Repairing Cracked Cedar Cladding on Garden Buildings

Why Cedar Cracks, and When to Take Notice

Western red cedar has been the timber of choice for British garden buildings for well over a century, and for good reason. It is naturally resistant to rot, light in weight, and takes on that silvery-grey patina that sits so comfortably beside a stone wall or a box hedge. Left untreated, it will still outlast most softwoods several times over.

What surprises many owners is how quickly the surface begins to split. Cedar is a soft, open-grained timber that swells and shrinks noticeably as moisture levels change across the seasons. A cladding board may gain and lose a few per cent of its width between a wet November and a dry July, and the surface simply has to give somewhere. The result is surface checking — fine hairline cracks running along the grain, typically no deeper than a couple of millimetres.

In most cases this is cosmetic and entirely normal. It is not a fault in the timber and it does not mean the building is failing. The trouble starts when a check opens into a genuine split that reaches a third or more of the way through the board, or when water can sit in a gap and feed fungal growth behind it.

Assessing the Damage Before You Touch It

Walk the building on a dry day and look along each face of the cladding rather than straight at it. Raking light from a low sun will show up splits that are invisible head-on. Carry a thin blade or a small flat screwdriver and a pencil.

• Hairline checks — surface only, less than 2mm deep. Leave these alone. Filling them is usually a waste of effort and the filler will simply crack again.
• Open splits — 3mm wide or wider, or deep enough to take the tip of a blade. These are worth filling, particularly on south and west elevations where driving rain hits hardest.
• Lifting or cupping boards — usually a fixing problem rather than a timber one. Check for corroded nails or screws; replace ferrous fixings with stainless steel.
• Soft, spongy timber or dark staining — this points to decay rather than simple cracking, and the affected board is normally better replaced than repaired.

Mark the splits you intend to treat with a pencil so you can work through them methodically. On a typical shed or summerhouse you are unlikely to find more than a dozen that warrant attention.

Cleaning and Preparing the Split

Filler will not bond to dirt, algae or loose fibres, so preparation matters more than the product you choose. Work when the timber is dry — ideally after three or four rain-free days, and with no rain forecast for the next twenty-four hours.

Begin by brushing the whole area with a stiff hand brush, working along the grain. For green algae or the grey-black mould that colonises shaded north walls, scrub with a dilute solution of a proprietary wood cleaner and rinse thoroughly, then allow to dry completely. A moisture meter is a worthwhile investment here; you want the reading below about 15% before you go any further.

Next, open each split slightly with a blunt scraper and use a knife, a thin hacksaw blade or a piece of folded abrasive paper to clear out any loose splinters and dust. Feather the sharp edges of the split so the filler has something to key into. Finally, blow or brush the dust out and wipe the surface with a cloth barely dampened with white spirit.

Filling the Split Properly

Two families of product work well on cedar. The first is a two-part epoxy wood filler, which sets hard, sands smooth and takes a finish. The second is a flexible exterior-grade resin filler, which moves with the timber and is the better choice for wide splits on boards that see a lot of sun.

Whatever you use, avoid rigid interior fillers, silicone sealant and general-purpose decorator's caulk. Silicone in particular will not accept an oil finish and will leave a shiny, water-trapping scar along the board.

• Press the filler into the split with a plastic spatula, working across the grain, then strike it off flush along the grain.
• Slightly overfill deep splits — most fillers shrink a little as they cure.
• Leave it proud by no more than a millimetre; anything more is hard to sand back without dishing the surrounding timber.
• Allow the full curing time stated on the tin, which may be longer in cool, damp autumn weather.
• Sand flush with 120-grit paper wrapped around a cork block, always along the grain.

If you would rather keep the repair invisible, mix fine cedar sawdust into the filler to tint it. It will never match perfectly, but it will weather down within a season or two.

Refinishing with a Breathable Oil

Cedar needs to breathe. A film-forming finish such as polyurethane varnish or a heavy exterior paint traps moisture inside the board, and when that moisture wants out it lifts the coating in sheets. A penetrating oil — linseed-based, or a modern alkyl-resin exterior oil — soaks into the grain, feeds the timber and lets it move naturally.

Apply with a brush or a lint-free cloth in thin coats rather than one heavy soaking. Work in the shade where possible, since oil applied to hot timber in direct sun can dry patchily and go tacky. Two coats are usually enough on vertical cladding; end grain and any cut edges will take three or four, as they are the thirstiest parts of the board.

Wipe off any surplus after twenty minutes, and give the surface a further day before exposing it to rain. Pigmented oils give more UV protection and hold their colour longer, while clear oils let the cedar silver down naturally — both approaches are perfectly respectable.

A Simple Routine for the Years Ahead

Once you have dealt with the deep splits, a little seasonal attention will keep them from returning. Check the cladding each spring and again in early autumn, and top up the oil every two to three years, or annually on sun-baked elevations. Keep gutters clear so water is not cascading down the boards, and make sure shrubs and climbing plants are kept a hand's width clear of the timber.

Do those small things and a cedar-clad outbuilding will quietly outlast most of the garden around it — cracking, as it always has, with good grace.