Building a terrace and installing cladding

A practical guide to building a Siberian larch terrace and installing cladding: base, joists, ventilation, fixings, maintenance and material quantities.

Why Siberian larch?

Siberian larch grows slowly in a cold climate, so the wood is dense with tight annual rings. The heartwood is naturally resistant to moisture and rot, which is why chemical impregnation is not required for ventilated, above-ground construction. That does not make it immune: in ground contact, with poor ventilation or unprotected cut ends, it can rot. Protect cut ends, keep the air gap and maintain the surface.

Before you start

• Store the boards flat on level supports, off the ground, covered from rain and direct sun, with air able to circulate.
• Let the boards rest at the building site for at least 2–3 days before installation so they can adjust. Leave them longer if time allows.
• Order about 10% extra for cuts and waste.
• Larch is a natural material: colour and grain vary from board to board, and small knots and resin pockets are normal.
• Check for underground services before digging.

Terrace base and joists

1. Prepare the base. Remove topsoil, lay a weed membrane, add a layer of gravel or crushed stone and compact it. Water must be able to drain away.
2. Lay the joists on supports. Use adjustable pedestals or concrete blocks, never directly on soil. Use 45×70 mm joists for a standard terrace; 45×45 mm is only suitable for small, light decks. Keep joists about 40 cm centre to centre, never more than 50 cm. For heavier loads or 27 mm boards, confirm the spacing and section with a structural engineer.
3. Slope. Give the terrace a 1–2% fall away from the building so water runs off.
4. Protect the joists. Put EPDM or bitumen joist tape on top of the joists. It protects the wood where water collects.
5. Ventilation. Leave at least 20 mm of open air space under the boards. Do not close the sides of the terrace tightly; air must circulate.

Installing decking boards

1. Lay the boards across the joists. Leave a 5–8 mm gap between boards (use spacers).
2. Where two boards meet end to end, leave about 5 mm. Each board end needs its own support: use a joist at least 70 mm wide or two joists side by side. A 45 mm joist is only enough to fix the middle of a board.
3. Leave a 10–15 mm expansion gap at walls, posts, thresholds and other fixed structures. Never push boards tight against a wall.
4. Fix each board with 2 screws at every joist, about 20–25 mm from the board edges and at least 25 mm from the board end.

Screws and fixings

Use only stainless steel screws: A2 for normal conditions, A4 near the sea or a pool. Larch contains natural acids and tannins that react with ordinary steel and zinc-plated screws, causing black stains and rust; the fixing can fail. Never use zinc-plated, yellow-passivated or drywall screws.

Cladding screw length. For 20 mm cladding on 25 mm battens, use 4×40 mm screws. If you use 4×50 mm, the battens must be at least 30 mm thick so the screw does not protrude.

• Drill a 4 mm pilot hole for a 5 mm screw, and 3–3.2 mm for a 4 mm screw. A pilot hole that is too small increases the risk of splitting in dense larch, especially near the ends.
• Countersink slightly so the screw head sits flush with the surface and water cannot collect in the hole.
• Use a Torx bit that fits the screw head exactly.

Installing cladding

• Fix vertical battens (at least 25 mm thick; at least 30 mm if using 4×50 mm screws) to the wall at 40–50 cm centres. Leave a ventilated air gap of at least 20–25 mm behind the cladding.
• Close the openings at the bottom and top of the air gap with insect mesh, and fit a breathable wind-barrier membrane on the wall.
• Start at the bottom and keep the cladding at least 20 cm above the ground so splashing water does not reach it.
• Tongue-and-groove boards (horizontal): install with the tongue pointing up so water cannot collect in the groove. Fix with hidden nails or screws through the tongue. Do not press the boards too tightly together; leave about 1–2 mm for movement.
• Rhombus boards are face-fixed with visible screws: work from the bottom up, with the angled edge directing water away from the wall. Fix with 2 screws per batten.
• For vertical boards, use horizontal battens fixed over vertical counter-battens so water and air can move freely behind the boards.
• Seal freshly cut ends with oil or end-grain sealer.
• At windows, doors and other openings, use suitable flashing or trim profiles so water cannot get behind the cladding.

Colour and care

• Fresh larch is light yellow-red to reddish brown. If left untreated, sunlight slowly turns it silver-grey. This is natural and does not harm the wood.
• To keep the warm colour, treat the wood with a UV-protective exterior oil. Apply it to clean, dry wood and repeat when the colour fades.
• In the first warm months some boards may release resin. This is natural and harmless; wipe it off with a cloth and white spirit or alcohol.
• Small surface cracks and small sound knots are natural features of larch.
• Clean with water and a soft brush. Remove leaves and dirt from the gaps so water can drain away.

How much material?

Number of boards = area ÷ (board length × (board width + gap)) × 1.10

Example: a 20 m² terrace with 24×140×4000 mm boards and a 6 mm gap. One board covers 0.584 m². 20 ÷ 0.584 = 34.25, so you need 35 boards before waste. Add 10% (34.25 × 1.10 = 37.7): about 38 boards.

• Joists at 40 cm centres: about 2.5 m of joist per m², so about 50 m for 20 m². That is roughly 13 joists of 4000 mm. Add extra for the terrace edges.
• Screws: about 35 per m², so about 700 for 20 m². Buy about 800.
• For cladding, ask us for the covering width of the profile; it depends on the profile and installation direction.

Related products: 24×140×4000 mm smooth board, 27×140×4000 mm ribbed board, 45×70×4000 mm joist, 20×120×4000 mm tongue-and-groove cladding and 20×95×4000 mm rhombus cladding.

Categories: terrace boards, cladding boards and structural timber.

Frequently asked questions

How many 24×140×4000 mm boards do I need for 20 m²?

One board covers 0.584 m² including a 6 mm gap. 20 ÷ 0.584 = 34.25, so you need 35 boards before waste. Add 10% (34.25 × 1.10 = 37.7): about 38 boards.

What joist spacing should I use?

About 40 cm centre to centre for a standard terrace, never more than 50 cm. Use 45×70 mm joists; 45×45 mm only for small, light decks. For heavier loads, confirm the spacing and size with a structural engineer.

Does Siberian larch need impregnation?

Siberian larch heartwood is naturally resistant to moisture and rot, so chemical impregnation is not required for ventilated, above-ground construction. Protect cut ends, keep the air gap and maintain the surface. Ground contact and poor ventilation shorten its service life.

Which screws should I use for a terrace?

A2 stainless steel; A4 near the sea or a pool. 5×50 mm for 24 mm boards, 5×60 mm for 27 mm. Drill a 4 mm pilot hole for a 5 mm screw. Do not use zinc-plated, yellow-passivated or drywall screws.

How long does a larch terrace last?

A properly built and ventilated terrace lasts for decades. Joist construction, ventilation, fixings and maintenance determine the service life; there is no guaranteed period.

How do I fix cladding boards?

Fix vertical battens at 40–50 cm centres, leaving a 20–25 mm ventilated gap. Install tongue-and-groove boards with the tongue up; rhombus boards with the angled edge away from the wall. Use 2 screws per batten, pre-drill and seal cut ends.

Safety and responsibility

Wear gloves, eye protection and a respirator when working with timber. Cut and drill in a well-ventilated area. Check for underground services before digging. This guide provides general advice. For balconies, roof terraces, heavy loads or unusual structures, consult a structural engineer and follow local building regulations.

Questions? Send your terrace dimensions through the quote form, email larchsupply@gmail.com or call +370 656 60569.