How to fit a door lining

Video by Tommy's Trade Secrets. This walk-through is based on the video "Tommy's Trade Secrets - How To Fit A Door Lining" from Tommy's Trade Secrets, a well-established UK trade DIY channel. The tips on using packing pieces to get the jambs perfectly plumb regardless of how wonky the rough opening is are especially useful — most openings in older UK houses are anything but square.

1. Measure the rough opening and choose the right lining

A door lining kit from a builders’ merchant consists of two side jambs and a head jamb, usually rebated (stepped) so the door closes against a stop. Standard internal door linings are sized for a 35 mm thick door, though thicker fire doors use a deeper rebate. Before you buy, measure the rough opening width and height, and the wall thickness — lining sets come in different widths to suit 90 mm, 100 mm, and 115 mm stud walls.

The rough opening should be roughly 30 mm wider and 20 mm taller than the finished door size you want. That gap is taken up by the lining, the packers, and a small clearance. If you are replacing an old lining, remove it completely and check the opening is still sound — especially in older Sandwich properties where walls can be lathe-and-plaster and the openings less predictable than a modern stud wall.

2. Cut the head jamb and mark the housing joints

The head jamb — the horizontal top piece — sits between the two side jambs, not on top of them. This means the side jambs run from floor to ceiling and the head is cut to fit between them. Most lining sets are pre-rebated; cut the head jamb to the internal width of your finished opening (the door width plus around 4 mm clearance each side).

Mark the housing positions on the side jambs at the correct height. The housing is a shallow channel cut across the inner face of each jamb that the head slots into, and it keeps the assembly rigid once fixed. Cut the housings with a tenon saw and a sharp chisel to about 12 mm deep — just enough to grip the head firmly. A neat housing makes for a solid, rattle-free frame; a sloppy one will work loose in a year.

3. Assemble the lining frame on the floor

Slot the head jamb into the housings on both side jambs and check that the frame is square. The quickest way to check for square is to measure diagonally from corner to corner — both diagonals should be the same length. If one is longer, gently rack the frame until they match. You can fix a temporary diagonal brace across the back to hold it in shape while you carry it to the opening.

Some joiners glue and nail the housing joint; others just nail. Either works as long as the fixings are secure. Do not use woodscrews alone through the side of the jamb into the head — it is not as strong as a proper housed joint. That said, if the opening is a tight fit, assembling the lining in the opening piece by piece is perfectly acceptable.

4. Offer up the frame and pack it plumb

Lift the frame into the rough opening with the rebate facing the room the door will open into. It will almost certainly not be a perfect fit — rough openings rarely are. This is where timber packers come in. Slip pairs of packing shims between the side jambs and the masonry or timber studs at roughly 400 mm intervals. Adjust them until the jamb reads plumb on a spirit level.

Check both side jambs independently. A jamb that is plumb when the frame is empty may shift once fixed, so keep the level on it as you tighten things up. The head should be level too — if your floor is slightly out (common in older properties), the head takes priority. A level head is what the eye reads; a sloping head looks wrong even if you cannot immediately put your finger on why.

5. Fix the lining to the wall

Once packed plumb and level, fix the jambs through the packing pieces and into the surrounding structure. Use 100 mm screws through the face of the jamb into the wall. In a masonry opening, you will need to plug the wall first — drill through the jamb and packer into the brickwork, then use a suitable plug and screw. Three fixings per jamb is the minimum; four is better on a heavy door.

Keep checking with the spirit level as you drive each fixing, since screws have a habit of pulling things slightly out of true. Once both jambs are fixed, check the internal width at the top, middle, and bottom — it should be consistent throughout. Any variation here will show up as an uneven gap when the door is hung. Small adjustments can be made by tapping the packers, but large discrepancies mean starting the packing process again.

6. Trim flush and prepare for hanging

Any packing pieces that stick out proud of the jamb face need to be cut or broken off flush. Timber packers snap cleanly with a Stanley knife scored across them a few times; plastic packers need a saw or knife. Run a length of timber across the jamb face to check it is flat — any high spots will cause the architrave to bow when you nail it on later.

The opening is now ready to hang a door. Check the clearance between the two jambs one final time, mark a pencil line 2 mm in from each jamb face to show where the door edge will sit, and you have a clean reference to work from when sizing the door. Mind you, do not cut the door until you have measured the actual opening rather than relying on the nominal size — rough openings and linings are rarely exactly as advertised.

When to call a handyman

Call Richard if the rough opening is in a load-bearing wall and there is any doubt about whether the lintel above is doing its job, if the opening is badly out of square and needs significant timber work to bring it right, or if you want the door hung at the same time. Fitting a lining and hanging a door in one visit is a common combination job, and doing both together saves time and ensures the clearances are set correctly from the start rather than discovered afterwards.