Damp and ventilation in a garden bar: keep the room dry and fresh

A bar that smells musty on the first warm evening of spring has spent the winter as a damp box. Damp in garden buildings comes from three places: water getting in, water rising from the ground and moisture from the air condensing on cold surfaces.

Water getting in

• Roof: check the covering, flashing and edges for gaps or damage.
• Windows and doors: look for failed seals and gaps around frames.
• Cladding: split boards or open joints let driven rain in.
• Gutters: overflowing or missing gutters soak walls and the base.

Water from below

• Keep the floor raised off the ground with a ventilated gap where the design allows.
• Make sure the base drains and does not let water pool.
• Keep soil, bark and plant pots away from the bottom of walls.
• Use a damp-proof membrane or barrier where the maker specifies it.

See garden bar base options.

Condensation

Cold walls and roof surfaces meet warm, moist air from people, drinks, damp coats and unflued heaters. The moisture condenses, soaks into timber and fabrics and encourages mould.

• Ventilate. Use vents high and low so air can move through. Open windows or the hatch after use.
• Avoid unflued fuel heaters inside; they add moisture and present carbon monoxide risks. See heating.
• Insulate walls and roof with a suitable vapour control approach so warm air meets a warmer surface. See insulating a garden bar.
• Keep things off cold walls, especially cardboard, upholstered seats and paper.

Winter routine

• Bring cushions, fabrics and paper goods indoors.
• Leave cupboard doors slightly open so air moves.
• Visit the bar occasionally, open it up on dry days and check for leaks.
• Consider a small, safe dehumidifier if the building has a suitable supply.

Signs to act on

• Black mould spots on walls or ceilings.
• Soft, darkened timber, particularly at the bottom of walls or around windows.
• Swollen doors that stick in winter.
• Rust on metal fittings indoors.

Deal with leaks first. No ventilation can keep up with a roof that lets rain in.

Ventilation and building rules

Approved Document F covers ventilation in buildings in England. Most garden bars are outbuildings that do not need Building Regulations approval, but ventilation still matters for health and timber. If you add fuel-burning appliances, ventilation requirements become a safety issue.

Next dry day, walk round the bar after rain and look for wet patches inside. Where water appears is where you start.