How do I stop my shed getting damp inside?

Most shed damp is condensation, and most shed damp is treated as if it were a leak. Sealing a shed with condensation makes it distinctly worse, because the moisture has nowhere to go.

What you'll need

• Torch
• Airbrick or vent covers
• Damp-proof membrane
• Timber treatment
• Gravel (optional)

Step by step

1. Work out where the water is.Stains and drips below the roof line point to a roof leak. Damp low down all the way round points to ground contact. Beads of water on the inside of panels and on tools, with a dry roof, is condensation.
2. Check the roof first if there are stains.Felt splits at the ridge and at the eaves, and around any fixings. A leak is worth fixing before anything else because it wets the structure permanently.
3. Check the base and clearance.A shed sitting directly on soil, or on a slab with no membrane, wicks water up into the floor and the bottom of the panels. A gap of at least 50mm below the floor for air is what prevents this.
4. Improve the ground around it.Clear soil, leaves and vegetation away from the walls, and make sure water drains away from the shed rather than towards it. A gravel margin around the base helps considerably.
5. Add ventilation for condensation.Two vents, low on one side and high on the opposite side, so air moves through. Proprietary shed vents or simple covered holes both work, and this is the actual fix for condensation.
6. Do not seal a shed up.An airtight shed with damp air and cold panels condenses on every surface. Ventilation is the counterintuitive but correct answer.
7. Keep wet things out.Wet tools, damp grass boxes, wet coats and bags of compost all release moisture into a closed shed. Let them dry before they go in.
8. Lift stored items off the floor.Pallets or shelving under boxes lets air circulate and keeps things out of any water that does get in.
9. Treat the timber.An annual or biennial coat of a breathable timber treatment on the outside, particularly the bottom two boards and any end grain. Do not use a film-forming paint on the inside.
10. Insulate carefully if you use it as a workspace.Insulation without a vapour barrier and ventilation moves the condensation into the wall build-up where you cannot see it. Insulate, vapour-check on the warm side, and still ventilate.
11. Check the guttering.Water pouring off a roof edge onto the ground splashes back onto the lower boards constantly. A gutter and a butt solves it and gives you free water.

Tips

• Water on the inside of the panels with a dry roof is condensation. Ventilate, do not seal.
• Fifty millimetres of air under the floor prevents most damp problems in a shed.
• A gutter on a shed stops splash-back and fills a water butt at the same time.

Common mistakes

• Sealing every gap to keep the damp out — In a condensation problem the moisture is already inside, coming off wet items and the ground. Sealing traps it and makes the condensation worse.
• Standing a shed directly on soil or grass — Moisture wicks straight into the floor and the bottom boards, and they rot within a few years. A proper base with airflow underneath is what makes a shed last.

If it doesn't work

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