Shed Ventilation: How to Stop Condensation, Damp and Mould

Damp, musty smells, black spots on the roof boards and rusty tools are some of the most common things people tell us about their sheds, and nearly all of them come back to one thing: the air inside isn't moving. The good news is that it's usually simple and cheap to fix.

Why sheds sweat

Every shed holds moisture. It comes up from the ground, off wet mowers, bikes and cushions, out of the timber itself as it takes in and gives off water through the seasons, and from people working inside. When that moist air hits a cold surface, usually the underside of the roof, the glass or metal tools, it condenses into droplets. On a cold, clear night after a mild day it can look as if the roof is leaking. If the moisture has nowhere to go, it soaks back into the timber and everything stored inside.

Signs your shed needs more ventilation

• Droplets on the underside of the roof or on the windows in the morning.
• A musty smell when you open the door.
• Mould or black spots on the roof boards, walls or stored items.
• Tools and bike chains rusting quickly.
• Cardboard boxes going soft, or paper and fabric feeling damp.

Before blaming condensation, check the roof covering and the base. A small tear in the felt or water pooling round the bottom of the walls can look very similar.

Vents, windows and gaps

• Fixed louvre vents let air move even when the shed is locked. The trick is to fit them low on one side and high on the opposite side so air flows right through, not just in and out of one hole.
• Opening windows are the easiest way to air a shed on dry days. If your shed only has fixed glazing, the Power Sheds Opening Window Kit adds a timber window that opens, and Suresheds timber windows can be ordered as sash (opening) rather than fixed when you buy the building.
• Eaves and ridge gaps: many sheds have small gaps where the roof meets the walls. They're there for a reason, so don't fill them with sealant or foam, and don't stack boxes against them.

Airflow under the floor

A timber floor sat flat on wet ground or a slab that collects puddles will stay damp from underneath. The floor should sit on bearers, timber rails that hold it clear of the base so air can pass below. A raised base adds more space for that air to move:

• The Suresheds Tanalised Airflow Base lifts a Suresheds building on 4x2 tanalised timber to let air circulate under the floor. It's made and fitted with your Suresheds building.
• The Power Sheds Shed Base Kit is UC4 pressure-treated timber rated for ground contact, and lifts the floor bearers clear of the ground.
• The Shedlands Timber Shed Base and the Sovereign Retail Base do the same job for those ranges.
• The Shed Boot clips onto the bearers and lifts them off a hard, firm surface.

Our guide to choosing a base compares every option. Whatever you choose, keep a small gap clear around the outside so soil, leaves and plants don't build up against the walls.

The roof: felt, EPDM and insulation

The roof covering's job is to keep rain out. Felt does that well when it's in good condition, but it's the part most likely to tear or lift, so check it every autumn. An EPDM rubber roof is a one-piece membrane with no joins to fail, and it's available as an upgrade on Power Sheds, Suresheds and Sovereign buildings.

Condensation forms on the inside of the roof whichever covering you have, because the roof boards are the coldest part of the shed. That's why ventilation matters even on a brand-new roof.

If you insulate, moisture can get trapped inside the insulation. A vapour barrier stops that. The Suresheds Vapour Barrier is a breathable membrane fitted beneath the internal cladding, and Suresheds don't recommend one if you aren't insulating. Their Internal Package includes the vapour barrier, insulation, internal cladding and a laminate floor. For Power Sheds 44mm log cabins, the Floor & Roof Insulation with DPC includes a damp proof course.

What to store, and what not to

• Let mowers, bikes, tools and cushions dry before you put them away.
• Keep cardboard, paper, fabric and anything precious in sealed plastic boxes, off the floor.
• Leave a hand's width between stored items and the walls so air can move.
• Don't dry washing in a shed, and don't use gas or paraffin heaters in a closed building, as they add a lot of moisture.
• Store tools on wall racks rather than in a heap on the floor.

Insulated sheds and garden rooms

An insulated building is warmer but more airtight, so moisture from people breathing, kettles and heaters has fewer ways out. Open a window or vent for a few minutes every day, even in winter, and use electric heating. Our guide to insulated sheds with electricity covers insulation, power and condensation in more detail.

Winter tips

• Air the shed on dry, bright days, even for half an hour.
• Clear leaves from the roof and gutters, and snow away from the walls.
• Check the felt after storms.
• Re-treat timber in a dry spell before winter. Our timber care guide explains how often, and what timber is best for sheds explains why pressure-treated timber copes better with damp.

Quick checklist

• Air can get in low and out high (vents or an opening window).
• Eaves and ridge gaps are clear.
• The floor sits on bearers, with air moving underneath.
• The roof covering is sound, with no tears or lifted edges.
• Nothing wet is stored inside, and items aren't pressed against the walls.
• If insulated: vapour barrier fitted, and the building is aired daily.

Common questions

Does a shed need ventilation?

Yes. Without airflow, moisture from the ground, damp items and the air inside condenses on the roof, glass and tools, which leads to mould, rust and eventually rot. A vent or opening window and air moving under the floor are usually enough.

How do I stop condensation in my shed?

Let air in low and out high, keep eaves gaps clear, lift the floor off the ground on bearers or a raised base, don't store wet items, and open a window or the door on dry days.

Should I insulate my shed to stop condensation?

Insulation on its own can trap moisture. If you insulate, fit a vapour barrier behind the internal lining and keep ventilating. If you're not insulating, ventilation is the answer.

Where should shed vents go?

Low on one side and high on the opposite side, so air flows right through the building rather than just in and out of one spot.

Is the water on my shed roof condensation or a leak?

Condensation usually appears as droplets across the whole underside of the roof on cold mornings. A leak tends to show in one place, often after rain, and gets worse in wet weather. Check the felt and edges first.

Choosing a new shed? Our best garden sheds guide explains what makes a shed last.

Extras and upgrades vary by range, and some can only be ordered with that brand's building, so check the product page before you order.