I'll let you in on a secret: the last time I looked inside my garden shed, people were still speculating whether Covid was going to be "a thing." After years of neglect, and watching it slowly turn a vibrant shade of green in places, I've honestly been too scared to unlock it and see the damage inside.
But after this year's heatwaves, who knows what winter has in store? So I figured it was time to bite the bullet and tackle the worst of any potential damp and mould before the cold sets in.
Below, I share the tips experts gave me for getting my shed in top condition ahead of a potential cold snap, so I could stop worrying about 2018's Christmas decorations, my exercise bike, and whatever else was lurking in there.
1. Empty it out and identify the real problem
If I'm honest, decluttering my shed felt like a monumental task, and my main reservation was what I thought I was going to find underneath the mess. But according to DIY expert, Georgina Burnett, that's exactly the point of doing it.
"First, empty it completely so you can see how extensive the mould is," she told me. "Before cleaning, you want to identify and fix any sources of moisture and poor ventilation."
"Don't think 'I'm sorting the shed' – think 'I'm finding the problem,'" she said. "Once you know what you're dealing with, the job becomes much less daunting."
That reframing definitely helped. I opened the doors, popped a pair of gloves on and hauled everything out (the bike, several rogue tins of paint that shouldn’t have been stored in the shed, and yes, 2018's Christmas decorations, looking worse for wear).
With the shed empty, what I found wasn't pretty, but it wasn't exactly quarantine-worthy either. The shed had a faint, musty smell that signalled damp and a light ‘dusting’ of green mould on the interior walls that came away with a wipe. It wasn't a great start, but at least now I knew what I was dealing with.
Georgina Burnett is a home and garden expert, TV presenter and property developer, best known for DIY and decluttering advice on ITV's This Morning and Love Your Weekend. She's an ambassador for DIY Toolshare.
2. Check windows and doors
According to Sam Jenkinson, garden shed expert at Tiger Sheds, doors and windows need a little TLC before winter. My shed doesn't have any windows (thankfully), so I turned first to the door frame, which has always felt a bit stiff to close. I'd put that down to age rather than anything more sinister, but Sam suggests otherwise.
"Checking the frames of your wooden doors and windows is integral in keeping your shed waterproof over the winter months, as the cold weather can make the frames more prone to cracking," he explained.
He recommended looking for gaps around the door and frame, and checking for soft, rotten patches. Sure enough, running my thumb along the base of the frame, one corner gave way more than it should have. For gaps, Sam's advice is straightforward: fill them with sealant, expanding foam, or builders' caulk. For actual rot, you have to get a bit more involved. "Dig them out with a screwdriver and fill the area with a high-quality wood filler," then sand it flush and repaint or re-stain once it's dry.
That's exactly what my door frame needed. I dug out the soft section with an old flathead screwdriver (more of it came away than I'd have liked, but at least it confirmed the rot hadn't spread beyond that one corner), filled it, sanded it back, and left it to dry before doing anything else.
Shop for filling cracks and repairing holes
This heavy duty expanding foam fills gaps up to 2.5cm, bonds to wood and masonry, and is tack-free in 8-12 minutes. It's also paintable and sandable once fully cured.
This ready-mixed wood filler is perfect for imperfections up to 5mm deep. It's sandable once dry and suitable for staining, painting or varnishing.
Sam Jenkinson has been at garden building retailer Tiger for over five years and has built up a vast knowledge and experience in all areas. He is a keen gardener and self-proclaimed “shed head”.
3. Check the roof for leaks and replace if needed
According to Sam Jenkinson, shed roofs take the worst of the weather over autumn and winter, making them one of the most common points of failure. "You should start by clearing the roof of any leaves or debris so that rain, sleet and hail can easily run off," he says. "Pay close attention to the edges and ridge line for any signs of wear and tear. You can then repair shingles with sealant or replace damaged ones as required."
Mine had months of debris built up, which I cleared with a stiff broom before checking the felt itself for tears, bubbling or lifted seams – the clearest signs that water has been getting underneath rather than running off. Thankfully, it was in better shape than I'd feared: a couple of small tears near one corner, rather than the widespread damage I'd braced myself for.
Patching a small section of felt is a manageable DIY job with the right materials and a dry day. If the damage is more extensive, it's worth remembering that a leaking or poorly fitted roofing is one of the most common causes of persistent damp, so it's not a step worth doing by halves.
Top tip: For small tears and lifted seams, a self-adhesive roofing repair tape is a quick, low-cost fix that doesn't require re-felting the whole roof.
Try these for shed roof repairs
This strong and durable black mineral felt is perfect for protecting and weatherproofing the roof of your garden shed or workshop. It can be easily fixed to the roof of your shed using clout nails.
This heavy-duty tape bonds instantly to most building materials to provide a permanent, watertight seal in all climates.
4. Improve ventilation
If there's one issue that came up time and again, it was ventilation. According to Sam Jenkinson, it's one thing most shed owners overlook entirely.
"Ventilation is essential in preventing wood rot in sheds as it controls moisture, which is a primary cause of wood rot," Sam explained. "Proper airflow removes excess moisture from the air and wood structure, reducing the buildup of dampness that promotes fungal growth."
It also helps prevent condensation forming on interior surfaces, something that becomes more of a problem as the gap between indoor and outdoor temperatures widens heading into winter. Without it, condensation sits on the wood and fungal spores thrive.
My shed had every red flag: no vents, a door kept shut most of the year, and boxes pushed against the walls. James Crane, director at Crane Garden Buildings, had flagged exactly this when I spoke to him. "You should avoid stacking belongings tightly against the walls as this restricts airflow," he says.
I haven't fitted vents yet, but when putting everything back, I made sure nothing touched the walls directly, leaving a gap around the base of each shelf too.
James Crane is a partner at Crane Garden Buildings, the family-run Norfolk timber buildings company he runs alongside his brother Nick, offering bespoke garden rooms, studios, garages and summerhouses handmade to order.
5. Reduce moisture inside
With better airflow sorted and the roof checked for leaks, the next job was looking at what was sitting inside the shed, and whether any of it was making things worse.
"Check for damp items inside your shed such as paper, cardboard, and fabrics as these can soak moisture from the walls and floors," warned Sam Jenkinson. "If you identify such items, remove them as soon as possible."
Uncomfortable self-reflection followed. A soggy box of decorations and a long-damp tarpaulin were clearly culprits, holding onto moisture rather than letting it escape. Anything cardboard or fabric-based that wasn't essential went straight to the tip. What was worth keeping went into plastic storage boxes instead, off the floor and away from the walls.
Sam's other suggestion was a dehumidifier: "If you have an electrical supply, a desiccant dehumidifier is a good option. This type is suitable for both cold and warm climates, so it's arguably more suitable for UK weather compared to a compressor dehumidifier."
I don't have power run to my shed, so this wasn't an option for me, but if you do, it's worth factoring in, especially for a shed used as an office or hobby space rather than just storage.
Top tip: If you don't own a moisture meter – like this Dryzone Moisture Detector from Amazon – Georgina Burnett suggests borrowing rather than buying one for a single project. Schemes like DIY Toolshare let you borrow tools locally at a fraction of retail cost.
This is a smart, powerful solution for tackling damp, condensation and excess humidity in cooler spaces. Its desiccant technology keeps working effectively even below 10°C.
6. Treat the exterior with a wood preserver
Last on the list, and in some ways the most satisfying step, is giving the outside of the shed a proper protective treatment. According to Sam Jenkinson, this is the step that makes the biggest difference to keeping problems from coming back.
"One of the best ways to prevent leaking wooden shed walls is to treat them with a wood preservative," Sam explains. "This should be carried out once a year to ensure it remains effective at keeping any fungi, dry rot, pests and wet rot at bay."
There's a right way and a wrong way to go about it, though. "Make sure your shed is completely dry as to not trap any moisture," Sam says. "Next, clean the surface and inspect for any damage, such as signs of rot, decay, or cracks in the timber cladding that may need to be repaired before treatment."
That meant waiting for a dry, mild day and giving the shed a proper once-over first: brushing off dirt, cobwebs and loose debris, and checking every panel for anything I'd missed earlier.
"Once this has been done, remove any hardware and lightly sand the surface to enhance the treatment's absorption," Sam continues. "Finally, you're ready to apply the wood preservative with either a brush or roller."
Essentials for shed exteriors
I went with a wax-enriched formula that colours and weatherproofs the wood for up to 5 years. It's non-drip, showerproof within the hour, and low odour too.
Trust me when I say you'll need a pair of stiff bristled brushed like these. They're really helpful for removing algae or moss from the outside of your shed ahead of painting.
FAQs
How do you know when a shed is beyond repair?
Working through this list, I was relieved to discover most of what was wrong with my shed was fixable with a weekend and a fairly modest shopping list. Not every mouldy shed gets off that lightly, though, and it's worth knowing the difference before you sink time and money into a lost cause.
I put this to James Crane, who's seen plenty of sheds in far worse states than mine.
"Some moisture is inevitable in any unheated garden building, but persistent damp shouldn't be," he told me. "The quality of the building and how it has been constructed can make a significant difference."
Mould that keeps coming back, no matter what you do to it, is usually a sign of something structural. "Mould and damp in a garden shed are usually caused by moisture getting in and then having nowhere to escape," James explains. "Common culprits include leaking or poorly fitted roofing, gaps around windows and doors, inadequate ventilation, and a shed base that allows moisture to rise from the ground."
It's also worth being honest about what you're working with in the first place. "More basic, lightweight sheds can sometimes lack the robust construction and weatherproofing needed for year-round British conditions, which may make problems with water ingress, condensation and damp more likely over time," James says.
So how do you actually tell if you're past the point of DIY fixes? A few signs to watch for:
- Rot that keeps spreading. If you've dug out and filled one soft patch, only to find another forming nearby a few months later, the timber itself may be failing rather than just a single damaged section.
- A shed base that's compromised. If moisture is rising up from the ground rather than coming in through the walls or roof, no amount of exterior treatment will fix it, the base itself needs attention or replacing.
- Structural give. Walls, floor or roof panels that flex, sag or feel spongy across a wide area, rather than one contained spot, usually point to damage that's gone beyond a filler-and-preserver job.
- It keeps coming back despite doing everything right. As James puts it, catching a small leak or condensation problem early is far easier than dealing with established mould or damaged timber later on. If you've ventilated, decluttered, treated and it's still returning, the source of the moisture likely hasn't actually been resolved.
It's worth learning how to clean mould off wood properly before you treat and repaint your shed. And once your shed is sound again, choosing the best exterior wood paint for the job will help keep it that way for years to come.
