- External corners: Corner post (most common), mitre joint (most refined), butt joint (simplest), metal angle trim (avoid on quality projects).
- Best corner post: 45x45mm Siberian larch PAR square post — same species as cladding, naturally durable, no treatment required.
- Window reveals: Matching species reveal board fixed before cladding boards. Drip detail essential at head. Boards butt cleanly into reveal on all four sides.
- Mitre corners: Most architectural finish — requires precision cutting, allowance for timber movement, and sealant in the joint. Not recommended for DIY on large elevations.
- Eaves detail: Open cavity at top for ventilation — insect mesh at eaves opening. Cladding boards run to soffit level or soffit board fixed to conceal cavity top.
- Movement: All corner details must allow for timber movement — particularly on untreated species with higher moisture movement. Sealant joints, not rigid adhesive, at all junctions.
Corner details and junction details are where timber cladding installations are most visibly differentiated — the difference between a quality installation and an amateur one is most clearly seen at the external corners, window reveals, door reveals, and eaves. Getting the board runs right on a flat elevation is straightforward; getting the junctions right requires planning, precision, and knowledge of how timber moves. This guide covers all the principal timber cladding junction details in the UK — external corner options, window and door reveals, eaves and soffit details, base details, and the key movement allowances that prevent joint failures over the cladding's service life.
Siberian larch rainscreen cladding external corner detail — precision corner junction on a commercial building. The quality of corner detailing is where cladding installations are most visibly differentiated between experienced and inexperienced installers. Clean corners, consistent joint widths, and accurate board alignment are the marks of a quality cladding installation.
Why Corner and Junction Details Make or Break a Cladding Installation
A flat run of cladding boards on an uninterrupted elevation is relatively straightforward to install — keep the first board level, maintain consistent fixing, and the boards look after themselves. Corners, reveals, eaves, and base details are where the complexity lies — and where the visible quality of the installation is most apparent. There are three reasons corners matter more than flat runs:
- Visibility: Corners are typically the most visible part of a cladding installation — viewed from multiple angles simultaneously. A poorly detailed corner reads as an error to any informed observer, while a well-detailed corner signals craftsmanship even before the board quality is assessed
- Weathertightness: Corners, reveals, eaves, and base details are the most vulnerable points in any cladding system from a weathertightness perspective — wind-driven rain exploits gaps and poorly sealed joints at junctions before it penetrates flat runs of board. Getting the detail right is as much about long-term performance as it is about appearance
- Timber movement: All timber movement in cladding UK occurs with changes in moisture content — boards expand in damp conditions and shrink in dry ones. This movement is greatest across the grain (width of the board) and must be accommodated at all fixed junctions. A corner or reveal detail that does not allow for movement will open gaps as the timber dries, or push boards out of alignment as it wets up
External Corner Options for Timber Cladding UK
A square timber post (typically 45x45mm) fixed vertically at the corner before cladding boards are run. Boards butt into the post face on each elevation. The simplest, most robust, and most widely used corner detail — accommodates timber movement naturally, no cutting precision required at the corner itself. Use matching species — larch corner post 45x45mm.
Boards on both elevations cut at 45° and joined at the corner — the cleanest, most architectural finish. Requires a precision mitre saw, allowance for timber movement in the joint, and sealant to prevent water ingress. Not recommended for DIY on large elevations or on species with high moisture movement (untreated spruce). The standard on high-specification architect-designed projects.
Boards on one elevation run past the corner and are butted by boards on the adjacent elevation. Simple to install but less refined in appearance — the end grain of one elevation's boards is visible at the corner. Acceptable on agricultural buildings and informal garden structures but generally avoided on residential and commercial projects where appearance is a priority.
Aluminium or stainless steel angle fixed at the corner with boards butting into it. Weatherproof and simple to install, but reads as a different material at the corner and interrupts the natural timber character of the cladding. Generally specified only where timber corner details have failed and need remediation, or on commercial projects where the metal trim is part of a wider metal cladding system.
For the vast majority of UK timber cladding projects — residential, commercial, garden rooms, extensions — the corner post is the correct and most practical specification. A 45x45mm Siberian larch PAR corner post fixed before the cladding boards, with boards butting cleanly into the post face, produces a clean, durable corner that accommodates timber movement naturally and requires no precision mitre cutting. The corner post should be the same species as the cladding boards to ensure consistent weathering — larch post with larch cladding, ThermoWood post with ThermoWood cladding.
Timber Cladding Window Reveal Detail UK
Siberian larch window reveal detail on a new build house — the reveal board covers the gap between the window frame and the face of the cladding boards, creating a clean, finished junction at every window opening. Matching species between the cladding boards and the reveal board ensures consistent weathering across the whole facade.
The window reveal is the most frequently occurring and most visible junction detail on any clad elevation — and it is the detail that most clearly distinguishes a quality specification from a basic one. The correct window reveal detail for timber cladding in the UK:
- Reveal board species and profile: The reveal board should be the same species as the cladding boards — larch reveal on a larch cladding elevation, ThermoWood reveal on a ThermoWood elevation. This ensures consistent weathering and colour matching over time. Use a PAR (planed all round) finish on the reveal board face. Our door and window reveal board PAR range is available in matching species
- Reveal board depth: The reveal board must be deep enough to cover the full gap between the window frame and the outer face of the cladding boards — typically 70–120mm depending on the wall build-up depth and the cladding profile thickness. Measure the actual depth on site before ordering
- Fix reveal board before cladding: The reveal board is fixed before the cladding boards are run up to the window opening — not retrofitted after the cladding is complete. Fitting the reveal after the cladding is the most common sequencing error and produces a less clean result
- Head drip detail: A projecting head board or metal flashing at the top of the window opening is essential — it throws water clear of the window frame and prevents water running behind the cladding at the window head. The head board should project minimum 30–40mm beyond the face of the cladding
- Sealant at reveal/cladding junction: A backer rod and paintable sealant at the junction between the cladding board face and the reveal board edge accommodates the small movement that occurs as the boards move with moisture content changes
Window reveal detail on a timber clad garden room — the matching species reveal board creates a clean, consistent junction between the window frame and the cladding boards. On garden rooms and garden offices, the window reveal detail is particularly visible given the typically smaller scale of the building and closer viewing distances.
Timber Cladding Eaves Detail UK
The eaves detail — where the cladding meets the roof overhang or soffit — is the least visible but most technically important junction in a cladding system. It determines whether the ventilated cavity behind the cladding boards can breathe correctly and whether the top of the cladding system is protected from weather ingress:
- Open cavity at eaves: The ventilated cavity behind the cladding boards must have an open top at the eaves — warm moist air from the cavity exits here. Without a top opening, the cavity cannot ventilate and moisture builds up behind the boards. The eaves opening must be covered with insect mesh (same specification as the base cavity opening) to prevent bird and insect ingress
- Soffit board option: On buildings with a projecting soffit, a timber soffit board can be used to conceal the top of the cavity while still allowing ventilation through a ventilated soffit strip or through the gap between the soffit board and the cladding face. The soffit board should be the same species as the cladding for consistency
- Roof overhang protection: A generous roof overhang (minimum 150–200mm on residential buildings) significantly reduces the amount of weather the top of the cladding elevation is exposed to — reducing the maintenance obligation on the head detail and protecting the top of the window and door reveals. Where roof overhang is limited, a deeper head flashing is required
- Top board: The top cladding board on each elevation should be fixed so its top edge is protected from water ingress — either by the soffit board, a separate capping piece, or a metal flashing. Exposed top edges of cladding boards are vulnerable to water absorption and premature deterioration regardless of species
Timber Cladding Base Detail UK — Ground Clearance and Drainage
The base of the cladding system — ground clearance cladding UK is one of the most critical base detail requirements — where the boards terminate at ground or floor level is the most moisture-exposed part of any cladding installation and requires specific attention to ground clearance, drainage, and cavity ventilation:
- Minimum 150mm ground clearance: The bottom face of the lowest cladding board must be minimum 150mm above finished ground or paving level. This prevents water splash-back from rain impact on hard surfaces, wicking of ground moisture into board ends, and mud contamination of the lower boards. On garden rooms and low floor-level buildings, achieving 150mm clearance may require raising the floor structure
- Insect mesh at cavity base: Stainless steel or plastic insect mesh fixed at the base of the ventilated cavity opening — preventing bird and insect ingress while allowing drainage and air circulation. Staple at 100mm centres and run the full width of the elevation
- Gravel board or DPC: On elevations close to ground level, a gravel board (a horizontal board at the base of the elevation below the cladding boards) prevents soil and vegetation contact with the bottom of the cladding system. A DPC (damp proof course) at the base of the cavity system prevents rising damp from the ground from tracking up the wall structure behind the cladding
- Drainage from cavity: Any water that enters the cavity (condensation, wind-driven rain through board joints) must be able to drain freely at the base. The base cavity opening — protected by insect mesh — is the primary drainage point. Ensure the cavity is not blocked at the base by the insect mesh installation or by debris
Timber Movement at Corner Details — Allowances by Species
Siberian larch cladding boards close up — the natural grain of Siberian larch is visible in the board face. Understanding timber movement in cladding UK — how each species moves with moisture content changes — is essential for specifying corner and junction details that remain watertight and gap-free over the cladding's 20–30 year service life.
| Species | Moisture movement | Equilibrium MC | Corner detail implication |
|---|---|---|---|
| Siberian Larch | Medium | 12–15% | Allow 2–3mm movement in mitre joints. Corner post detail is preferred — accommodates movement without sealant failure |
| ThermoWood | Very low | 4–6% | Minimal movement — most suitable for mitre joint corners. Sealant joints at reveals remain stable. Best species for tight corner details |
| Nordic Spruce | High | 16–20% | Significant movement — mitre joints will open and close seasonally. Corner post detail essential. Generous sealant joints at all reveals and junctions |
| Douglas Fir | Medium-low | 12–14% | Similar to larch — corner post preferred. Allow 2mm movement in mitre joints. Pre-drill all fixings at corners to prevent splitting |
Corner Details by Cladding Profile UK
Different cladding profiles require different approaches to corner detailing — particularly at the board end treatment and the junction between profiles at corners:
- Shadow gap cladding corner detail: Shadow gap corner detail UK — the recessed shadow line of the shadow gap profile should ideally continue at the corner — either through a mitre joint that preserves the shadow line, or through a corner post sized so the shadow gap reads consistently when viewed from the front of each elevation. A 45x45mm corner post with shadow gap boards butting flush to the post face is the standard detail
- Shiplap corner detail: The overlapping profile of shiplap boards means careful attention is needed at the corner to maintain consistent overlap depth. Corner post detail is standard — boards butt into the post and the overlap is cut back to the post face on each elevation. A slightly wider post (50x50mm) accommodates the thicker shiplap profile neatly
- Rainscreen corner detail: Rainscreen corner detail UK — open-joint rainscreen cladding requires the most precise corner detailing — the open joints and clean board faces mean any inaccuracy at the corner is immediately visible. Mitre joints or purpose-made aluminium corner extrusions are the standard on high-specification rainscreen corners. See our rainscreen cladding guide for the full corner specification
- Charred/SSB corner detail: Charred cladding boards require careful handling at cut ends — the carbonised layer on the cut face should be sealed with a compatible dark sealant or the end treated with a proprietary SSB end-grain treatment to prevent moisture absorption at the exposed end grain. See our SSB range and our charred cladding guide for the full specification
The three most common corner and junction detail errors on UK timber cladding installations: (1) No movement allowance in mitre joints — mitre joints on untreated spruce or larch that open seasonally as the timber dries down from installation MC. Always allow 2–3mm and seal with flexible sealant. (2) Reveal boards fitted after cladding boards — produces an inferior junction that cannot be properly sealed. Always fit reveals first. (3) Mismatched species at junctions — a treated pine corner post on a Siberian larch elevation weathers differently (grey vs green/brown) and is immediately visible. Always match species throughout the junction details.
What are the options for timber cladding external corners UK?
Four options: corner post (45x45mm matching species — most common and most practical), mitre joint (45° cut — most refined, requires precision), butt joint (simple — less refined), metal angle trim (avoid on quality projects). Corner post is the recommended specification for most UK residential and commercial cladding projects.
How do you detail a window reveal with timber cladding UK?
Fix a matching species PAR reveal board around the window opening before running the cladding boards up to it. Fit the reveal board first — never retrofit after cladding is in place. Add a drip detail at the window head. Seal the junction between cladding boards and reveal with backer rod and flexible sealant to accommodate timber movement.
What size corner post for timber cladding UK?
Standard 45x45mm square PAR post in matching species — available in Siberian larch. For profiles 25mm+ thick, use 50x50mm to provide adequate bearing for the board ends. Fix the post vertically before running cladding boards, using A4 stainless fixings throughout. Same species as the cladding boards is essential for consistent weathering.
How do you allow for timber movement at cladding corners UK?
Corner post detail accommodates movement naturally — boards butt into the post and can move freely without opening the corner joint. For mitre joints, allow 2–3mm and fill with flexible sealant that can move with the joint. ThermoWood has minimal moisture movement and is the most stable species for tight corner details. Nordic spruce has the highest movement and should always use corner post detail with generous sealant at reveals.
Timber Cladding Corner Details — Supply UK Wide
We supply Siberian larch corner posts (45x45mm PAR), door and window reveal boards, and all timber cladding profiles including shadow gap, shiplap, rainscreen, and Shou Sugi Ban charred cladding across the UK. Matching species throughout. FSC and PEFC certified. 7–14 day delivery.