Why a wooden summerhouse needs looking after

Timber is a wonderful material for a garden building. It is warm, workable and, treated properly, it will outlast most of us. What it is not is self-maintaining. A summerhouse sits outside all year in our famously changeable climate, taking rain on the roof, driving drizzle against the walls and damp rising from the soil. Left alone, unprotected timber will turn grey, soak up moisture and eventually rot from the joints outwards.

The good news is that keeping a summerhouse weathertight is not difficult. It is a matter of a few hours in spring and a few hours in autumn, done properly, plus a quick walk round after any big storm. Do that and the building stays dry, sound and pleasant to sit in for decades.

Here is how to work through it, from the top of the building down.

Start with the roof, where most damp gets in

Water running downhill is responsible for the great majority of summerhouse problems, so inspect the roof first. Look for lifts, splits, wrinkles and bald patches, and pay close attention to the laps and the edges where the covering wraps over the fascia.

• Replace any torn or brittle felt. The old material may be nailed or glued, but it should lift off with a scraper and a little patience.
• When re-covering, work from the bottom upwards so each sheet overlaps the one below by at least 75mm, with laps facing away from the prevailing wind.
• Fix with galvanised clout nails every 100mm along the edges and 150mm across the middle, then bed the laps down with bitumen adhesive.
• If the roof has had its day, consider upgrading to EPDM rubber or a metal sheet. Both last far longer than felt and need almost no attention.
• Clear moss and leaf litter from the roof surface and from behind the fascias, and fit guttering if the run-off currently pours down the walls.

Treat the timber before the weather turns

Preservative treatment is the single most valuable job you can do. It should be applied to bare or previously treated timber that is clean, dry and free of flaking finish. Ideally, pick a dry spell in spring or early autumn with a temperature above 10°C and no rain forecast for 24 hours.

Use a good-quality wood preserver or oil-based stain rather than a thin decorative paint. A preservative sinks into the grain and protects against fungal decay and insect attack, while a surface film alone will trap water once it cracks.

• Sand weathered surfaces lightly, then brush away dust and any algae or mould with a stiff brush and a mild fungicidal wash.
• Apply with a wide brush, working along the grain, and pay special attention to end grain, door and window edges, and the bottom 300mm of every wall.
• Two coats are far better than one. Allow the first to dry fully before the second.
• Treat the inside of the building too, particularly floor bearers, the underside of the roof and any exposed frame.
• Re-treat every two to three years, or annually if the summerhouse catches the full weather.

Seal joints, windows and doors

Even a well-built summerhouse develops small gaps as the timber expands and contracts through the seasons. Those gaps let in wind-driven rain.

• Check the joints between panels and where the walls meet the roof and floor. Fill deep cracks with a flexible exterior wood filler rather than a rigid one, which will simply crack again.
• Run a bead of exterior-grade silicone or polymer sealant around window frames, door frames and any glazing bars. Smooth it with a wet finger for a neat finish.
• Fit a drip bar or flashing above doors and windows, so water is thrown clear of the opening rather than running into it.
• Fit draught excluder and a brush seal to the door if the gap is wide. It keeps rain out and warmth in.
• Check hinges and locks. A swollen door that no longer closes is a sign that water is getting into the edge of the timber, which needs dealing with promptly.

Keep the base dry and the air moving

Rot almost always starts at the bottom. If the base of your summerhouse sits on damp earth, or if water pools underneath it, no amount of surface treatment will save the timber.

• Stand the building on a solid, free-draining base — paving slabs, a concrete pad or treated bearers laid on gravel.
• Keep a clear 25mm gap between the base and the underside of the floor to allow air to circulate.
• Clear soil, gravel and planted borders away from the bottom edge of the walls. Aim for at least 150mm of clearance all round.
• Fit air vents or leave small gaps at each gable end if your building is well sealed, so damp air can escape rather than condensing on the inside of the roof.

A simple seasonal routine

Little and often beats a big rescue job every ten years. In spring, wash down the exterior, inspect the roof and re-treat any bare patches. In autumn, clear leaves and debris from the roof and guttering, check the seals around openings, and make sure nothing is stacked against the walls. After storms, walk round looking for lifted felt, loose fixings or water pooling at the base.

If you spot dark staining or a soft, spongy feel in the timber, deal with it straight away. Cut out the affected section, treat the surrounding wood with a preservative and let it dry thoroughly before repairing. Caught early, rot is a minor repair. Left alone, it becomes a new summerhouse.