A garden room is a wonderful thing. It gives you a place to think, to make, to work without the washing machine rumbling in the background. But because it sits outside the protective envelope of your house, it takes the full brunt of the British weather. A few hours of attention now, before the wet season properly sets in, will save you a great deal of trouble later. The checks below are the ones I'd carry out on any studio, whatever its age or construction.

Start With a Slow Walk Around the Outside

Before you touch a single tube of sealant, spend fifteen minutes simply looking. Go out on a dry day with a notebook and a torch, and work your way around the building methodically. You are looking for the small clues that water leaves behind long before it becomes a visible problem.

• Dark patches on cladding, particularly under windows and at corners, suggest water is tracking where it shouldn't.
• Flaking paint or varnish usually means moisture is sitting in the timber beneath.
• Green or black staining near the base points to splashback and poor drainage.
• Rippled or lifted edges on a flat roof are an early warning sign of trapped water.
• Gaps you can see daylight through from inside — check from indoors too, ideally on a bright day.

Note everything down, even the things that look trivial. It is far easier to plan the work in one go than to keep coming back to it.

Inspect and Renew Seals Around Doors and Windows

Seals are the first thing to fail and the easiest thing to fix. Silicone mastic has a working life of perhaps five to ten years, and it hardens, shrinks and pulls away from the frame as it ages. Run a finger along every joint between frame and wall, and look closely at the base of the door where it meets the threshold — this is where most draughts and damp get in.

Where the sealant has failed, cut it out properly rather than smearing new mastic over the top. Use a sealant remover or a sharp blade, clean the joint with a spirit wipe, and let it dry fully. Then apply a good quality low-modulus silicone or an acrylic frame sealant that stays flexible. Low-modulus products cope better with the movement a timber or composite frame experiences through the seasons.

Don't forget the moving parts. Check the brush or rubber strip on the bottom of the door, the draught excluder around the frame, and the seals on any opening windows. These cost very little to replace and make an immediate difference to both warmth and comfort.

Clear Gutters, Downpipes and Drainage

If your garden room has guttering, it is doing one of the most important jobs on the whole building. Blocked gutters are the single most common cause of damp in garden structures, because water backs up, spills over the back edge, and runs straight down the wall or into the roof structure.

• Scoop out leaves by hand, then flush the run with a hose to check the fall.
• Check every joint and stop end for drips while the water is running.
• Test downpipes by pouring a bucket of water into the top — if it backs up, the pipe is blocked.
• Make sure the outlet discharges well away from the base, ideally into a soakaway or water butt rather than onto the patio.
• Look at the ground around the building. If it slopes towards the studio, consider a shallow French drain or a gravel border to carry water away.

If there is no guttering at all, a simple drip edge or a small run of guttering along the exposed side is a worthwhile addition, particularly where the roof overhangs by less than 150mm.

Treat Timber Before the Wet Arrives

Timber cladding, posts, fascias and decking all benefit from a protective treatment, and late summer or early autumn is the ideal time to apply it. The wood needs to be dry, and you need a couple of rain-free days for the product to cure.

Start by cleaning. A stiff brush and warm soapy water will remove surface dirt; for green algae or mildew, use a proprietary wood cleaner and rinse thoroughly. Allow at least 48 hours to dry in good conditions. Then sand back any rough or greyed patches and dust off.

For bare or weathered timber, a solvent-based wood preserver soaks in and protects against fungal decay and insect attack. Follow this with a stain, oil or paint that suits the look you want. Water-based microporous stains are popular because they let the timber breathe while repelling water. Pay particular attention to end grain at the bottom of posts, cut edges and any joint where water can sit. These are where rot begins.

Check the Roof, Fixings and Ventilation

From ground level, inspect the roof covering for lifted edges, cracks, blisters or loose fixings. On a felt or EPDM roof, a lifted edge can be re-bonded with contact adhesive or a proprietary lap sealant, but it must be done while the surface is completely dry. Check the flashing where the roof meets any wall or upstand, and look at the trim along the eaves.

Inside, look at the ceiling for any brown staining or soft patches, and check the corners of the room. While you're in there, think about airflow. A sealed, unheated garden room will trap moisture from your own breathing and from damp materials, and that moisture has to go somewhere. Trickle vents, a small extractor, or simply leaving a window slightly open on dry days will help enormously. A hygrometer costs very little and tells you what's actually happening — aim to keep relative humidity between 40% and 60%. If it creeps higher, a small dehumidifier will keep tools, paper, fabrics and electronics in good order through the winter.

Finally, walk around and check the fixings: hinges, handles, door closers and any brackets holding shelving or awnings. A quick tighten now prevents bigger movement later, and it is far more pleasant than doing it in January with cold hands.