Summary
Architrave is one of the most visible pieces of joinery in any room — it frames every door opening, and a poorly fitted mitre at eye level is one of the fastest ways for a customer to judge the overall quality of a second-fix job, regardless of how good the rest of the work is. Unlike skirting, where an internal corner can be scribed to hide minor inaccuracy, architrave's mitred corners have nowhere to hide — any gap, misalignment, or inconsistent reveal is immediately visible.
The technical challenge is that a "perfect" 45° mitre only produces a tight joint when both pieces are cut from truly square reference lines and the door lining itself is square and plumb. In real buildings — especially refurbishment work — linings are rarely perfectly square, walls are rarely perfectly flat, and plaster thickness varies from one side of an opening to the other. Good architrave fitting is as much about diagnosing and compensating for these real-world inaccuracies as it is about operating a mitre saw accurately.
This guide covers the reveal line, marking and cutting the mitre, compensating for out-of-square openings, fixing methods, and troubleshooting the most common mitre faults. It complements skirting architrave for the wider second-fix trim picture and second fix carpentry for how architrave fits into the overall second-fix sequence.
Key Facts
- Standard reveal (margin) — architrave is set back 5-6mm from the edge of the door lining on both sides and the head; this consistent reveal is what gives a professional, "machine-made" appearance
- Standard architrave widths — 44mm, 57mm, 69mm, and 94mm are common UK stock sizes; wider architrave suits larger rooms and higher ceilings, narrower suits smaller openings and modern minimal detailing
- Standard architrave thickness — 15mm or 19mm PAR (planed all round) softwood or MDF
- Mitre angle — 45° at both top corners for a standard rectangular opening; this is only correct where the lining is genuinely square — always test the actual angle before cutting
- Fixing method — adhesive (grab adhesive/panel adhesive) plus panel pins is standard for MDF or softwood architrave onto a timber door lining; pins are punched below the surface and the holes filled before decoration
- Pin spacing — panel pins typically at 300-400mm centres along the length, plus one close to each end near the mitre joint to hold it tight while the adhesive cures
- Mitre saw accuracy — a mitre saw's 45° detent gives an accurate cut angle, but the resulting joint is only accurate if the reveal lines on both legs are marked identically — a mitre saw does not correct for an out-of-square lining
- Test-cut practice — always cut an offcut at 45° and hold it against the actual corner before committing to the finished pieces, particularly on older properties where linings have moved
- Compensating for an out-of-square opening — where a lining is out of square, the mitre angle must be split unevenly (e.g. 43°/47° instead of 45°/45°) so the two cut faces still meet in a straight line at the corner, even though neither individual cut is exactly 45°
- Coping as an alternative — a scribed (coped) joint is the standard solution for skirting internal corners but is rarely used for architrave top corners, which are almost always mitred; architrave joints at an unusual junction (e.g. meeting a run of coving or a non-standard angle) may require a scribed or a mitred-and-scribed hybrid approach
- MDF vs softwood behaviour — MDF architrave is dimensionally stable and holds a tight mitre well over time, but is more brittle at a thin mitre point and can chip during cutting or fixing if not supported; softwood is more forgiving to work but moves seasonally, which can open a mitre joint slightly over time — see mdf vs solid timber skirting for the wider material comparison
- Caulking finish — a fine bead of paintable decorator's caulk (never silicone) along a slightly imperfect mitre line disguises minor gaps before painting — see caulking before painting — but should be a finishing touch, not a substitute for accurate cutting
- Matching existing profiles — refurbishment and period property work often requires matching an existing ogee, torus, or bullnose profile exactly; a mismatched modern pencil-round profile installed alongside original period architrave is immediately obvious and undermines the whole room's appearance
- Workmanship standard — BS 8000-5 (Workmanship on building sites — Code of practice for carpentry, joinery and general fixings) sets the general quality benchmark referenced for second-fix trim work
Quick Reference Table
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Try squote free →| Element | Standard Dimension | Notes |
|---|---|---|
| Reveal (margin) | 5-6mm | Consistent on both sides and head |
| Architrave width | 44 / 57 / 69 / 94mm | Match to room proportion and existing profiles |
| Architrave thickness | 15mm or 19mm PAR | 19mm for heavier/period-style sections |
| Top corner joint | 45° mitre (adjusted if lining out of square) | No scribe alternative at top corners |
| Fixing | Adhesive + panel pins | Pins punched and filled before decoration |
| Pin spacing | 300-400mm centres + near each mitre | Closer spacing near joints for clamp-up strength |
| Workmanship reference | BS 8000-5 | General carpentry/joinery quality benchmark |
Detailed Guidance
Marking the Reveal Line
Before any cutting happens, mark the reveal line on the door lining itself. Use a combination square, a marking gauge, or a simple scrap gauge block cut to the reveal dimension (5-6mm) and run it around both legs and the head of the lining, keeping the line consistent.
This reveal line is the single most important reference for the whole job — every subsequent measurement for architrave length and mitre position is taken from it, not from the raw edge of the lining. An inconsistent reveal — wider on one side than the other, or wandering along the length of a leg — is one of the most common snagging points flagged on a final inspection, even when the mitres themselves are cut well.
Cutting the Mitre — Standard Square Opening
- Measure the vertical leg length from the reveal line at the floor (or the top of skirting, whichever the leg architrave meets) to the point where the head architrave's reveal line crosses it
- Cut the first 45° mitre on the leg piece, angled so the longer edge of the cut faces outward (away from the wall) — this is the standard orientation for a top-outward mitre
- Offer the piece up to the lining, aligned to the reveal line, and mark the second end for length (typically square-cut for the floor end, or mitred if meeting a horn/skirting detail)
- Repeat for the second leg and the head piece, always cutting the head piece last once both legs are confirmed fitted, since the head length depends on the exact position the legs sit at
- Dry-fit all three pieces before any fixing — check both mitres close tightly with no gap on the face or in the corner, and that the reveal is consistent all round
Compensating for an Out-of-Square Lining
Not every door lining is square, especially in older properties where settlement, historic movement, or an imprecise original installation has left the opening slightly out of true. Forcing two standard 45° cuts onto an out-of-square opening produces a mitre that is tight on the face but open at the back, or vice versa — visible as a hairline gap that widens with time and movement.
Method for an out-of-square opening:
- Use a sliding bevel to capture the actual angle of the corner where the leg and head reveal lines meet, rather than assuming 90°
- Divide the actual angle in half to find the correct mitre angle for each piece — for example, an 88° actual corner needs two 44° cuts (not 45°/45°) to close correctly
- Set the mitre saw to the calculated angle for each cut rather than the default 45° detent
- Test-cut on scrap and offer up before committing to the finished architrave length
This adjustment is a small deviation from "standard" 45° but is the difference between a mitre that looks factory-made and one that has a visible, nagging gap a customer will notice every time they walk through the door.
Fixing Sequence and Technique
Fix architrave working from one leg, across the head, to the other leg, checking the reveal continuously as you go rather than fixing each piece fully before moving to the next:
- Apply a bead of grab adhesive to the back of the leg piece and press into position against the reveal line
- Pin near the top and bottom first to hold position, then fill in intermediate pins at 300-400mm centres
- Bring the head piece into the mitre joint, apply adhesive, and pin close to the mitre corner on both ends before filling in along the length — this clamps the joint tight while the adhesive sets
- Fit the second leg last, checking the second mitre closes cleanly against the head before final pinning
- Punch all pins below the surface and fill with an appropriate filler (matching the finish — painted MDF/softwood typically uses a standard wood filler or caulk; stained or oiled timber needs a colour-matched filler)
Troubleshooting Common Mitre Faults
| Fault | Likely Cause | Fix |
|---|---|---|
| Gap at the face of the mitre, tight at the back | Opening slightly more than 90° (obtuse), 45°/45° cuts used | Recut with adjusted angle per the sliding-bevel method |
| Gap at the back of the mitre, tight at the face | Opening slightly less than 90° (acute), 45°/45° cuts used | Recut with adjusted angle per the sliding-bevel method |
| Mitre closes on first fit but opens after a few weeks | Timber movement (more common in softwood, less in MDF); pins alone without adequate adhesive coverage | Refix with full-length adhesive bead, not just pin-point contact; consider MDF for stability-critical situations |
| Reveal inconsistent along the length of one leg | Reveal line not marked accurately, or lining itself bowed | Re-mark reveal with a straightedge check; pack behind the architrave at the bow point if the lining itself is not straight |
| Mitre corner chipped or crumbled at the point (MDF) | Insufficient support during cutting, or the point over-thinned by inaccurate marking | Support the offcut during the cut; consider a slightly wider reveal to keep more material at the mitre point on fragile profiles |
| Visible saw marks or a slightly open joint after painting | Cut quality not fine enough for a painted finish, or gap filled with the wrong product | Use a fine-tooth blade appropriate for the material; fill with paintable decorator's caulk before final coat, not during first-fix stage |
Matching Architrave to Period Properties
Where a room contains original period architrave — commonly ogee or torus profiles in Victorian and Edwardian properties — new architrave for an altered or added opening should match the existing profile as closely as possible. Off-the-shelf modern profiles (typically pencil-round or simple ovolo) look conspicuously wrong next to original period mouldings. Options include sourcing matching stock from a specialist joinery supplier, having a short run custom-machined by a local joiner, or salvaging matching sections from an architectural reclamation yard. Always confirm whether the property has any relevant planning constraints (listed building consent) before altering or replacing original architrave in a protected property.
Frequently Asked Questions
Why doesn't my 45° mitre close tightly even though I cut it accurately on the saw?
The saw's 45° detent is only correct if the door lining itself is genuinely square. If the opening is even slightly out of true — common in older properties — two accurate 45° cuts will not meet cleanly. Check the actual angle with a sliding bevel and split it between the two pieces rather than assuming 45°/45° will always work.
Should I use pins alone, or pins and adhesive?
Both. Adhesive provides the long-term bond and holds the reveal line consistently along the full length of the piece; pins hold the piece in position while the adhesive cures and add mechanical strength at the ends and near the mitre joints. Pins alone, without adhesive, are more prone to the mitre opening slightly as the timber moves with seasonal humidity changes.
Is a wider or narrower architrave easier to mitre accurately?
Width itself doesn't change the mitre-cutting method, but wider architrave (69mm, 94mm) shows any inaccuracy more obviously because there is more surface for the eye to track along the joint line. Narrower architrave (44mm) is more forgiving to the eye but the mitre point itself is more delicate and prone to chipping, particularly in MDF.
Can I use the same reveal measurement on every door in a property?
Yes — consistency of reveal (5-6mm) across every door opening in a property is what reads as "properly finished" to a customer, even more than any individual mitre being perfect. Mark and check the reveal on every opening rather than eyeballing it door to door, since small variations accumulate into a noticeably inconsistent result across a whole property.
Regulations & Standards
BS 8000-5 — Workmanship on building sites, Code of practice for carpentry, joinery and general fixings; the general quality benchmark for second-fix trim work including architrave
BS 8214:2016 — relevant only where architrave surrounds a fire door opening (FD30/FD60), as fire door tolerances and intumescent detailing take precedence over standard reveal/mitre practice at that opening
BSI: BS 8000-5 — carpentry and joinery workmanship code of practice
TRADA: Timber Frame and Joinery Technical Guidance — technical reference for timber moisture movement and joinery detailing
British Woodworking Federation — Joinery Best Practice — industry good practice for site-fixed joinery including architrave and skirting
skirting architrave — companion guide covering skirting board fitting alongside architrave
second fix carpentry — full second-fix sequence including door clearances, staircase work and window boards
mdf vs solid timber skirting — material comparison relevant to choosing architrave stock
caulking before painting — finishing technique for disguising minor mitre imperfections before decoration
skirting board quantity calculator — linear metreage calculator applicable to architrave ordering by extension
skirting board fitting pricing guide — pricing context for second-fix trim work including architrave