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Larch decking boards: what to know before you plan
Larch decking boards are a softwood for outdoor use with a warm, reddish-brown colour and a fine growth-ring pattern. According to our product data, the Canadian larch in our range is classified in durability class 3–4 under EN 350. That puts it on a par with Douglas fir, and like any timber deck it needs good structural wood protection. Without oil, the surface weathers to a silver grey over time.
Autumn and winter are a good time to plan your deck for spring. In this guide we explain the pros and cons of larch, the difference between Canadian and Siberian larch, the choice between 27 and 43 mm thickness, how larch compares with Douglas fir and hardwood, and how to install and care for it. You will find all wood species in our wooden decking boards collection.
Quick decision guide
- Larch is a good fit if you want a softwood with a lively, fine grain and plan to screw it visibly, oil it or let it turn silver grey.
- 27 mm is the usual thickness for residential decks with joists spaced no more than 50 cm apart (centre to centre).
- 43 mm is the choice for a particularly stiff surface, for example steps, platforms or heavily used areas.
- Douglas fir has the same durability class according to our product data. If you want a higher class, choose thermo ash (class 1).
- Always stainless steel: A2 stainless steel screws at least 5 mm in diameter, pre-drilled boards and a gap of about 7 mm.
Matching products from Gunreben:
Canadian larch A/B smooth 27 × 143 mm – the classic decking board with a smooth planed surface.
Canadian larch A/B fine-ribbed 27 × 143 mm – with fine ribbing if you prefer a textured surface.
Canadian larch A/B smooth 43 × 143 mm – the thick board for steps, platforms and commercial areas.
Canadian larch joists 44 × 70 mm – the matching timber substructure in the same wood species.
Canadian or Siberian larch: what is the difference?
Canadian larch is also known as western larch. According to the product description, our timber comes from the high elevations of Interior British Columbia. The climate there produces fine-grained, densely grown wood with few knots. According to our product data it is visually and technically comparable to Siberian larch, which is also widely sold as decking timber.
For your decision, grading, drying and dimensional accuracy matter more than the name. Our larch is available in grade A/B and KD (kiln-dried). The boards are 143 mm wide and supplied in fixed lengths.
Larch decking boards: pros and cons
Pros
- Durability class 3–4 under EN 350 according to our product data, suitable for outdoor use as long as the structure can dry well.
- Fine, lively look: the contrast between lighter earlywood and darker latewood gives the surface texture.
- Easy to work: larch can be sawn, planed and sanded and absorbs oils well.
- Two thicknesses and three surfaces: 27 mm smooth, fine-ribbed or coarse-grooved, plus 43 mm smooth or fine-ribbed.
- Matching joists in the same wood species.
Cons
- Greying: like all timber outdoors, larch turns silver grey without care and the top layer develops fine cracks. According to our installation guide this does not affect the load-bearing capacity.
- Resin: as a softwood, larch contains resin, which can seep out mainly at knots and in warm weather.
- Harder latewood: sanding can leave small irregularities. Because of the silica content, tools blunt faster.
- Avoid iron: screws or metal parts that are not rust-proof can cause dark discolouration.
- Cracks and warping cannot be completely avoided with outdoor timber, especially at knots and board ends.
Which thickness? 27 or 43 mm larch
The thickness depends on the joist spacing and how the deck is used. For 14 cm wide boards, our installation guide specifies a minimum thickness of 2.5 cm at 50 cm support spacing and 2.7 cm at 60 cm. Regardless of this, we recommend limiting the joist spacing to no more than 50 cm.
- 27 mm: for most residential decks, balconies and roof terraces. Choose the surface to suit your taste: smooth, fine-ribbed or coarse-grooved.
- 43 mm: for steps, platforms, boardwalks and areas with higher loads. The larch fine-ribbed 43 × 143 mm is available in addition to the smooth version.
These values are a guide for simple decks. For steps, raised decks or significant heights, please plan the substructure with a specialist contractor.
Larch or Douglas fir? And when to choose hardwood?
The most common question with softwood decks is: larch or Douglas fir? According to our product data, both are in durability class 3–4. The difference lies mainly in appearance and format: we offer larch in 27 and 43 mm thickness and 143 mm width, Douglas fir in 26 and 40 mm thickness and 145 mm width. If you want a higher durability class, choose thermo ash or a hardwood. A detailed comparison is available in our guide Douglas fir or hardwood.
| Wood species | Durability according to product data | Thicknesses at Gunreben | Special features |
|---|---|---|---|
| Canadian larch A/B | Class 3–4 (EN 350) | 27 and 43 mm, width 143 mm | Fine-grained with few knots, softwood containing resin |
| Douglas fir I/III | Class 3–4 (DIN EN 350-2) | 26 and 40 mm, width 145 mm | Reddish-brown colour, kiln-dried to 16–18 % moisture content |
| Thermo ash Prime | Class 1 (DIN EN 350-2) | 21 mm, width 130 mm | Thermally modified, deep brown, system board with tongue and groove |
| Garapa Prime | Class 3 (DIN EN 350-2) | 21 and 25 mm, width 145 mm | Hardwood with high density, A4 stainless steel recommended |
Installing larch decking boards: the key points
A larch deck follows the rules in our installation guide for timber decks (PDF, German). The most important points in brief:
- Substructure: joist spacing of no more than 50 cm, two joists at every butt joint of the boards. The boards should overhang the outer joist by no more than 8 to 10 cm. The matching option is the larch joist 44 × 70 mm. More options can be found in the substructure collection.
- Clearance from the ground: the joists rest on rubber granulate pads or adjustable pedestals so that no moisture rises and air can circulate underneath.
- Slope and clearance: about 2 % slope for water run-off and at least 25 mm clearance to walls and fixed building parts.
- Gap: a gap of about 7 mm is considered sufficient for dried timber, because a 145 mm wide board can move by up to 10 mm in width in extreme cases. 7 mm spacers make this easy.
- Screwing: two A2 stainless steel screws per crossing point, at least 5 mm in diameter, with a length of about 2.5 to 3 times the board thickness. For 27 mm larch, the stainless steel screws 5.3 × 70 mm A2 are suitable. Always pre-drill and countersink the boards, for example with the HELPO drill countersink. Use a stainless steel bit so that no abrasion discolours the surface.
- Protect board ends: chamfer the cut edges and treat the end grain with end grain wax to reduce cracking.
According to our installation guide, concealed fastening with DielenFix is designed for boards of 19 to 25 mm and is therefore not suitable for 27 or 43 mm larch. The available fastening methods are explained in our guide fastening systems for decking boards. For planning the substructure, read choosing a substructure for decking boards. All other accessories can be found under accessories for wooden decking.
Oil your larch deck or let it turn grey?
Whether you oil larch is mainly a question of appearance. According to our installation guide, oil does not improve the durability of the wood. Applied regularly, however, it delays greying and helps the surface dry faster, which gives moss and algae less of a foothold. Film-forming coatings such as varnish are not suitable for decking boards.
- Before the first oiling: clean the surface with water and a brush or with the G² deck wood cleaner, then smooth raised fibres with a sanding pad once dry.
- Choosing oil: the Gunreben decking oil in the shade larch/Douglas fir/oak is matched to these species. According to the product data, consumption is about 50 ml/m² per coat. You will find it in the care products for wooden decking collection.
- Dark stains: discolouration caused by iron-containing objects can be lightened with oxalic acid. Please follow the instructions for use.
- No pressure washer: the water jet can roughen the surface and cause colour differences.
When and how often to re-oil is explained in our guide oiling your deck in autumn or spring? For the cold season, read preparing your deck for winter: remove leaves and dirt regularly, keep the gaps clear and sweep snow early.
Which larch suits which project?
- Residential deck next to the house: larch smooth 27 × 143 mm on larch joists, visibly screwed with stainless steel.
- Textured surface: the fine-ribbed or coarse-grooved board if you prefer the look of ribbing.
- Steps, platform or boardwalk: larch 43 × 143 mm for a stiff surface; plan the structure with a specialist contractor.
- Matching a larch façade: if you want the façade and deck in the same wood, you will find suitable profiles in our guide rhombus cladding compared.
- Higher durability class wanted: thermo ash (class 1) as an alternative.
Conclusion: plan your larch deck well
Larch decking boards made of Canadian larch combine a lively timber look with durability class 3–4. What matters for your deck is a well-ventilated substructure with joists no more than 50 cm apart, stainless steel screws, pre-drilled boards and a gap of about 7 mm. Whether you oil the wood or let it turn grey is up to your taste.
Choose your board now: take a look at the Canadian larch 27 × 143 mm and order the joists and stainless steel screws at the same time.
Large area or commercial project? Landscapers, carpenters and other businesses can send their enquiry via our business contact form or by email to info@gunreben.de. Private customers can reach us via the contact page.
FAQ on larch decking boards
Answers to common questions about durability, care and installation.
According to our product data, our Canadian larch is classified in durability class 3–4 under EN 350. How long a deck lasts depends heavily on its construction: water must drain away, the joists need clearance from the ground and the surface must be well ventilated. A general figure in years is therefore not possible.
No. According to our installation guide, oil does not improve durability, but it delays greying and helps the surface dry faster. If you like the silver-grey patina, leave the larch untreated and clean it regularly.
According to our product data, both are in durability class 3–4. The choice is mainly a matter of appearance and format: we offer larch in 27 and 43 mm and Douglas fir in 26 and 40 mm. For a higher durability class, thermo ash is the alternative.
A2 stainless steel screws at least 5 mm in diameter and about 2.5 to 3 times as long as the board is thick, two per crossing point. For 27 mm boards, 5.3 × 70 mm screws are suitable, for example. Pre-drill and countersink the boards; the hole may be 0.5 to 1 mm larger than the screw diameter.
Our installation guide generally specifies about 7 mm for dried timber. For decks that are not directly exposed to the weather, gaps down to 4 mm are possible. At butt joints, allow a gap of 6 to 10 mm.