External Shiplap Cladding
How to fit PVC-U shiplap cladding to external walls, including batten framework, ventilated cavity, and window reveal detailing.
Safety Warning — Working at Height & Scaffolding
External cladding installation requires scaffold access for all areas above 2 metres. Do not attempt to clad upper storeys from a ladder. Scaffold must be erected by a competent person and inspected before use. Be aware of overhead power cables. Wear hard hat, safety boots, and high-vis when working at height. Secure all loose boards and materials — PVC boards can act as sails in wind.
What You'll Need
Tools
- Scaffold tower or full scaffold
- SDS drill (for masonry fixing)
- Drill/driver with countersink
- Circular saw or fine-tooth handsaw
- Spirit level (1200mm minimum)
- Tape measure and chalk line
- Snips or multi-tool
- Caulking gun
Materials
- PVC-U shiplap cladding boards (5m lengths)
- Starter trim (base profile)
- J-trims and F-trims (window reveals)
- External corner trims
- Treated softwood battens (25 × 50mm)
- Masonry fixings (75mm frame fixings)
- Stainless steel pins or screws
- Exterior silicone sealant
Technical Specifications
Step-by-Step Installation
1 Inspect the Wall and Plan
Check the existing wall for damp, cracks, or structural issues — cladding will conceal these, so address them first. Plan the batten layout around windows, doors, and services. Mark pipe positions, vents, and any penetrations. Consider insulation — if adding rigid insulation behind the cladding, increase batten depth accordingly. External cladding on residential buildings may require Building Regulations approval — check with your local authority, especially for fire-rated zones.
2 Install the Batten Framework
Fix 25 × 50mm treated softwood battens vertically at 400–600mm centres using masonry fixings. The battens create the ventilated cavity behind the cladding — this is essential for moisture management. Use a spirit level to ensure each batten is perfectly plumb. Add horizontal battens around windows, doors, at the base, and at the top of the cladded area. Where walls are uneven, pack behind battens with timber offcuts to maintain a flat plane.
3 Fit Trims and Starter Profile
Fix the starter trim along the base of the cladded area — this receives the bottom edge of the first board. Fix J-trims around window and door reveals. Fix external corner trims on all outside corners. All trims are fixed to the batten framework with stainless steel pins. Leave the base of the cavity open (or use a ventilation grille) to allow air to enter the cavity from below. The top should also be vented.
4 Fit the First Board
Hook the bottom edge of the first cladding board into the starter trim. Check level — the first board sets the line for every board above. Fix through the top nail flange into each vertical batten using stainless steel pins or screws. Do not pin through the decorative face of the board. Leave a 5mm expansion gap at each end where the board meets corner trims or window reveals.
5 Continue Up the Wall
Each subsequent board hooks over the top of the board below (the shiplap interlock). Pin through the nail flange at every batten. Work up the wall, checking level every 4–5 boards. Where boards need joining end-to-end on long walls, stagger the joints (do not align joints vertically) and use an H-section trim or leave a 3mm gap with a backing strip for weather protection. Around windows, cut boards carefully to fit tight against J-trims.
6 Detail Windows and Doors
Window and door reveals are the most time-consuming part. Cut boards to length so they butt neatly into J-trims above and below each window. For the head (top) and sill (bottom) of windows, cut cladding boards to fit around the opening. Use a multi-tool or jigsaw for notch cuts. Seal all cut edges where they meet trims with exterior silicone. Ensure the head flashing (above the window) directs water away from the window and onto the cladding face.
7 Top Edge and Ventilation
At the top of the cladded area (meeting the soffit, fascia, or a higher wall), the last board may need trimming to width. Fit into an F-trim or J-trim at the top edge. Ensure the ventilated cavity remains open at the top — air must circulate from the base vent, up through the cavity, and out at the top. This prevents moisture build-up behind the cladding. A ventilation gap of 10mm at the top, protected by the trim, is usually sufficient.
8 Final Sealing and Inspection
Apply exterior-grade silicone sealant around all window and door trim perimeters, at any board penetrations (pipes, vents, cables), and along the base starter trim if exposed to driving rain. Stand back and inspect the full elevation — check for consistent board alignment, even shadow lines, and neat trim joints. Clean all boards with warm soapy water to remove handling marks. Remove scaffold only when fully satisfied with the finish.
Common Mistakes to Avoid
- No ventilated cavity — Fixing cladding flat against the wall traps moisture, causing damp, mould, and rot to the wall behind. The 25mm batten cavity is non-negotiable.
- Untreated battens — Battens in the cavity will be exposed to moisture. Untreated softwood will rot within a few years. Always use tanalised or pressure-treated timber.
- Pinning through the face — Fix through the nail flange only. Face pins create visible holes, allow water ingress, and look unprofessional.
- Blocking cavity ventilation — The cavity must be open at base and top. Blocking either end defeats the purpose of the ventilated cavity and traps moisture.
- No expansion gaps — Boards tight against trims will buckle when they expand in warm weather. Always leave the 5mm gap.
- Poor window detailing — Rushed window reveals are the most common aesthetic failure. Take time with cuts and trim junctions — they're at eye level and highly visible.
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