You open the shed on a cold morning and the underside of the roof is beaded with water. Tools are spotted with rust, cardboard boxes have gone soft, and the roof has been dripping onto things. Nothing leaks. It has not rained.
This is condensation, it is entirely predictable, and it is fixable. Here is the mechanism and then the fixes in the order that costs least.
The mechanism, in one paragraph
Air holds water vapor, and how much it can hold depends on its temperature. Warm air holds more. When air touches a surface colder than its dew point, the vapor it can no longer hold turns into liquid on that surface.
A metal shed roof is the perfect surface for this. It is thin, uninsulated, and it loses heat to the night sky faster than the air inside cools. So on a clear night the roof drops below the dew point of the air underneath it, and the water comes out on the metal.
Three inputs: moisture in the air, a cold surface, and still air. You can attack all three, and two of them are free.
Where the moisture comes from, because it is not the weather
This is the part most guides skip, and it is where the cheap wins are.
The ground. A shed on bare soil, gravel or an unsealed slab sits on top of a continuous supply of moisture rising by evaporation. This is frequently the largest source and it is invisible.
Wet things you put in there. A mower brought in damp, a lawn sweeper with wet leaves in the hopper, tarpaulins, a coiled hose, wet firewood, a bag of soil, damp clothing. Every one of these evaporates into the air inside for days.
Fuel and liquids. Containers vent, and open bags of compost, salt or fertilizer are both hygroscopic and damp.
The shed itself, when new. A slab or a base takes a long time to dry, and a shed erected over a fresh slab will be damp for months.
Air exchange with outside. Warm humid air comes in during the day and condenses at night.
The fixes, cheapest first
1. Take the wet things out, and stop putting them back
Dry equipment before it goes in. Do not store wet tarpaulins, damp firewood or a hose full of water. Do not put a mower or a sweeper in still wet from the grass; let it dry outside first. The general principle applies to any off-season storage, and our guide on storing a tow-behind lawn sweeper for the other ten months sets out the same discipline for a machine that comes in wet by definition.
Free, and on many sheds this alone halves the problem.
2. Ventilate, properly
Ventilation removes the humid air before it can condense, and it is the single most effective structural measure.
Two openings, not one, at different heights. Air needs somewhere to come in and somewhere to leave. A single vent does very little. Low on one side and high on the opposite side gives you a path.
High and low matters more than large. Warm moist air rises, so the high vent does the exhausting and the low one does the supplying.
Do not block them. Shelving pushed against a vent, or boxes stacked over one, removes the only airflow you had. This is extremely common.
Many sheds ship with two small gable vents that are inadequate for a shed full of damp equipment. Adding vents is usually straightforward on a metal or resin shed, and if you are not comfortable cutting into your own shed, that is a reasonable job to hand to someone who is.
3. Deal with the ground
A moisture barrier under the shed floor stops ground moisture rising into the shed. On a new build this is a plastic sheet under the slab or base, and it is far easier to do now than later.
Gravel around and under helps drainage and keeps the base from sitting in standing water.
Move water away from the shed. Check that the ground slopes away and that no downpipe discharges beside it. This is often the actual cause and it is fixed with a shovel.
4. Raise the shed off the wet
A shed sitting directly on soil is wetter than one on a level base with drainage. If your shed is on grass or dirt and you are already fighting damp, the base is a large part of the answer, and it is the part shed sellers are quietest about because it costs real money.
5. Insulate the cold surface
If you have addressed moisture and ventilation and the roof still runs, the remaining lever is the cold surface itself. Insulation or an anti-condensation lining raises the surface temperature above the dew point.
Some metal sheds are available with a factory anti-condensation membrane bonded to the roof panels, which is a coating designed to hold moisture and release it as conditions change rather than let it drip. Retrofitting insulation is possible and is a bigger job.
6. Absorbers and dehumidifiers, last
Chemical moisture absorbers and small dehumidifiers do work in a small, closed space, and they are last on this list for a reason: they treat the symptom continuously, forever, while the ground, the ventilation and the wet mower keep supplying moisture.
If you use them, they are a supplement to ventilation, not a substitute. And a powered dehumidifier in a shed means an outdoor electrical supply, which is its own consideration and, beyond a properly protected existing outdoor circuit, is work for a licensed electrician rather than an extension cord run under the door.
What to do with what is already in there
Get metal tools off the floor and away from the walls. Airflow around them matters as much as the shed's overall humidity.
Stop using cardboard. It absorbs moisture, holds it against whatever is inside, and collapses. Sealed plastic bins are better in every way here.
Lightly oil bare metal before winter storage. This is standard practice for garden tools and it works.
Leave gaps. A shed packed solid has no airflow anywhere, and the damp appears in the middle of the stack where you cannot see it.
The one that surprises people
Resin and plastic sheds condense too. They are marketed as maintenance-free and rot-proof, which is true and is a different question. Plastic is a poor conductor, so the roof does not get quite as cold as bare metal, but a sealed plastic box over damp ground with a wet mower in it will still run with water.
Metal sheds get the reputation because the drips are dramatic and the rust is visible. The mechanism is the same in all of them.
The order, one more time
Remove the wet things. Open two vents at different heights and keep them clear. Fix the ground and the drainage. Then, and only then, consider insulation or absorbers.
Most sheds are fixed by the first two, and both are free.
Frequently asked questions
Why is my shed wet inside when it has not rained? Condensation. The thin, uninsulated roof loses heat to the night sky faster than the air inside cools, drops below the dew point of the air underneath it, and the water vapor in that air comes out as liquid on the metal.
What is the cheapest way to stop shed condensation? Remove the moisture sources and ventilate. Dry equipment before it goes in and stop storing wet tarpaulins, damp firewood or a hose full of water, then open two vents at different heights, low on one side and high on the opposite side, and keep them clear. Both are free and most sheds are fixed by these two.
Where does the moisture actually come from? Mostly not the weather. The ground under the shed is frequently the largest source, followed by wet equipment brought inside, venting fuel containers and open bags of compost, salt or fertilizer, a still-drying new slab, and warm humid air that comes in during the day and condenses at night.
Do resin and plastic sheds get condensation too? Yes, the mechanism is the same. Plastic is a poor conductor so the roof does not get quite as cold as bare metal, but a sealed plastic box over damp ground with a wet mower in it will still run with water.
Do moisture absorbers or a dehumidifier solve it? They work in a small, closed space, but they treat the symptom continuously, forever, while the ground and the wet equipment keep supplying moisture. Use them as a supplement to ventilation, not a substitute, and note that a powered dehumidifier beyond a properly protected existing outdoor circuit means work for a licensed electrician.
Yard Insight has not run a controlled comparison of shed ventilation arrangements or anti-condensation linings, and this article makes no claim otherwise. No humidity figure, dew point, vent area or product is named above. Cutting new openings in a shed and any electrical supply to an outbuilding are jobs to hand over if you are not confident; an outdoor electrical circuit is licensed work.
