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Why a pergola takes a beating in a British garden

A pergola is one of the most exposed structures you can put in a garden. It has a large surface area, plenty of end grain, dozens of joints where water lingers, and it often sits in the shade of the very plants it supports. Add mild, wet winters and humid summers where timber never quite dries out, and you have ideal conditions for fungal decay. Rot rarely announces itself. It creeps in at the joints, works along the grain, and only shows when a beam feels spongy under your thumb. Looked after properly, a timber pergola can last thirty years or more; left alone, it may need replacing within eight. An hour or two each year is all it takes.

Inspect the timber every spring

Late spring is the best time to look your pergola over. The timber has been through the worst of the wet weather, the climbers are not yet dense enough to hide problems, and there are months of drying weather ahead for any treatment you apply.

Use your hands as much as your eyes. A few checks will tell you most of what you need to know:

• Press a screwdriver into the wood at every post base, beam end and joint. Sound timber resists; decayed timber gives way with a dull softness.
• Look for colour changes where rain runs off. Dark damp streaks, or white and grey cottony growth, are early warning signs.
• Check the feet of the posts where they meet soil, paving or a metal shoe. Rot almost always starts here, because water collects and air cannot circulate.
• Examine exposed end grain on beam ends and post tops. It soaks up water like a bundle of straws and softens first.
• Turn a few screws and bolts. Loose, blackened or crumbling fixings usually mean hidden damp behind them.

If you find soft wood, probe gently to find where the sound timber begins. You are looking for the extent of the damage, not a diagnosis on the spot.

Clear the joints and let water drain away

Most pergola rot begins with trapped moisture, and most trapped moisture is held in place by debris. Leaf litter, moss, grass clippings and soil in a bracket all act like a sponge, holding water against the timber for weeks.

• Brush out every joint, notch and lap with a stiff hand brush, then use a scraper or old chisel for compacted material.
• Clear the channel where beams sit on the posts. A small packer or notch that keeps timber off timber is worth adding if yours has none.
• Check the tops of the posts. If water can stand on a flat top, it will. Angle it slightly or cap it.
• If the ground beneath stays boggy, improve drainage around the base or lift the soil away from the posts.
• Trim back climbers that smother the frame. A thick mat of foliage keeps timber permanently damp and hides problems until they are serious.

Always clear debris before you treat, never after. Preservative applied over moss and grime cannot reach the wood.

Treat with a suitable preservative

Once the timber is clean, dry and free of loose material, it is time to feed and protect it. Choose a preservative made for exterior timber, ideally offering both fungicidal and insecticidal protection. Solvent-based products penetrate well and cope with timber that is not perfectly dry. Water-based ones are kinder to nearby plants and lower in odour, but need several dry days either side of application.

• Apply with a brush rather than a sprayer, working the product into end grain, joints and any splits.
• Give end grain two or three coats. It absorbs far more than a flat face and is where water enters first.
• Work in dry weather above about 10°C, with no rain forecast for at least 24 hours.
• On a new pergola, treat bare timber before assembly. Every joint protected before it is closed up will last years longer.
• Reapply a wood oil or stain every two to three years to keep the surface shedding water.

Preservative is a treatment, not a cure. It will not restore timber that has already rotted; it only slows the process. Rotten sections need cutting out and replacing.

Protect the feet, fixings and base

The bottom 300mm of any post is the danger zone. If a post is set into soil or concrete, raise it if you can — a metal post shoe or paving slab lifting the timber clear of the ground will transform its lifespan. Where a post is already buried, dig the soil away to check for softness and keep mulch and plants clear of it.

Use galvanised or stainless steel fixings and check them yearly. Ordinary steel screws rust, stain the timber and hold a pocket of damp against the wood. Where a fixing has rusted, remove it, let the hole dry, and replace it with a treated dowel or a new corrosion-resistant screw.

Deal with early damage before it spreads

If you find a small area of soft wood, cut back to sound timber, shape a replacement piece from treated timber and fix it with waterproof adhesive and stainless fixings. Treat the cut faces with preservative before closing the joint. Repairs like this are quick, cheap and can add many years to a pergola.

For anything more than a shallow patch — a post gone soft at the base, a beam that crumbles when probed — replacement is usually the sensible answer. Wet rot is a moisture problem, so diverting the water matters as much as replacing the timber. Once the source is dealt with and the frame is treated, a pergola will reward you with decades of shade, scent and structure.