How do I keep a garage dry and usable?

A damp garage is nearly always one of two quite different problems, and treating the wrong one is why people run dehumidifiers for years without improvement.

Safety

Step by step

1. Diagnose it before spending anything.Tape a half-metre square of polythene tightly to the floor and leave it a couple of days. Water on top means condensation from the air; water underneath means moisture coming up through the slab.
2. Look for the obvious water routes.Gaps under the door, a failed roof, blocked or missing gutters, ground sloping towards the building, a wall shared with a garden bed. Fixing these costs little and does most of the work.
3. Deal with the roof and gutters first.Flat garage roofs fail at the edges and around any penetration. Missing gutters send water straight down a wall. This is where most garage damp actually originates.
4. Sort out the ground level.Ground should slope away from the building, and soil should not be piled against a wall. A garage floor below the surrounding ground will take water in whatever else you do.
5. Fit or clear ventilation.Air movement is what removes condensation, and most garages have almost none once the door is shut. Vents at high and low level on opposite walls give cross-ventilation.
6. Do not seal it up.The instinct is to close every gap. A sealed damp garage stays damp, because the moisture has nowhere to go. Ventilate rather than seal.
7. Insulate the cold surfaces.Condensation forms on whatever is coldest — usually a metal door, a single-skin wall or the underside of the roof. Insulating those raises their temperature above the dew point, which stops it forming.
8. Deal with a metal up-and-over door.It is the classic condensation surface in a garage. Insulation board bonded to the inside makes a large difference, and a brush seal along the bottom keeps out water and leaves.
9. Seal the floor if it is a bare slab.An old garage slab usually has no damp-proof membrane, so moisture comes up through it continuously. A floor sealer or a damp-proof coating stops it, and it is a large improvement.
10. Add a dehumidifier only after the rest.It is the right final step, not the first. A small unit with a continuous drain, set to about 55%, in a ventilated and insulated garage, works well. In a leaky one it runs forever.
11. Keep everything off the floor and away from walls.Pallets, shelving or a rack. Cardboard, fabric and paper on a concrete floor wick moisture continuously and are the first things to go.
12. Store the right things there.Tools, garden equipment, hard goods. Not paint, which freezes; not documents, photographs or electronics; not fabric or upholstered furniture, which will grow mould.
13. Never run an engine or fuel heater inside.Carbon monoxide accumulates fast in a closed garage. This includes generators, camping stoves and paraffin heaters, and it is the genuinely dangerous garage mistake.
14. Check it after heavy rain.Go and look while it is raining hard. Water routes are obvious then and almost impossible to trace afterwards.

Tips

• The polythene test costs nothing and tells you which of the two problems you have. Everything else follows from it.
• Insulate the metal door. It is usually the coldest surface in the garage and where most of the condensation forms.
• Look at it during heavy rain. Leaks are obvious then and invisible the next day.

Common mistakes

• Sealing every gap to keep damp out — Moisture already inside then has nowhere to go, and condensation gets worse. Garages need ventilation, not sealing.
• Running a dehumidifier without fixing the source — It runs continuously and never gets ahead of water coming in through a roof, a wall or the slab. Fix the ingress first.

If it doesn't work

Questions people ask