastering, the carpenter returns to fit the door stop, hang the door, and nail on the architrave (the decorative moulding that covers the gap between the lining and the wall). That sequence is rigid. Get it wrong and something doesn't fit.

Types and sizes

Width is everything

The width of the lining must match the total thickness of your wall, including plaster on both sides. This is where most mistakes happen. A wall that's "100mm block" is actually 100mm block plus plaster coats on each face, and the total finished thickness depends on whether it's a dry-lined stud wall, a plastered blockwork wall, or something else entirely.

Measure at three points: top, middle, and bottom of the opening. Use the largest measurement. Walls aren't perfectly uniform, and a lining that's too narrow is a much bigger problem than one that's 2mm too wide (the architrave hides small discrepancies).

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Standard vs fire-rated

Standard lining kits use 27-28mm thick softwood or MDF. They suit every internal doorway that doesn't require fire resistance.

FD30 fire door linings are thicker (32mm) and include a groove routed into the rebate for an intumescent strip (a material that expands when heated, sealing the gap between door and frame during a fire). Fire door linings are required wherever building control demand an FD30 fire door:

• Between an integral garage and the house
• On escape routes in homes with loft conversions (three or more storeys)
• Flat entrance doors
• HMO shared areas (FD30S, which adds smoke sealing)

Softwood vs MDF

Softwood linings are the traditional choice. They accept screws well, take stain or varnish if you want a natural wood finish, and cope with damp areas better than MDF.

MDF linings give a consistently smooth surface for painting. No knots to bleed through, no grain to raise. They're dimensionally more stable than cheap softwood, which matters because budget lining kits from DIY stores are notorious for cupping (warping across the width). Carpenters on trade forums regularly complain that big-box-store softwood kits are made from poorly graded timber with random grain direction.

The decision is straightforward: MDF for any lining that's going to be painted white (which is most of them). Softwood for stained or oiled finishes, for period properties where you want real timber, or for any humid location like a bathroom where MDF's moisture sensitivity is a risk. If you go MDF, seal all six faces before installation, including the cut ends.

How to work with it

Before you start

Assemble the lining on the floor first. The head slots into housing notches at the top of each leg. Some kits come with the notches pre-cut; cheaper ones need you to cut them. Check the head length against your door width plus clearances: you need roughly 6mm more than the door width (3mm gap each side).

Cut the legs to length. Standard kits come with 2100mm legs. For a standard 1981mm door, you'll cut the legs to approximately 2016mm (door height plus 3mm head gap, plus 25-30mm floor gap, plus the head thickness). The exact figure depends on your finished floor level, so check that before cutting.

Fitting the lining

Pack the lining into the opening using plastic packers or timber wedges. Place packers at every fixing point, concentrating them at the hinge positions where load is greatest. Five pairs of fixings per leg is the professional standard: 100mm from the top, 100mm from the bottom, and three evenly spaced between.

Use a 1200mm spirit level on both legs and the head. Both legs must read plumb. The head must read level. Then check the diagonals: measure corner to corner both ways. If the two diagonal measurements aren't identical, the frame isn't square and the door won't fit without binding.

The quickest way to verify square is the 3-4-5 method. Measure 300mm along the head from one inside corner and mark it. Measure 400mm down the leg from the same corner and mark it. The diagonal between those two marks should be exactly 500mm. If it's not, the corner isn't a right angle. Tap the base of the leg sideways until you hit 500mm on the diagonal, re-check plumb, then fix. This works at any scale as long as the ratio holds: 600-800-1000 is easier to measure accurately on a full-size lining.

Identify the block type before buying fixings

Before you drill, work out what the wall is made of. Tap it with a knuckle. Dense concrete block sounds solid and barely dents. Thermalite or aircrete feels softer, sounds hollow, and you can scratch it with a nail. Red engineering brick rings when tapped. This matters because each material needs different fixings.

For dense blockwork walls, use 80-100mm screws through the packers and into the block with standard brown wall plugs (7mm for No.10 screws). Stagger your fixings so they land in the middle of the blocks, not in the mortar joints. For thermalite or aircrete blocks, standard plugs won't grip because the material is too soft. Use Fischer Hammerfix-N nails or Torx concrete screws, which cut their own thread into lightweight blocks. Some carpenters prefer nylon frame fixings (Fischer SXR or similar) for aircrete because they spread the load over a wider area and resist pull-out better than point fixings.

For stud walls, screw through the lining and packer into the studs. Position fixings 30-35mm from the jamb edge so the architrave will cover them. If a fixing lands between studs, add a nogging (a short piece of timber nailed between the studs) to give the screw something solid to bite into.

Fitting sequence step by step

The full sequence runs like this. First, dry-fit the assembled lining into the opening. Push it tight to one side and check how much packing you need on the other. Split the difference so the lining sits roughly centred. Second, pack both legs at the top (hinge side first) and fix one screw per leg at the top. Third, hang a plumb line or hold a long level against each leg and pack the bottom to bring it plumb, then fix the bottom screw on each leg. Fourth, pack and fix the intermediate points, checking plumb after each one. Fifth, check diagonals with the 3-4-5 method. Sixth, check "in wind" by sighting down each leg. Only when all six checks pass should you add the remaining fixings and drive them home. Rushing this sequence is how linings end up twisted or racked.

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After fixing, check plumb and square one final time. Tightening screws can pull the lining out of position, so this last check catches any movement introduced during the fixing stage.

The reveal

The "reveal" is the strip of lining visible between the architrave and the door. When you fit the architrave, it overlaps the lining edge by 8-10mm, leaving the rest of the lining face exposed. This strip needs to be consistent all the way around the door for a clean finish. Mark the reveal on the lining with a marking gauge set to 8mm before nailing the architrave, and check that the mitre joints at the corners come together tightly. A perfect mitre matters more than a perfectly consistent reveal, so adjust by a millimetre if needed to get the corners right.

Timing with plastering

Fit the lining before the plasterer arrives. The plasterer uses the lining edge as a datum (a reference surface) and finishes the plaster flush with the lining face. If the lining goes in after plastering, the plasterer has to guess where it will sit, and the architrave won't sit flat against the wall.

How many do you need

One lining kit per internal doorway. Count every opening that will have a door, including cupboard doors if they use standard-size linings.

A typical extension adds two to four new doorways: the opening from the existing house into the new space, possibly a utility room door, a WC door, and maybe a door to the garden (though that takes an external frame, not a lining).

Order the correct width for each opening individually. Don't assume every wall in your build is the same thickness. The partition between a kitchen and a utility room might be a 100mm stud wall (132mm lining), while the wall between the extension and the existing house might be original 230mm masonry (needs a 250mm+ lining from a timber merchant). Measure each one.

Cost and where to buy

Standard softwood or MDF lining kits in the most common width (132mm) cost £21 – £46. Trade-oriented suppliers like Doordeals sit at the bottom of that range. Wickes and B&Q sit in the middle. MDF primed kits that need no undercoat are at the top.

FD30 fire door lining kits cost £40 – £65. The premium over standard linings reflects the thicker timber (32mm vs 27mm) and the intumescent groove machining.

Alternatives

An external door frame is the alternative for exterior doorways. It's heavier, rebated (the door closes into a step machined in the frame rather than against a planted stop), and includes a threshold piece and weatherseal. Door frames cost roughly double the price of a lining kit. They're specified by the door manufacturer, not something you choose independently.

For internal use, there's no real alternative to a lining kit. You could build a bespoke lining from lengths of PAR (planed all round) timber, and some carpenters prefer this for non-standard openings or high-quality work. Robin Clevett's Skill Builder videos show the process. It's slower and costs more in labour, but gives you complete control over timber quality and width.

Door sets (door leaf pre-hung in its frame, sold as a single unit) eliminate the need to buy a separate lining kit. The door comes already hinged into a lining with the stop positioned and the gaps set correctly. A carpenter can fit one in twenty minutes rather than the hour or more that fitting a lining and hanging a door separately takes.

The trade-off is flexibility. Door sets are manufactured for specific wall thicknesses, and the common ones suit standard stud walls (around 100mm total) or standard plastered blockwork (around 132mm total). If your walls are non-standard, especially dry-lined blockwork with dabs where the total wall thickness might be 150mm or more, finding a door set to match is difficult. You'd need to order a bespoke set from a specialist like Howdens or JB Kind, which adds lead time and cost.

Door sets also limit your choice of door design. You pick from whatever doors the manufacturer offers in set form, which is usually a narrower range than the standalone leaf options. For fire doors, however, door sets have a real advantage: the complete assembly (leaf, lining, intumescent strips, hinges) arrives pre-certified, which removes the risk of assembling incompatible fire-rated components yourself. If building control asks for proof of FD30 certification, a door set comes with paperwork covering the whole assembly rather than individual component certificates you have to match up.

Where you'll need this

• Plastering - door linings must be fitted before plastering so the plaster finishes neatly to the lining edge
• Decoration - door linings are painted or stained as part of finishing works
• Kitchen Installation - new doorways in the extension need linings before doors can be hung

Door linings appear in every extension and renovation project that involves new internal doorways. They're part of first-fix carpentry on the build programme, sitting between the structural work being complete and the plasterer arriving.

Common mistakes

Buying the wrong width. This is the single most common error. Homeowners buy 132mm because the shop stocks it, without measuring the actual wall thickness. If your wall is dry-lined blockwork with dabs, 132mm will be 20mm too narrow. The architrave won't sit flat and you'll see a gap behind it. Measure the wall at three points, use the largest measurement, and buy accordingly.

Not checking plumb and square. A lining that's 2mm out of plumb at the head translates to a door that touches one side of the stop at the top and gaps on the other side at the bottom. Use a long spirit level (1200mm minimum) on both legs. Check diagonals. Check "in wind." This takes five minutes and saves hours of fiddling with hinges later.

Fitting after plastering. It seems logical: plaster first, keep the linings clean, fit them into a finished opening. But the plasterer needs the lining as a reference surface. Without it, the plaster won't finish flush with the lining edge, the architrave won't sit flat, and you'll spend time filling or shimming. Fit the lining first. Protect it with tape.

Using expanding foam behind fire door linings. Standard expanding foam is not fire-rated. The gap between a fire door lining and the wall must be packed with mineral fibre or sealed with intumescent mastic. Building control will check this, and if they find foam, the doorset fails.

Too few fixings. Budget lining kits sometimes suggest three screws per leg. That's not enough. The timber cups, the lining moves, and the door starts catching. Five pairs per leg is the professional minimum. Use pairs (two screws side by side) to resist cupping.