Why Autumn Is the Moment to Act

A shed takes more punishment between October and March than at any other time of year. Driving rain, falling leaves, frost expanding every tiny gap and a general drop in temperature all combine to push moisture into timber, tools and anything stored on the floor. The good news is that a couple of hours spent now, while the weather is still merely damp rather than grim, will save a great deal of trouble later. Work through the jobs below in order and you close off the main routes that water and rot use to get in.

Clear the Gutters and Check Where Water Goes

If your shed has guttering, autumn leaves will have filled it by now. Blocked guttering sends water down the side of the shed instead of away from it, soaking the base of the walls and the floor plate — exactly where rot starts.

• Put on gloves and scoop out the debris by hand, then rinse the run with a hosepipe.
• Check the fall of the gutter. Water should run gently towards the downpipe with no flat spots where it pools.
• Clear the downpipe too. A short length of hose pushed up from the bottom will shift most blockages.
• Make sure the outlet discharges onto a soakaway, gravel bed or water butt — not straight onto the shed base or a path that floods back towards it.
• If there is no gutter at all, consider fitting a few metres. It is a cheap job and it transforms how dry the base of the shed stays.

While you are up there, look at how the roof sheds water. If the felt overhangs the sides only slightly, wind-driven rain can run down the walls. A small drip edge formed with a batten or a strip of flashing will help.

Inspect the Roof Felt Before the Frosts

Roof felt rarely fails suddenly. It lifts, tears and cracks at the edges and around fixings, and once water gets underneath, the roof boards swell and the whole covering has to come off.

• Look along the edges for lifted or torn felt and feel for soft, spongy boards underneath.
• Check the ridge and any overlaps, plus the area around nails or screws.
• Seal small tears with a bitumen repair compound and patch larger ones with a piece of spare felt bedded in adhesive, overlapping the joint by at least 100mm.
• Re-fix lifted edges with galvanised clout nails into new battens, never into the old nail holes.
• If the felt is brittle and cracking across the whole roof, plan to replace it next spring with a heavier grade covering.

Small repairs now cost a few pounds and an hour. A full re-roof costs a weekend and considerably more.

Sweep Out, Dry Out and Beat the Damp

Take everything out, brush the walls and floor, and sweep the lot out the door. This is also your chance to see what has been quietly going mouldy at the back since spring.

• Scrub patches of green or black mould with a diluted bleach solution, then let everything dry with the door open on a dry, breezy day.
• Deal with damp before you put anything back. If the floor is bare earth, lay a damp-proof membrane and cover it with ply or a couple of pallets.
• Stand everything on battens, pallets or shelving. Nothing should sit directly on the floor.
• Raise the shed itself if the base sits below the surrounding ground — water needs to run away, not in.
• Check the door and window seals. A draught excluder and a bead of sealant around the frame keeps driving rain out.

Treat the Timber While It Is Still Dry Enough

Wood preservative needs dry timber and a dry day. Once the autumn rain sets in properly, you have lost your window until spring.

• Brush down the walls and check for soft patches, especially the bottom 300mm of each panel and along the floor plate.
• Poke suspect areas with a screwdriver. If it sinks in easily, cut the rot out and splice in new timber treated with preservative.
• Apply a fresh coat of wood preserver to bare or weathered patches, paying attention to end grain, joints and bottom edges.
• Re-fix loose hinges, hasps and trim with stainless or galvanised screws, and oil the hinges and lock before the cold makes them stiff.

Do not paint over damp timber. It traps moisture and makes the rot worse. If in doubt, leave it to dry and come back to it.

Store Smart and Set Up for Winter

How you put things back matters as much as the repairs themselves. Sheds rot from the contents outwards when air cannot move around stored items.

• Keep a clear 50mm gap between stored items and the walls so air can circulate.
• Lift garden furniture, mowers and bikes onto blocks or pallets, and cover them with a breathable sheet rather than a sealed plastic tarpaulin.
• Store paint, adhesives and anything that dislikes frost in a frost-free place, or accept that they may be ruined.
• Empty and dry out watering cans, hoses and pressure sprayers before the first frost splits them.
• Protect hand tools by wiping metal parts with an oily rag and hanging them rather than leaving them in a damp box.
• Put a couple of moisture absorbers or a tub of cat litter on a shelf to soak up humidity, and check them monthly.

Half a day's work on a bright Saturday in early autumn will keep your shed watertight, your tools usable and your stored belongings dry right through to spring.