How to Install External Shiplap Cladding: Full Guide

To install shiplap cladding outside, you need to fix treated timber battens vertically to the wall at 400–600 mm centres, creating a minimum 25 mm ventilated air gap behind the boards. Starting from the bottom of the wall, the first board is levelled and fixed, then each subsequent board is interlocked via its rebated joint and secured through the face or secret-nailed at every batten crossing. External corners, internal corners, window reveals, and top edges are finished with purpose-made PVC-U trims.

The process is straightforward for anyone with basic carpentry skills, but attention to detail — particularly around ventilation, thermal expansion, and weatherproofing at openings — is what separates a professional finish from a problem installation. This guide covers every step in detail.

Before You Start: Planning and Preparation

Check Whether You Need Permission

In most cases, adding cladding to a residential property falls under permitted development rights and does not require planning permission, provided the materials are similar in appearance to the existing exterior. However, you will need to check with your local planning authority if:

• The property is in a conservation area or a National Park
• The building is listed (Grade I, II*, or II)
• An Article 4 direction is in place restricting permitted development
• The cladding significantly alters the building’s appearance from the highway

Building Regulations approval may also be required — particularly if the cladding affects the building’s thermal performance (Approved Document L) or fire safety (Approved Document B). On buildings over 18 m, combustibility requirements under Regulation 7 apply.

Survey the Substrate

Before fixing any battens, assess the existing wall:

• Structural integrity — the wall must be capable of supporting the weight of battens, fixings, and cladding. On masonry walls, check for loose pointing or crumbling mortar and repair as necessary.
• Damp-proof course (DPC) — identify the position of the DPC. Cladding should not bridge the DPC, and the bottom edge of the cladding should sit at least 150 mm above external ground level.
• Services — locate any electrical cables, gas pipes, or water pipes running within the wall or on its surface. Use a cable detector and check service drawings where available.
• Existing finishes — remove any loose render, flaking paint, or decayed timber that could compromise the batten fixings.

Calculate Materials

Measure the total area to be clad in square metres, then deduct windows and doors. Add 10–15% for cutting waste, particularly on elevations with multiple openings or irregular shapes.

You will also need:

• Battens — treated softwood, typically 25 mm × 50 mm or 38 mm × 50 mm
• Fixings for battens — masonry screws or frame fixings (minimum 50 mm penetration into the substrate)
• Fixings for cladding — stainless steel nails (30–40 mm) or stainless steel screws
• Trims — external corner trims, internal corner trims, J-channels for window and door reveals, starter strips, and ventilation strips
• Breather membrane — if the wall construction requires it (timber-frame walls always require a breather membrane; masonry walls may not)

Tools Required

Tool	Purpose
Spirit level (1200 mm)	Checking batten and board alignment
Chalk line	Marking horizontal reference lines
Tape measure (5 m minimum)	Measuring boards and openings
Mitre saw or fine-tooth circular saw	Cutting PVC-U boards cleanly
Cordless drill/driver	Fixing battens and (optionally) boards
Hammer	Nailing boards if not screw-fixing
Nail punch	Setting nail heads below the surface
Tin snips or utility knife	Trimming and notching boards
Pencil or fine marker	Marking cut lines
Safety glasses and gloves	Personal protection when cutting
Scaffold or tower	Safe working at height

Step 1: Install the Breather Membrane (If Required)

On timber-frame walls, a breather membrane must be fixed to the sheathing board before battens are installed. This membrane allows water vapour to pass outward from the building while preventing liquid water from penetrating inward.

• Fix the membrane horizontally, working from the bottom up
• Overlap each horizontal run by at least 100 mm
• Tape all overlaps and penetrations with compatible sealing tape
• Ensure the membrane drapes freely between battens — do not stretch it tight, as this can compromise its draping action and create water pathways

On solid masonry walls in good condition, a breather membrane is not normally required, but it can provide an additional layer of protection.

Step 2: Fix the Battens

Battens create the ventilation cavity and provide the fixing substrate for the cladding boards.

Batten Layout

• Fix battens vertically at 400–600 mm centres (400 mm is preferred for additional rigidity)
• Use additional battens around all openings — windows, doors, vents, and meter boxes — so that every board end is supported
• Fix a batten tight to each external and internal corner

Fixing Method

• On masonry walls: drill, plug, and screw with fixings achieving minimum 50 mm penetration into the masonry
• On timber-frame walls: screw directly into the studs (locate studs with a detector if necessary)
• Check each batten is plumb with a spirit level before fully tightening fixings
• If the wall surface is uneven, pack behind battens with offcuts to create a flat fixing plane

Ventilation Cavity

The cavity behind the cladding must be a minimum of 25 mm deep as recommended by BS 8104 and Approved Document C. This cavity allows air to circulate from bottom to top, removing any moisture that penetrates behind the boards or migrates through the wall construction.

• Leave the bottom of the cavity open (or use a ventilated starter strip/insect mesh) to allow air intake
• Leave the top of the cavity open (or use a ventilated trim) to allow air to exhaust
• Do not block the cavity at any point — for example, by filling it with insulation or packing it with sealant

Step 3: Install Trims and Starter Strip

Before fixing any cladding boards, install the trim profiles that frame the cladding area:

1. Starter strip — fix horizontally at the base of the cladding area. The starter strip supports the bottom edge of the first board and kicks it out to the same angle as subsequent boards.
2. External corner trims — fix vertically at each external corner. These receive the cut ends of boards from both sides of the corner.
3. Internal corner trims — fix vertically at each internal corner.
4. J-channels — fix around window and door reveals. J-channels receive the cut ends of boards and create a neat, weathertight junction.
5. Top trim or soffit junction — fix at the top of the cladding area where it meets the soffit, fascia, or roof junction.

Secure all trims to the battens using stainless steel screws at approximately 300 mm centres.

Step 4: Cut and Fix the First Board

The first board sets the line for the entire installation, so accuracy here is critical.

1. Measure the run — measure the horizontal distance between the trims or corners where the first board will sit. Deduct 5–8 mm from each end for thermal expansion gaps (check the manufacturer’s guidance for exact allowances).
2. Cut the board — use a mitre saw or fine-tooth circular saw. Cut with the decorative face up to minimise chipping on the visible surface.
3. Position the board — slot the bottom edge into the starter strip and the ends into the corner trims or J-channels.
4. Check level — use a spirit level to confirm the board is perfectly horizontal. The starter strip should hold it at the correct angle, but verify before fixing.
5. Fix the board — drive stainless steel nails or screws through the face of the board at each batten crossing. Position fixings approximately 15–20 mm from the top edge of the board, where the next board’s overlap will conceal them.

Step 5: Fix Subsequent Boards

With the first board fixed and levelled, the remaining boards interlock via the rebated joint:

1. Engage the joint — slide the bottom rebate of the next board over the top edge of the previous board until it clicks or seats firmly
2. Check alignment — every 4–5 boards, check the horizontal with a spirit level and measure the distance from the board edge to a fixed reference point (such as a window head). This catches any cumulative drift before it becomes visible.
3. Fix at every batten — secure each board at every batten crossing. Do not skip battens to save time, as unsupported spans can flex in wind and may crack in cold weather.
4. Stagger end joints — where boards are not long enough to span the full width of a wall, butt-join them over a batten. Stagger these joins between rows so they do not form a visible vertical line. Leave a 2–3 mm gap at butt joints for expansion, and use an H-section jointing strip if specified by the manufacturer.

Step 6: Cut Around Openings

Windows and doors require careful measurement and cutting:

• Measure each piece individually — do not assume all windows are the same size
• Cut notches using a combination of saw cuts and a utility knife or tin snips
• Allow expansion gaps at all cut ends where boards enter J-channels
• Ensure sills are properly flashed — water must be directed away from the wall, not behind the cladding

For meter boxes, vents, and external taps, cut the board to fit around the obstacle and seal any gaps with a compatible silicone sealant.

Step 7: Install the Final Board and Top Trim

The final board at the top of the cladding area often needs to be cut down to width:

1. Measure the remaining gap between the last full board and the top trim
2. Deduct the expansion allowance
3. Cut the board lengthwise using a circular saw with a fine-tooth blade
4. Slot the board into position and fix through the face
5. Snap or slide the top trim over the cut edge to conceal it

Common Mistakes to Avoid

Mistake	Consequence	Prevention
No expansion gaps	Boards buckle in hot weather	Allow 5–8 mm at all abutments
Blocked ventilation cavity	Moisture trapped behind cladding; rot risk to battens	Keep cavity clear top and bottom
Fixing too tightly	Boards cannot expand; stress cracks develop	Leave 0.5 mm clearance under screw heads
Steel fixings used instead of stainless	Rust staining on the cladding face	Always use stainless steel or hot-dip galvanised fixings
Boards not checked for level	Progressive drift creates visible misalignment	Check level every 4–5 courses
Cladding bridging the DPC	Moisture bypasses the damp-proof course	Maintain 150 mm clearance above ground level

Maintenance After Installation

PVC-U shiplap cladding requires minimal ongoing maintenance:

• Annual wash — use warm soapy water and a soft cloth, sponge, or soft-bristle brush. Work from the top down to avoid dirty water running over clean areas.
• Stubborn marks — use a non-abrasive PVC-U cleaner. Avoid solvents, bleach, or scouring pads.
• Annual inspection — check fixings, trims, and sealant around openings. Tighten or replace any loose fixings. Reseal any cracked or deteriorated sealant.

Frequently Asked Questions

Can I install shiplap cladding vertically?

Shiplap boards are designed for horizontal installation. Vertical installation would prevent the rebated joint from shedding water effectively, as rain could track along the joint rather than dripping away. If you want a vertical board effect, consider a tongue-and-groove profile designed for vertical use or a slatted cladding system.

How do I handle ventilation grilles behind cladding?

If the wall has airbricks or ventilation grilles, you must maintain the airflow through the cladding. Cut the cladding boards around the grille position and fit a matching vent cover on the cladding face. Never block airbricks, as this can cause condensation and damp problems inside the building.

What is the maximum unsupported span for PVC-U shiplap?

Most manufacturers recommend a maximum batten spacing of 600 mm for standard profiles. In exposed locations or at height, reducing this to 400 mm provides better wind resistance and reduces the risk of boards flexing.

Can I install PVC-U cladding in winter?

Yes, but be aware that PVC-U becomes slightly more brittle in very cold temperatures (below 5°C). Handle boards carefully to avoid impact damage, and store them flat in a sheltered location before installation. Cutting may produce slightly rougher edges in cold weather — a sharp, fine-tooth blade minimises this.

Do I need to insulate behind the cladding?

The ventilated cavity behind cladding is not an insulation space. If you need to improve the wall’s thermal performance, add insulation to the inner face of the wall or use an external wall insulation system with a cladding-compatible finish. Do not fill the ventilation cavity with insulation, as this blocks airflow and can cause moisture problems.

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