Using Larch Timber for Fencing and Gates: A UK Buyer's Guide

Larch has quietly become the workhorse of British fencing. It grows here, it is hard and resinous, and it holds a nail far better than the cheap imported softwood that dominates the fencing aisle. Sawmills in Scotland, Wales and the north of England turn out miles of it every year as boards, rails and posts.

None of that makes it foolproof. Larch has a perishable sapwood, it moves as it dries, and "larch" on a merchant's ticket can cover a wide range of quality. Here is what actually matters when you are buying it for a fence or a field gate.

Why larch works for fencing and gates

Larch sits in a useful middle ground. It is denser and stiffer than most home-grown softwoods, and it weathers to a good silvery grey without the surface flaking you get from some treated timbers. Above ground it holds up well.

Its resin is part of the appeal. Resin slows water uptake and makes the timber more resistant to fungal attack than pine or spruce of the same age. It also makes larch slightly awkward to work: blades blunt faster, and fresh sawn boards can bleed resin in warm weather.

The catch is sapwood. The pale band just under the bark has almost no natural durability. On a fast-grown British larch it can account for a fair proportion of a board's width, and it will rot within a few years if it is left wet and untreated. Good specification starts with knowing how much of the piece is heartwood.

Species and origin: what is on the stack

Three larches do most of the work in the UK. European larch, Japanese larch and the hybrid between them are all grown here and sold as British larch. Siberian and Russian larch arrives as imported stock.

British larch is usually faster grown. You get wider growth rings, more knots and a higher proportion of sapwood. It is often cheaper and has not travelled far. For fence boards, rails and post-and-rail runs it does the job well.

Imported Siberian larch is slower grown, denser and generally more stable, with more heartwood. That makes it a better candidate for gate frames and anything that needs to stay straight. It costs more, and you should check whether it has been kiln dried or shipped green.

Ask two questions at the counter: where was it grown, and what proportion is heartwood? A merchant who cannot answer both is one to be wary of.

Grades, and how to read them

There is no single grading system for fencing timber in the UK, which is why labels vary so much. You will see "fencing grade", "industrial grade", "sawn treated", "cladding grade" and "joinery grade", among others.

Roughly speaking, fencing grade is sorted for strength and price rather than appearance. Knots, wane and some distortion are acceptable. Cladding and joinery grades are sorted for appearance, with limits on knots, splits and sapwood. For a gate, buy joinery grade, and ideally straight-grained dried stock for the stiles.

What to check before you buy:

• Moisture. Fresh sawn larch is heavy with water and will shrink and cup as it dries. For gates and close-boarded panels, buy dried stock.
• Sapwood. Look for the pale band. More than a thin edge on a board that will stay wet is a problem.
• Knots. Tight, small knots are fine. Loose, dead knots and clusters of them will fall out.
• Ring shake and splits. Hairline splits running along the growth rings mean the piece is already failing. Reject it.
• Straightness. Sight down each length. Larch twists, and a twisted rail pulls a whole fence out of line.
• Blue stain. Mostly cosmetic, but it usually points to poor stacking and slow drying.

Rot resistance and where larch belongs

Larch heartwood is classed as moderately durable. That is a broad label, and the practical translation matters more than the category.

For fence boards, rails and cladding — timber kept above ground and able to dry between wettings — untreated larch heartwood gives a good service life. It will not outlast oak, and it will not match a properly pressure-treated softwood post, but for the money it is sound.

In the ground, untreated larch is a poor bet. Posts sit in the one place decay fungi thrive: damp, oxygenated and permanently wet. Use pressure-treated timber rated for ground contact, use oak, or keep the larch out of the soil altogether with a metal post shoe or a concrete collar with a drainage gap.

If you are specifying treatment, the useful reference in the UK is the use class system, which runs from interior dry conditions up to permanently wet. Fence boards and rails generally sit in the exterior above-ground class. Posts in ground contact sit in a higher class and need preservative treatment rated for it.

Treatments that actually help

Pressure treatment is the only reliable way to get preservative deep into the timber. Copper-based treatments are the norm now, and treated larch performs well above ground and reasonably in the ground, though the heartwood resists penetration more than the sapwood does.

Creosote still has a following for agricultural fencing. It is restricted in the UK to professional use, so if you want it, buy pre-treated stock rather than trying to apply it yourself.

Brush-on preservatives, dips and coloured stains are surface treatments. They slow weathering and improve appearance. They do not turn a perishable sapwood board into a durable one, and no coating will save a post sitting in wet clay. Whatever you use, treat every cut end and drilled hole on site — sawn ends are the open door for water and fungi.

Installation notes for a fence that lasts

A post is only as good as its footing. Concrete gives the most rigid fixing; rammed hardcore and soil is faster and adequate for a light garden fence. Either way, bed the base on gravel so water drains away from the end grain, and slope the top of the post so rain runs off instead of sitting on it.

1. Keep timber out of the soil. Rails and boards should sit clear of the ground, ideally 50mm or more.
2. Use hot-dip galvanised or stainless fixings. Larch is acidic and will corrode plain steel and thin electroplated coatings, staining the timber as it goes. Pre-drill near ends and edges to avoid splitting.
3. Leave gaps. Boards swell when wet. A few millimetres between

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